A systematic review of secondhand tobacco smoke exposure and smoking behaviors: Smoking status, susceptibility, initiation, dependence, and cessation.

Addictive Behaviors 47 (2015) 22–32

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Addictive Behaviors

A systematic review of secondhand tobacco smoke exposure and smoking behaviors: Smoking status, susceptibility, initiation, dependence, and cessation Chizimuzo T.C. Okoli a,⁎, Jonathan Kodet b a b

College of Nursing, University of Kentucky, Lexington, KY, USA Department of Counselling Psychology, University of Kentucky, Lexington, KY USA

H I G H L I G H T S • • • •

Secondhand Secondhand Secondhand Secondhand

smoke smoke smoke smoke

a r t i c l e

exposure exposure exposure exposure

is associated with being a smoker. increases smoking susceptibility and initiation. may increase nicotine dependence in adults. hinders smoking cessation

i n f o

Available online 24 March 2015 Keywords: Secondhand smoke Smoking status Smoking susceptibility Smoking initiation Nicotine dependence Smoking cessation

a b s t r a c t Objectives: To examine the association between secondhand tobacco smoke exposure (SHSe) and smoking behaviors (smoking status, susceptibility, initiation, dependence, and cessation). Methods: Terms and keywords relevant to smoking behaviors and secondhand tobacco smoke exposure were used in a search of the PubMed database. Searches were limited to English language peer-reviewed studies up till December 2013. Included papers: a) had clearly defined measures of SHSe and b) had clearly defined measures of outcome variables of interest. A total of 119 studies were initially retrieved and reviewed. After further review of references from the retrieved studies, 35 studies were finally selected that met all eligibility criteria. Results: The reviewed studies consisted of thirty-five (89.7%) studies with differing measures of SHSe (including questionnaire and biological measures) and varying definitions of main outcome variables of interest between studies. The majority of the studies (77%) were cross-sectional in nature. The majority of studies found that SHSe was associated with greater likelihood of being a smoker, increased susceptibility and initiation of smoking, greater nicotine dependence among nonsmokers, and poorer smoking cessation. Conclusions: The review found positive associations between SHSe and smoking status, susceptibility, initiation and nicotine dependence and a negative association with smoking cessation. In light of design limitations, future prospective and clinical studies are needed to better understand the mechanisms whereby SHSe influences smoking behaviors. © 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . 3.1. Description of studies . . . . . . . . 3.2. Secondhand smoke and smoking status

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⁎ Corresponding author at: Tobacco Treatment and Prevention Division, Tobacco Policy Research Program, University of Kentucky College of Nursing, 315 College of Nursing Building, Lexington, KY 40536-0232, USA. Tel.: +1 859 323 6606 (office), +1 859 866 8508 (cell); fax: +1 859 323 1057. E-mail address: [email protected] (C.T.C. Okoli).

http://dx.doi.org/10.1016/j.addbeh.2015.03.018 0306-4603/© 2015 Elsevier Ltd. All rights reserved.

C.T.C. Okoli, J. Kodet / Addictive Behaviors 47 (2015) 22–32

3.3. Secondhand smoke and smoking susceptibility 3.4. Secondhand smoke and smoking initiation . 3.5. Secondhand smoke and nicotine dependence 3.6. Secondhand smoke and smoking cessation . 4. Discussion . . . . . . . . . . . . . . . . . . . 5. Limitations . . . . . . . . . . . . . . . . . . . 6. Conclusions . . . . . . . . . . . . . . . . . . . Role of funding sources . . . . . . . . . . . . . . . . Contributors . . . . . . . . . . . . . . . . . . . . . Conflict of interest . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Secondhand tobacco smoke exposure (SHSe) remains a global health problem with 40% of children and 34% of non-smoking adults exposed and resulting in 603,000 attributable deaths from ischemic heart disease, respiratory infections, asthma, and lung cancer (Öberg et al., 2011). This global impact continues to spur the need for tobacco control measures that aim to limit and protect individuals from SHSe. Particularly, Article 8 of the WHO Framework Convention recommends effective measures to protect exposure from tobacco smoke in both indoor and public spaces (World Health Organization, 2009). These recommendations come from extensive studies suggesting that there is no risk free level of SHSe (U.S. Department of Health and Human Services, 2006). Indeed, even brief and transient SHSe confers health risks (Flouris et al., 2010; Raupach et al., 2006). Although the adverse physical health consequences of SHSe is extensively recognized, (Barnoya & Glantz, 2005; Jones et al., 2011; U.S. Department of Health and Human Services, 2010; Zhong et al., 2000) a growing body of research has begun to assess its behavioral effects. Notably, studies have demonstrated the neurocognitive detriments (Herrmann, King, & Weitzman, 2008; Llewellyn et al., 2009; Swan & Lessov-Schlaggar, 2007; Yolton et al., 2005) and mental health effects (Bandiera, 2011; Bandiera et al., 2010, 2011; Hamer et al., 2011; Hamer, Stamatakis, & Batty, 2010) associated with SHSe. Given its acknowledged psychological and neurocognitive effects, the risks conferred by SHSe may extend to behaviors which reinforce smoking. For instance, environments of tobacco smoke exposure are associated with increased smoking cues that may potentiate smoking behaviors among smokers (Field et al., 2007; McRobbie, Hajek, & Locker, 2008). Additionally, through the psychoactive effect of nicotine exposure from SHSe, novice smokers may be at increased susceptibility to initiate tobacco use (Anthonisen & Murray, 2005). Indeed, clinical studies have found that SHSe results in increased occupancy of nicotineacetylcholine receptors in the brains of adult smokers and nonsmokers, (Brody et al., 2011) suggesting increased neural vulnerability to nicotine exposure from SHS. Despite the putative mechanisms of action whereby SHSe may influence smoking behavior, it is important to first determine to what extent SHSe is associated with such behaviors. Therefore, our study aims to systematically examine the literature for studies assessing the associations between SHSe and smoking behaviors; particularly smoking status, susceptibility, initiation, dependence, and cessation. The finding of this review will be informative in strengthening policies to limit SHSe, prevent smoking uptake, and to promote strategies for cessation. 2. Methods We performed a literature search for published articles in the English language that reported on the relationship between SHSe and smoking related behaviors (i.e., smoking status, smoking susceptibility, smoking initiation, nicotine dependence and smoking cessation) prior to December 2013, through PubMed electronic database. Key terms

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used for the search were as follows: Secondhand tobacco smoke OR Environmental tobacco smoke AND smoking initiation OR smoking susceptibility OR risk for smoking AND nicotine dependence OR cravings OR addiction AND smoking cessation OR quit smoking. Only studies that had clearly defined measures of SHSe and of the main outcome variables of interest were included. A total of 115 studies were initially retrieved and reviewed. Studies that examined the effect of smoking bans on smoking behaviors, (Edwards et al., 2008; Ma et al., 2010; Muilenburg et al., 2009) assessed the ‘exposure to smoking’ but not SHSe, (Bricker et al., 2012; Choi et al., 2002; Okoli et al., 2009) examined the effect of sensitivity to SHSe on behaviors (Lessov-Schlaggar et al., 2011a, 2011b) or examined SHSe but did not examine the relationship with smoking behaviors (AlBedah & Qureshi, 2011) were further excluded. In addition, studies that examined the effect of prenatal tobacco smoke exposure and their relationship to other substance use disorders (Goldschmidt, Cornelius, & Day, 2012) were further excluded from our analysis. After further review of references from the retrieved studies, 35 studies were finally selected that met all eligibility criteria. Meta-analytic techniques could not be adequately performed because of the heterogeneity in the definitions of measures and designs across studies. However, to facilitate our analysis and discussion, we have grouped the results of the review to examine the associations between SHSe and smoking status, smoking susceptibility, smoking initiation, nicotine dependence, and smoking cessation. 3. Results 3.1. Description of studies The retrieved studies consisted of 27 (77%) cross-sectional analyses, seven (20%) longitudinal analyses, and one (3%) case–control study; representing findings from a population of 1,138,101 participants. The samples represent 14 countries with thirteen (37.1%) studies from the United States, six (17.1%) from Canada, four (11.4%) from China or Hong Kong, two (5.7%) from Turkey, one combined Global Health Survey data from Cambodia, Laos, and Vietnam, one combined Global Youth Tobacco Survey data from 168 countries, and one study each from Sweden, Norway, Spain, Italy, Taiwan, Ukraine, and New Zealand. Nineteen (54.3%) studies were among youth, 13 (37.1%) used data solely from adults, and three (8.6%) included data from both youth and adults. Twelve studies (34.3%) examined the association between SHSe and smoking status; (Darling & Reeder, 2003; Glover et al., 2011; Homish et al., 2011; Hong, Kam, & Kim, 2013; Hopenhayn et al., 2013; Kalkhoran, Neilands, & Ling, 2013; Lee et al., 2007; Manzoli et al., 2005; Seo, Bodde, & Torabi, 2009; Skorge et al., 2007; Twose et al., 2007; Tyc, et al., 2004) seven (20.0%) examined the association between SHSe and smoking susceptibility; (Chen, Huang, & Chao, 2009; Guindon, Georgiades, & Boyle, 2008; Kalkhoran et al., 2013; Racicot, McGrath, & O'Loughlin, 2011; Schultz, Nowatzki, & Ronson, 2013; Veeranki et al., 2013; Yang, Leatherdale, & Ahmed, 2011) five (14.3%) examined the

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association between SHSe and smoking initiation; (Andreeva, Krasovsky, & Semenova, 2007; Becklake, Ghezzo, & Ernst, 2005; Glover et al., 2011; Voorhees et al., 2011; Wang, Ho, & Lam, 2011) ten (28.6%) examined the association between SHSe and nicotine dependence; (Bélanger et al., 2008; Grana et al., 2012; Okoli et al., 2008; Okoli & Khara, 2014; Okoli, Rayens, & Hahn, 2007; Racicot et al., 2011; Seo et al., 2009; Vançelik, Beyhun, & Acemoğlu, 2009; Wang et al., 2012; Wilson-Frederick et al., 2011) and ten (28.6%) examined the association between SHSe and smoking cessation (Grana et al., 2012; Helgason et al., 2004; Kalkhoran et al., 2013; Kashigar et al., 2013; Manzoli et al., 2005; Okoli et al., 2008; Okoli & Khara, 2014; Şahbaz et al., 2008; Wang et al., 2013; Wen et al., 2007). With regard to measurement of SHSe, half of the studies assessed SHSe inside of the home. Four studies specifically measured SHSe both at home and at work (Kalkhoran et al., 2013; Kashigar et al., 2013; Skorge et al., 2007; Twose et al., 2007) while other studies only assessed for familial or home SHSe (Darling & Reeder, 2003; Hopenhayn et al., 2013; Manzoli et al., 2005). Ten studies included assessment of SHSe in vehicles (Bélanger et al., 2008; Glover et al., 2011; Homish et al., 2011; Kalkhoran et al., 2013; Okoli et al., 2008; Schultz et al., 2013; Seo et al., 2009; Voorhees et al., 2011; Wilson-Frederick et al., 2011; Yang et al., 2011). Some studies asked subjects to report if they had been close to an active smoker outside or in the same room (Homish et al., 2011; Hong et al., 2013; Seo et al., 2009; Voorhees et al., 2011). Four studies assessed for SHSe at public places such as bars, clubs, or indoor places of leisure (Chen et al., 2009; Kalkhoran et al., 2013; Vançelik et al., 2009; Wilson-Frederick et al., 2011). Some studies asked research participants to respond whether they had been exposed to SHS at all or to rate themselves as rarely or frequently exposed (Andreeva et al., 2007; Chen et al., 2009; Helgason et al., 2004; Hopenhayn et al., 2013; Lee et al., 2007; Okoli & Khara, 2014; Şahbaz et al., 2008). Frequency of SHSe was quantified by studies differently. Some studies assessed SHSe frequency through the number of days exposed (Darling & Reeder, 2003; Guindon et al., 2008; Homish et al., 2011; Veeranki et al., 2013; Voorhees et al., 2011; Wang et al., 2011, 2012, 2013; Yang et al., 2011) while others assessed for the total hours of SHSe (Kashigar et al., 2013; Twose et al., 2007). Still, others measured the total number of situations or sources of SHSe (Okoli et al., 2007; Racicot et al., 2011; Tyc et al., 2004). One study asked subjects to rate their SHSe in places where they spend most of their time (Grana et al., 2012). Finally, five studies measured SHSe or smoking status through biological means (i.e., hair nicotine or salivary cotinine) (Glover et al., 2011; Homish et al., 2011; Okoli et al., 2007; Racicot et al., 2011; Wang et al., 2012).

3.2. Secondhand smoke and smoking status Smoking status describes both frequency (e.g., daily smokers vs. non-daily smokers) and identity (e.g., current vs. non-current smoker) aspects of smoking. Categorizations of smoking status are more clearly established among adult populations but may be more dynamic among children, adolescents, and younger adults (Berg et al., 2010; Delnevo et al., 2004; Okoli et al., 2008, 2009). Twelve studies from our review examined the association between SHSe and smoking status (see Table 1), of which ten examined the relationship between SHSe and smoking identity and two studies examined frequency of smoking (Darling & Reeder, 2003; Kalkhoran et al., 2013). Seven studies examined data from adult populations (Homish et al., 2011; Hopenhayn et al., 2013; Kalkhoran et al., 2013; Lee et al., 2007; Manzoli et al., 2005; Skorge et al., 2007; Twose et al., 2007) and five examined data from youth (Darling & Reeder, 2003; Glover et al., 2011; Hong et al., 2013; Seo et al., 2009; Tyc et al., 2004). Six studies found that SHSe at home was associated with smoking identity; (Darling & Reeder, 2003; Hopenhayn et al., 2013; Kalkhoran et al., 2013; Manzoli et al., 2005; Seo et al., 2009; Skorge et al., 2007) and of the four studies examining

SHSe in vehicles, all but one (Homish et al., 2011) found that SHSe in vehicles is associated with smoking identity. Among youth, all studies found that SHSe is associated with smoking identity. Both a study of New Zealand adolescents and a study of Korean adolescents found significant dose–response relationships between SHSe and the likelihood of being a current or established smoker and of daily smoking (Darling & Reeder, 2003; Seo et al., 2009). A crosssectional study among children in the U.S., (Tyc et al., 2004) and a more recent study of Korean rural youth, also found that SHSe was associated with being a current smoker (Hong et al., 2013). However, a longitudinal study among children (10–13 years of age) in the U.S. found that after controlling for demographic and parental smoking status, car SHSe, rather than home SHSe, was predictive of a three-fold increased risk of being a current smoker (Glover et al., 2011). These studies suggest that among youth SHSe is a salient factor in identity as a smoker. Among adult populations, all studies found that SHSe from various sources and in the various settings (including the home, car, and work environments) is associated with smoking identity and smoking frequency. For example among adults in Spain, SHSe distinguished being an occasional smoker from a never smoker (Twose et al., 2007). Among adult lung cancer patients in the U.S., living in a home with regular SHSe was predictive of a two-fold risk of being a current smoker; (Hopenhayn et al., 2013) and among pregnant women in the U.S., SHSe was predictive of being a current smoker in the first trimester and postpartum (Homish et al., 2011). These studies reinforce the influence of SHSe in continued smoking behavior even within groups for which smoking poses risks for worsened health outcomes. 3.3. Secondhand smoke and smoking susceptibility Smoking susceptibility is a measure of the increased risk for or likelihood to engage in smoking behaviors (Pierce et al., 1996). Strong factors influencing smoking susceptibility include receptivity to tobacco marketing materials and promotion (Feighery et al., 1998; Paynter & Edwards, 2009) and parental and peer attitudes towards smoking (Distefan et al., 1998; Leonardi-Bee, Jere, & Britton, 2011). In our review, all seven studies examining the association between SHSe and smoking susceptibility (Chen et al., 2009; Guindon et al., 2008; Kalkhoran et al., 2013; Racicot et al., 2011; Schultz et al., 2013; Veeranki et al., 2013; Yang et al., 2011) were cross-sectional designs (see Table 2). Six examined the association between SHSe and smoking susceptibility among youth and one study (Kalkhoran et al., 2013) examined this association in adults. Five studies found SHSe to be positively associated with increased smoking susceptibility with two notable exceptions. Among Canadian children, Racicot et al. (2011) found that although biological exposure (using salivary cotinine levels) did not predict smoking susceptibility, the number of smokers in an adolescent's social environment did. In addition, among Canadian youth, Yang et al. (2011) reported that whereas peer and sibling smoking was associated with susceptibility to initiate smoking, parental smoking, SHSe in a car or in the home, and the prevalence of smoking at school were not. These reviewed studies support the direct influence of SHSe on smoking susceptibility; however, the relationship may be mediated by social influences rather than biological. 3.4. Secondhand smoke and smoking initiation Smoking initiation involves the actual action of beginning smoking. The decision to initiate smoking can be influenced by several factors such as observing smoking in movies, (Charlesworth & Glantz, 2005) parental, peer, and sibling smoking behavior, (Tyas & Pederson, 1998) and context, culture, and gender (Okoli, Greaves, & Fagyas, 2013). Five of our reviewed studies examined the association between SHSe and smoking initiation; (Andreeva et al., 2007; Becklake et al., 2005; Glover et al., 2011; Voorhees et al., 2011; Wang et al., 2011) of which


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Table 1 SHS exposure and smoking status. Authors (year)

Participants (country), and design (sample size)

Hong et al. (2013).

Rural adolescent males (Korea) Cross-sectional analysis (N =

Hopenhayn et al. (2013).

Kalkhoran et al. (2013)

Homish et al. (2011)

Glover et al. (2011)

Seo et al. (2009).

Lee et al. (2007)

Measures

Results

SHS exposure: Close to an active smoker for one day or more in the most recent seven days. Smoking status: Current smoker (smoked on one or more 526,520) days in the most recent 30 days) Adults lung cancer patients (USA) ETS exposure: “Do you currently live in a home where you Longitudinal analysis (N = 89) are regularly exposed to secondhand smoke indoors?” (Yes/No). Smoking status: Smoke currently? (Yes/No). Young adult bar patrons aged 18 to 26 SHS exposure: Exposed to other people's tobacco smoke in the past 7 days (choices were home, car, indoors at work, a years (USA) bar or club). Cross-sectional analysis (N = 3819) Smoking status: Nondaily smokers (smoked for 1–29 days in past 30 days) or daily smokers (smoked for 30 days in past 30 days). Adult pregnant women (USA) Social network smoking: Spouse or partner who smoked, Longitudinal analysis (N = 316) number of smokers in the household (excluding the partner), proportion of relatives who smoked, and proportion of friends who smoked. ETS exposure: Frequency of SHS exposure in the past 7 days (by number of days in the past week in the same room, car, or outside with someone who was smoking). Smoking status: Self-report and saliva cotinine. Children 10 to 13 years olds (USA) SHS exposure: “Who was smoking around you in your Longitudinal analysis (N = 3607) home during the past seven days?” “Who was smoking around you in your car during the past seven days?” Subject was categorized as exposed to SHS if at least one person was smoking around them. Smoking status: Current smoking (irrespective of how frequent). Adolescent smokers (USA) ETS exposure: Number of days in the same room with Cross-sectional analysis someone who was smoking cigarettes or was in a car with (N = 8355) someone who was smoking during the past 7 days. Smoking status: Current smoker (smoked at least 1 day during the past 30 days) and established smoker (smoked

Adult African Americans (USA) Cross-sectional analysis (N = 388)

Skorge et al. (2007)

Adults (Norway) Cross-sectional analysis (N = 2819)

Twose et al. (2007)

Adults (Spain) Cross-sectional analysis (N = 1059)

Manzoli et al. (2005)

Adult women (Italy) Cross-sectional analysis (N = 9708)

Tyc et al. (2004).

Pre-adolescents and adolescents treated for cancer (USA) Cross-sectional (N = 47)

Darling et al. (2003). Darling and Reeder (2003)

Adolescents (New Zealand) Cross-sectional (N = 3434)

N 20 of the past 30 days and has smoked N 100 cigarettes during their lifetime). SHS exposure: Based on questionnaire (No vs. Yes). Smoking status: Current tobacco use (No vs. Yes).

Current smoking was significantly increased in the group who had been exposed to secondhand smoking. ETS was an independent predictor of being a current smoker.

Significant associations were found between SHS exposure at home and in a car and being a daily or nondaily smoker.

–Partner smoking was significantly associated with first trimester smoking. –ETS exposure (in a room or outside) was associated with increased risk for first trimester smoking. –A greater proportion of friends who smoke was associated with first trimester smoking. SHS exposures in cars and at home were each associated with a significant 3-fold increase in risk of current smoking.

–ETS exposure was associated with current and established smoking. –A dose–response relationship was found between ETS exposure (in home or in cars) and the likelihood of current or established smoking. Those with SHS exposure were 3.8 (95%CI =

2.1–6.8) times more likely to be current tobacco users than individuals reporting not being exposed to SHS. ETS exposure: Current exposure to passive smoking at –Compared to never smokers, current smokers home (every day, sometimes, and no) and current exposure were more likely to report ETS exposure in the to passive smoking at work (every day, sometimes, and no). home and at work. Smoking status: Current smokers (1–9, 10–19, and over 20 cigarettes smoked daily). SHS exposure: Participants who stated they were not –Among males, no association between former exposed to SHS were coded as “None”, those who stated smokers and home, leisure, work, or overall SHS they were exposed during b 1 h per day from Monday to exposure relative to never smokers. No Thursday or from Friday to Sunday were grouped in the significant association between occasional “Low” intensity category; those who were exposed 1 to 4 h smokers and home, leisure and work SHS per day were classified in the “Intermediate” intensity exposure, but occasional smokers were 3.6 (95% category, and participants who were exposed N 4 h per day CI = 1.2–10.7) times more likely than never on any day of the week were classified in the “high” smokers to have overall SHS exposure. intensity category. Smoking status: Never, former, and Occasional. –Compared to never smokers, current smokers SHS exposure: Exposed to smoking relatives who smoked were significantly 1.7 (95%CI = 1.5–2.0) times in participants' presence. Smoking status: Current smokers (smoked at least one more likely to report extra-familial SHS cigarette per day), ex-smokers (had quit smoking at least exposure. 1 year before the survey), and never smokers. ETS exposure: Cumulative ETS index score based on –Ever smokers had higher levels of ETS number of sources and intensity of exposure (based on exposure than never smokers. parental report). –Adolescents who had smoked in the past or Smoking status: Ever smoker (has ever used tobacco in the who were current smokers had higher past month) vs. never smoker. cumulative ETS exposure index scores than children who had never used tobacco. SHS exposure: Number of days in the past seven days –A clear dose–response relationship with people have smoked around individual in their own home. increased SHS exposure in the home associated Smoking status: Daily smoking (smoked at least once a day). with increased daily smoking.

two were cross-sectional, (Andreeva et al., 2007; Voorhees et al., 2011) two were longitudinal, (Becklake et al., 2005; Glover et al., 2011) and one included both cross-sectional and longitudinal analyses (Wang

et al., 2011) (see Table 3). Four studies examined the association between SHSe and smoking initiation with samples of children or youth while one study (Andreeva et al., 2007) studied a sample of both

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Table 2 SHS exposure and smoking susceptibility. Authors (year)


Measures

Results

Veeranki et al. (2013)

Adolescents, 13–15 years (Global). Cross-sectional analysis (N = 356,414).

–SHS exposure inside or outside the home was significantly associated with increased susceptibility to smoking.


Adult bar patrons, aged 18 to 26 years (USA). Cross-sectional survey (N = 3819).

Schultz et al. (2013)

Youth in grades 5–12 (Canada). Cross-sectional survey (N = 71,003).

SHS exposure: 1) During the past week, on how many days have people smoked in your home, in your presence? 2) During the past 7 days, on how many days have people smoked in your presence, in places other than in your home? Susceptibility to smoking: Four-point ordinal scale: (1) definitely not; (2) probably not; (3) probably yes; and (4) definitely yes, as based on the following three questions: 1) “If one of your best friends offered you a cigarette, would you smoke it?” 2) “Do you think you will be smoking cigarettes 5 years from now?” 3) “At any time during the next 12 months do you think you will smoke a cigarette?” SHS exposure: Exposure to other people's smoke “in your home,” “in a car,” “indoors at work,” and “at a bar or club” in past 7 days. Smoking susceptibility: Nonsmokers' responded to the question, “Do you think you will smoke a cigarette in the next year?” on a 5-point Likert scale; any response other than “definitely not” was coded as being susceptible to smoking initiation. SHS exposure:1) Exposed to tobacco smoke in a vehicle on at least 1 day in the past 7 days Smoking susceptibility: (1) “Do you think in the future you might try smoking cigarettes?”; (2) “If one your best friends offered you a cigarette would you smoke it?”; (3) “At any time in the next year do you think you will smoke a cigarette?” Strong intention = response to all 3 questions was definitely not. Weak intention =

Racicot et al. (2011)

Youth never smokers (Canada). Cross-sectional analysis (N = 327).

Yang et al. (2011)

Youth never smokers (Canada). Cross-sectional analysis (N = 41,886).

Chen et al. (2009).

Adolescents in junior and senior high school (Taiwan). Cross-sectional analysis (N = 8607).

Guindon et al. (2008)

Adolescents (South East Asia). Cross-sectional analysis (n = 21,238).

SHS = secondhand smoke, ETS = environmental tobacco smoke.

at least one of these questions was answered with a response other than definitely not, which indicates thoughts of possibly smoking in the future. Social smoke exposure: 1) Number of current smokers in the social environment, 2) Number of situations where SHS exposure occurs. Pharmacological exposure to nicotine: Salivary cotinine. Smoking susceptibility: 1) “Have you ever been curious about smoking a cigarette?: 0 = No, 1 = Yes”. 2) If one of your best friends were to offer you a cigarette, would you smoke it? (0 = Definitely no, 3 = Definitely yes”). A composite score was created by summing the five items, and the total score ranged from 0 (no susceptibility) to 11 (highest susceptibility). SHS exposure: The number of days exposed to smoking in a car during the previous week, the number of people who smoke inside their home daily. Smoking susceptibility: “Do you think in the future you might try smoking cigarettes?”, “If one of your best friends were to offer you a cigarette, would you smoke it?”, and “At any time during the next year, do you think you will smoke a cigarette?” Those who answered ‘definitely not’ to all three questions were considered non-susceptible; they were considered susceptible to future smoking if they responded positively to at least one item. SHS exposure: At home and in public places (yes vs. no). Smoking susceptibility: The absence of a firm decision not to smoke. SHS exposure: A five-point ordinal scale from 0 days/week to 7 days/week. Smoking susceptibility: derived using the following questions (using a four-point ordinal scale): 1. Do you think you will be smoking cigarettes five years from now? 2. If one of your best friends offered you a cigarette, would you smoke it? 3. At any time during the next 12 months do you think you will smoke a cigarette? Students who answered “definitely not” to all three questions were considered non-susceptible; all other students were considered susceptible.

–SHS exposure was associated with susceptibility to initiation.

-Being exposed to tobacco smoke in a vehicle on at least 1 day in the past 7 days increased the odds of being a susceptible nonsmoker (vs. a non-susceptible nonsmoker).

–Social smoke exposure was a predictor for smoking susceptibility but not pharmacological exposure to nicotine.

–Exposure to smoking in a car or in the home was not significantly associated with the odds of being susceptible to smoking

–Significantly more susceptible junior high students than non-susceptible students of both genders were exposed to secondhand smoke at home and in public places. SHS exposure at home increased the odds of being susceptible (OR = 1.08, 95% CI 1.04 to 1.12).


27

Table 3 SHS exposure and smoking initiation. Authors (year)


Measures

Results

Wang et al. (2011)

Children (Hong Kong). Longitudinal analysis (n = 2051).

SHS exposure: “How many days in the past 7 days did you breathe in secondhand smoke at home?” (“none,” “1–3

–Significant associations were found between SHS exposure and smoking initiation.

Glover et al. (2011)

Children 10 to 13 years old (USA). Longitudinal analysis (N = 3607).

Voorhees et al. (2011)

Adolescents (USA). Cross-sectional analysis (n = 57,123).

Andreeva et al. (2007)

Young adults, 15–29 years (Ukraine). Cross-sectional analysis (N =

days/week,” and “4–7 days/week”). Smoking initiation: Never smoking at baseline but ever smoking at follow-up. SHS exposure: “Who was smoking around you in your home during the past seven days?” “Who was smoking around you in your car during the past seven days?” If at least one person was smoking around student, subject was categorized as exposed to SHS. Smoking initiation: All students who had ever had a cigarette, even just a few puffs, but reported not being current smokers were classified as initiated smokers. SHS exposure: Frequency (range, 0–7) of exposure during the past 7 days, lived with a smoker, how many days in the same room with someone who was smoking cigarettes, and how many days rode in a car with someone who was smoking cigarettes. Smoking initiation: Starting smoking in the past year and meeting the current smoking criteria of smoking 1 + days in the past 30 days. SHS exposure: Questionnaire (rare vs. frequent exposure). Smoking initiation: The reported age of the first cigarette smoked and the reported age of daily smoking initiation.

609).

Becklake et al. (2005)

Children (Canada) Longitudinal analysis (N = 191)

SHS exposure: Saliva cotinine Having become a smoker: Has smoked at least 1 cigarette a week for a month.

–SHS exposure in cars was associated with a significant threefold increase in risk of smoking initiation. –SHS exposure at home was not significant after including all variables in analysis.

–Smoking initiation was positively associated with reporting SHS exposure. –SHS exposure at home or in the car was associated with both smoking initiation and initiation of regular smoking.

–Males with rare exposure to SHS were less likely to initiate smoking at the same age (earlier age) than their counterparts frequently exposed to SHS (OR = 0.8, 95%CI = 0.7–0.9). –The same was true for women (OR = 0.8, 95%CI = 0.7–0.9). –Salivary cotinine in childhood was an independent predictor of adolescent smoking, even after adjustment for amount smoked at home and other relevant factors.


youth and adults. In the cross-sectional analyses, studies among young adults in Ukraine (Andreeva et al., 2007) and adolescents in the U.S. (Voorhees et al., 2011) both found that SHSe was associated with retrospective reports of earlier age of smoking initiation and past year initiation. Longitudinal studies among adolescents from Hong Kong, U.S.A. and Canada found that SHSe at home was predictive of smoking initiation (Becklake et al., 2005; Glover et al., 2011; Wang et al., 2011). Specifically, Wang et al. (2011) found a dose response relationship such that each additional day of SHSe at home was associated with a 1.2 times greater odds of smoking initiation. Glover et al. (2011) found that smoking initiation significantly and independently predicted 1.5 times the odds of SHSe from the home and 1.9 times the odds of SHSe from the car. Finally, Becklake et al. (2005) found that salivary cotinine level was an independent predictor of becoming a smoker for both postpubertal (Odds ratio = 1.9, 95% CI = 1.2–3.0) and prepubertal (Odds ratio = 2.1, 95% CI = 1.0–4.5) adolescents. These reviewed studies all support that SHSe is an important predictor of smoking initiation. 3.5. Secondhand smoke and nicotine dependence Nicotine dependence is a measure of both physical and psychological dependence on smoking (Shadel, Shiffman, Niaura, et al., 2000). Nicotine dependence is associated with several bio-psycho-social factors which promote the use and desire to continue smoking through behavioral reinforcements that are both negative (such as withdrawal and craving) and positive (such as relaxation and euphoria) (Benowitz, 2010). A little studied factor for nicotine dependence is SHSe. All nine of our reviewed studies assessing the associations between SHSe and nicotine dependence were cross-sectional designs. Four studies used samples of only adults, (Okoli & Khara, 2014; Okoli et al., 2007, 2008; Wilson-Frederick et al., 2011) four used samples of only children and adolescents, (Bélanger et al., 2008; Racicot et al., 2011; Vançelik et al., 2009; Wang et al., 2012) and one study used a sample of both

adolescents and adults (Grana et al., 2012). Moreover, three studies assessed the effect of SHSe on reports of nicotine dependence or withdrawal symptoms among nonsmokers (Bélanger et al., 2008; Okoli et al., 2007; Racicot et al., 2011) and seven examined these associations among smokers (Grana et al., 2012; Okoli et al., 2008; Okoli & Khara, 2014; Okoli et al., 2007; Vançelik et al., 2009; Wang et al., 2012; Wilson-Frederick et al., 2011). Seven studies found that SHSe was significantly associated with nicotine dependence symptoms in their respective populations; specifically, SHSe was significantly associated with reporting of nicotine dependence symptoms in pre-adolescent nonsmokers, (Bélanger et al., 2008) behavioral symptoms akin to nicotine withdrawal in adult nonsmokers, (Okoli et al., 2007) nicotine dependence in adolescents smokers, (Vançelik et al., 2009; Wang et al., 2012) and nicotine dependence in adult smokers in general (Okoli et al., 2008; Wilson-Frederick et al., 2011) and in male adult smokers specifically (Okoli & Khara, 2014). However, Grana et al. (2012) found no significant association between SHSe and nicotine dependence in a sample of 60 adolescent and young adult smokers; and Racicot et al. (2011) found no significant association between SHSe and reported nicotine dependence symptoms among nonsmoking children. Finally, Okoli et al. (2007) found that hair nicotine levels were not significantly associated with nicotine dependence scores among adult smokers. These reviewed studies provide mixed evidence regarding the association between SHSe and nicotine dependence (Table 4). 3.6. Secondhand smoke and smoking cessation Smoking cessation is affected primarily by motivation to quit, past quit attempts, and cigarette dependence (McCaul, Hockemeyer, Johnson, et al., 2006; Vangeli, Stapleton, Smit, et al., 2011). Although demographic and socioeconomic factors have not been consistently associated with cessation (Vangeli et al., 2011), few studies have systematically examined the contribution of SHSe. Of the ten studies that

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Table 4 SHS exposure and nicotine dependence. Authors (year)

Participants/sample (country), and design (sample size)

Measures

Results

Wang et al. (2012)

Adolescent smokers (Hong Kong). Cross-sectional analysis (N = 2182).

SHS exposure: Exposure for 0, 1–4 and 5–7

–Increasing days of SHS exposure at home was associated with nicotine dependence.

Grana et al. (2012)

Adolescents and adults with mental illnesses (USA). Cross-sectional analysis (N = 60).

Wilson-Frederick et al. (2011)

African Americans adult smokers (USA). Cross-sectional analysis (N = 246).

Racicot et al. (2011)

Adolescent never smokers (Canada). Cross-sectional analysis (N = 327).

Vançelik et al. (2009)

Adolescent smokers (Turkey). Cross-sectional analysis (N = 536).

Okoli, Browning, Rayens, et al. (2008)

Adult smokers (USA) N = 822 Cross-sectional analysis

Bélanger et al. (2008)

Children, never smokers (Canada). Cross-sectional analysis (N = 1488).

Okoli et al. (2007)

Adult nonsmoking and smoking bar and restaurant workers (USA). Cross-sectional analysis (N = 105).

Okoli and Khara (2014)

Adults with substance use and or mental illness (Canada). Cross-sectional analysis (N = 502).

days/week. Nicotine addiction: Question adapted from the Fagerström Test of Nicotine Dependence (FTND) Secondhand smoke (SHS) exposure: 1) In the home 2) In the place they spend most of their time (both coded as 0 = smoke free, 1 = occasionally or usually smoky, 2 = always smoky). Nicotine dependence: Seven items from the Modified Fagerström Tolerance Questionnaire (mFTQ) for use with adolescents. SHS exposure: SHS exposure at home or at home and in other settings (reported as hours and/or days). Nicotine dependence: Diagnostic and Statistical Manual of Mental Disorders IV, Text Revision (DSM-IV-TR) and 2. Three items from the Fagerström Test for Nicotine Dependence (FTND) in this analysis: (1) time to first cigarette, (2) heaviness of smoking, and (3) difficulty in refraining from smoking. SHS exposure: 1) Number of current smokers in the social environment and 2) Number of situations where SHS exposure currently occurs. Pharmacological exposure to nicotine: Salivary cotinine. Perceived nicotine dependence: Nicotine Dependence Scale for Adolescents. SHS exposure: At home or during indoor leisure time activities (verified by expired carbon monoxide levels). Nicotine dependence: Fagerström Test for Nicotine Dependence. SHS exposure: Summary score of no source (neither car nor home), one source (either car or home), or both sources (both car and home) of exposure with a range of possible scores from 0 to 2. Nicotine dependence: Fagerström Tolerance Questionnaire (FTQ). SHS exposure: Number of people who smoke inside the home every day or almost every day? (none, 1, 2, 3, 4, and 5 or more people) and number of days ridden in a car with someone who was smoking cigarettes (0, 1 or 2, 3 or 4, 5 or 6, all 7 days). Nicotine dependence symptoms: Seven items drawn from the Nicotine Dependence Scale for Adolescents. SHS exposure: Hair nicotine & number of sources of SHS exposure. Behavioral symptoms: Among nonsmokers assessed by DSM-IV nicotine withdrawal symptoms. Nicotine dependence: Among smokers assessed by Fagerström Test for Nicotine Dependence (FTND). Secondhand smoke (SHS) exposure: Asked question: “Are you exposed to secondhand smoke?” Nicotine dependence: Fagerström Test for Nicotine Dependence (FTND).

–Nicotine dependence was not associated with secondhand smoke exposure.

–After adjustments, SHS exposure at home only, and in both current home environment and other settings, were associated with clinically-defined nicotine dependence.

–Social and pharmacological exposure to SHS did not predict perceived nicotine dependence

–Adolescents with SHS exposure had higher nicotine dependency than non-exposed adolescents, regardless of the level of nicotine dependency. –Greater SHS exposure was significantly associated with higher nicotine dependence scores even when controlling for demographic and other smoking related variables.

–After controlling for sibling, peer smoking, and susceptibility to initiating smoking, SHS exposure in a motor vehicle was independently associated with ND symptoms, but not exposure in the home.

–Nonsmokers with higher hair nicotine levels were 2.2 times more likely (per ng/ml of nicotine) to report nicotine withdrawal symptoms based on DSM-IV criteria. –SHS exposure was not significantly associated with nicotine dependence among smokers. –Greater nicotine dependence scores were significantly associated with SHS exposure for men but not for women.


examined the association between SHSe and smoking cessation in our review, seven studies were cross-sectional designs, (Grana et al., 2012; Kalkhoran et al., 2013; Manzoli et al., 2005; Okoli & Khara, 2014; Okoli et al., 2008; Wang et al., 2013; Wen et al., 2007) two were longitudinal, (Helgason et al., 2004; Kashigar et al., 2013) and one was a case–control study (see Table 5) (Şahbaz et al., 2008). Two studies examined this association with samples of youth; (Wang et al., 2013; Wen et al., 2007) six studied this association with adults (Helgason et al., 2004; Kalkhoran et al., 2013; Kashigar et al., 2013; Manzoli et al., 2005; Okoli & Khara, 2014; Okoli et al., 2008). The majority of findings supported a significant association between SHSe and poor smoking cessation. Five

studies (Helgason et al., 2004; Kalkhoran et al., 2013; Okoli & Khara, 2014; Okoli et al., 2008; Wang et al., 2013) specifically found a significant association between SHSe and quit attempts. In particular, SHSe in the home was significantly associated with a lower likelihood of quit attempts among adolescent smokers in Hong Kong and China (Wang et al., 2013; Wen et al., 2007). Okoli et al. (2008) found that adult smokers in the U.S. reporting exposure from both their home and a vehicle were less likely to successfully quit smoking for a day in the previous year and to be less inclined to quit smoking in the next 6 months. In contrast, Kalkhoran et al. (2013) found that SHSe in a car was associated with having made an attempt to quit smoking for


29

Table 5 SHS exposure and smoking cessation. Authors, title, and year


Measures

Okoli and Khara (2014)

Adult smokers with substance use and mental illnesses (Canada). Cross-sectional analysis (N = 502).

SHS exposure: Asked question: “Are you exposed –SHS exposure was not significantly associated with to secondhand smoke?” number of past quit attempts. Smoking cessation: Number of past quit attempts were obtained (with response choices: never =

Wang et al. (2013)

Adolescent smokers (Hong Kong). Cross-sectional analysis (N = 4.361).

Kashigar et al. (2013)

Adult head and neck cancer patients (USA). Longitudinal analysis (N = 298).


Adult bar patrons aged 18 to 26 years (USA). Cross-sectional analysis (N = 3819).

Grana et al. (2012)

Adolescents and adults, 13–25 years old (USA). Cross-sectional analysis (N = 60).

Okoli, Browning, Rayens, et al. (2008)

Adult smokers (USA). Cross-sectional analysis (N = 822).

Şahbaz et al. (2008)

Adults (Turkey). Case–control study (N = 169).

Wen et al. (2007))

Adolescent pupils (China). Cross-sectional analysis (N = 3957).

Manzoli et al. (2005)

Women (Italy). Cross-sectional analysis (N = 2582).

Results

0, 1 to 5 times = 1, 6 or more times = 2). SHS exposure: Number of days in the past seven days that someone has smoked near them at home. Quit attempts: In the past 12 months, have you ever tried stopping smoking?” (Yes vs. No) Smoking cessation: Quitting smoking for at least three months in the past 12 months. SHS exposure: Hours of smoking exposure at home or work (0 to 5+ hours). Smoking cessation: Self-reported smoking behavior at the time of follow-up: “Are you currently smoking? Yes/No.” SHS exposure: Exposure to other people's smoke “in your home,” “in a car,” “indoors at work,” and “at a bar or club” in past 7 days. Smoking cessation: Had stopped smoking tobacco for 1 day or longer because they were trying to quit as having made a quit attempt. SHS exposure: 1) In the home 2) In the place they spend most of their time (both coded as 0 = smoke free, 1 = occasionally or usually smoky, 2 = always smoky). Smoking cessation: Stage of change (precontemplation, contemplation, and preparation). SHS exposure: 1) A summary score of exposure was calculated with individuals having no source (neither car nor home), one source (either car or home), or both sources (both car and home) of exposure with a range of possible scores from 0 to 2. Smoking cessation: Measured by 1) length of abstinence on last quit attempt (range from less than 1 day to greater than 1 year), 2) intention to quit smoking in the next 6 months (yes vs. no), and 3) successful abstinence for more than 24 h in past year (yes vs. no). ETS exposure: Assessed by 1) exposure to environmental tobacco smoke in the home and 2) number of persons smoking in the home. Smoking cessation: Defined as to stay away from cigarette and other tobacco products at least six months. Passive smoking: Have inhaled the exhaled smoke by others over 15 min per day. Attempting to quit: Have attempted to quit in the past year. SHS exposure: Exposed to smoking relatives who smoked in participants presence AND extrafamilial environmental smoke exposure (exposed vs. not exposed). Smoking cessation: Had quit smoking at least 1 year before the survey.

Helgason et al. (2004),

Adults from quit line (Sweden). Longitudinal analysis (N = 1594).

ETS exposure: Exposed to second hand smoke (yes vs. no). Abstinence: Remained abstinent from 12–14 months after contact with quitline.

–SHS exposures inside and outside the home were each associated with lower prevalence of quit attempts and smoking cessation.

–Among current smokers, patients were more likely to quit by the time of follow-up if they had no SHS exposure at home. –The odds of smoking cessation were highest with the lowest number of hours exposed. –No association between quit attempts and any SHS exposure in the past 7 days or between quit attempts and SHS exposure at a bar, home, or indoors at work. –For car SHS exposure, in Oklahoma City, SHS exposure in a car was associated with an attempt to quit smoking –SHS exposure was not associated with stage of change.

–A greater number of sources of exposure were significantly associated with intentions and successful abstinence from smoking for 24 h in the last year. –However, greater sources of exposure was not associated with length of abstinence on last quit attempt.

–ETS exposure in the home did not differ between cases and controls on quitting. –However having a greater number of persons who smoked in the home was significantly associated with inability to quit –Attempting to quit smokers were more likely to be passive smokers than not attempting to quit smokers but this was not significant. –A positive association was found between smoking cessation and having smoking relatives who do not smoke in the presence of participant (OR = 1.7, 95%CI = 1.1–2.5). –Individuals who quit smoking were 0.9 (95%CI = 0.7–1.06) times less likely to be exposed to extra-familial SHS; but this was not significant. –Individuals who reported no ETS exposure were 1.9 (95% CI = 1.1–3.3) times more likely to remain abstinent from smoking than individuals with ETS exposure.


adult bar patrons in the U.S. Non-significant associations were also reported in three studies between quit attempts and SHSe at a bar, (Kalkhoran et al., 2013) at home, (Grana et al., 2012; Kalkhoran et al., 2013; Şahbaz et al., 2008) indoors at work, (Kalkhoran et al., 2013) in the place they spend most of their time, (Grana et al., 2012) or any

SHSe over the past 7 days (Kalkhoran et al., 2013). Likewise, Manzoli et al. (2005) reported that adult women in Italy who quit smoking were less likely to be exposed to extra-familial SHS as compared to those who did not; but the difference between groups was not statistically significant. However, two longitudinal studies among adult

30


smokers in the U.S. and Sweden found that SHSe was predictive of poorer smoking cessation success (Helgason et al., 2004; Kashigar et al., 2013). These reviewed studies provide mixed evidence regarding the influence of SHSe on smoking cessation, but are suggestive that SHSe is negatively associated with successful smoking cessation. 4. Discussion The association between SHSe and smoking behaviors has been elucidated through several clinical and epidemiological studies. Relationships between SHSe and smoking behaviors include: a) an increased likelihood of being a smoker; b) increased susceptibility to smoking; c) increased likelihood of smoking initiation; d) greater nicotine dependence symptoms; and e) reduced attempts and success in smoking cessation. These findings provide important considerations for future research and directions to inform policies to reduce harms associated with SHSe. As a group, these findings suggest several means whereby SHSe potentiates smoking behaviors. In the case of smoking status, smokers are more likely than nonsmokers to be exposed to SHS (particularly in the home and work environments), and SHSe is predictive of the future status of being a smoker (Glover et al., 2011; Homish et al., 2011; Hopenhayn et al., 2013). The ‘smoker’ status has much to do with the intentions surrounding, identification with, and exposure to smoking behavior (Okoli et al., 2008; Wakefield et al., 2004; Zhang et al., 2000). Hence, it is not surprising that SHSe is associated with smoking behaviors by virtue of the fact that smoking is often synonymous with the production of SHS. However, based on our findings, SHSe also predicts the changing of smoking status from nonsmoker to smoker. This finding is potentially explained through SHS's influence on smoking susceptibility and initiation which in turn influence smoking status (Gritz et al., 2003). Future studies are needed to examine the direct influence of SHSe on the formation of smoking identity and in the frequency of smoking. For example, studies may examine if SHSe exposure distinguishes between different categorizations of smoking identities (such as experimenter, former smoker, current smoker, established smoker) (Okoli et al., 2009). For susceptibility to smoking, nonsmokers exposed to SHS report greater intentions to future smoking; however this susceptibility may be due to psychosocial aspects of SHSe as opposed to the influence of psychoactive elements of nicotine exposure among children (Racicot et al., 2011). Studies have shown that increased exposure to scenes of smoking in movies and from persons in an adolescents life is associated with greater susceptibility to future smoking, (Tyas & Pederson, 1998; Gritz et al., 2003; Sargent et al., 2002; Unger et al., 2001) suggesting primarily psychosocial aspect linking SHSe with smoking susceptibility. However, recent studies have found that greater ‘sensitivity to SHSe’ is predictive of smoking susceptibility; (Lessov-Schlaggar et al., 2011a; Lessov-Schlaggar et al., 2011b) which posits a biological mechanism whereby SHSe may be linked to smoking susceptibility. Thus, in reference to smoking susceptibility, the question remains whether SHSe is a proxy for psychosocial ‘exposure to smoking’ or whether it constitutes some biological exposure that confers vulnerability to smoking. It is quite conceivable that SHSe is a proxy for social cues and pressures from smoking peers, siblings or parents which may foster smoking susceptibility. Future studies may employ simultaneous measurement of psychosocial and biological measures of SHSe to better understand the nature of this relationship. For smoking initiation, our review found that SHSe from the home and car is associated with early age of initiation and is predictive of initiating smoking. In conjunction with psychosocial factors, based on the sensitization-homeostasis model of nicotine craving, withdrawal, and tolerance, (DiFranza & Wellman, 2005) it is possible that nicotine exposure from SHS can stimulate neural pathways which increase the brains sensitivity to nicotine prior to active smoking. As a result, nicotine exposure from SHS may potentiate smoking initiation by increasing nicotine-

acetylcholine receptor activation and upregulation in the brain of nonsmokers as has been observed in other studies (Brody et al., 2011). This neural activation may explain the greater sensitivity to and positive reports associated with initial symptoms of smoking among youth who are exposed to smoking from their peers and family members (Okoli, Richardson, & Johnson, 2008). To better understand the relationship between SHSe and smoking initiation, prospective studies may examine the mediatorial role of biological (such as hair nicotine) or atmospheric measures (such as passive air nicotine monitors) of SHSe on smoking initiation while accounting for other contextual (such as culture), intrapersonal (such as gender), and interpersonal (such as familial and peer smoking behavior and attitudes) factors (Charlesworth & Glantz, 2005; Tyas & Pederson, 1998; Okoli et al., 2013). In addition, reports of nicotine dependence symptoms may not be associated with nicotine exposure from SHS among never smoking children (Racicot et al., 2011) and adults smokers (Okoli et al., 2007). It is important to be clear that by definition, nonsmokers cannot experience nicotine dependence. Thus the reports of nicotine dependence symptoms among never smokers may either be viewed as a misclassification, or may represent some psycho-behavioral effects experienced by nonsmokers as a result of SHSe. A recent study suggests that the reports of nicotine dependence symptoms among never smokers are associated with several psychosocial factors such as peer and teacher smoking (Racicot et al., 2013). Nevertheless, it is quite clear (with the exception of Okoli, Rayens, and Hahn (2007)) that SHSe is an independent predictor of nicotine dependence in epidemiological studies among smokers. Because studies have shown that SHSe confers additional lung health risks for active smokers, (Lam et al., 2005) it is conceivable that smokers with greater SHSe may have adverse behavioral consequences than smokers without SHSe. For example, studies may examine the additional effect of SHSe on the development of nicotine dependence among novice smokers and quantify the additional effect of SHSe on nicotine dependence among established smokers. Such future studies may also provide important directions in the treatment of tobacco use and dependence. Finally, smoking cessation attempts are associated with SHSe and such exposure is predictive of failed smoking cessation (Helgason et al., 2004; Kashigar et al., 2013). Possible mechanisms involve smoking related cues (Field et al., 2007; McRobbie et al., 2008) within environments of SHSe increasing the difficulty in maintaining abstinence or stopping smoking. In addition, as mentioned previously, SHSe may be a proxy for the social pressures and cues from significant others who smoke (Dickter & Forestell, 2012; Kleinjan et al., 2009; Kong, Camenga, & Krishnan-Sarin, 2012) which hinder cessation. Future studies are needed to better examine the influences, within environments of SHSe, which may affect smoking cessation. 5. Limitations Several limitations need to be considered in interpreting the findings of this review. First, the variability in SHSe measures and outcome measures reported in each study precluded the possibility of meta-analytic procedures in summarizing the studies. Second, the majority of studies were cross-sectional in nature which limits the causal inferences in explaining the relationship between SHSe and main outcomes of interest. Among the prospective studies, the majority relied on self-report measures and few used biomarkers of SHSe. As such, we lack the ability to distinguish between the effects of psychosocial aspects of SHSe as compared to more biological aspects. Third, differences in the studies may have been affected by unaccounted differences in countries, population groups, policy environments, and variable measured. Because of the lack of experimental design and random allocation of subjects, the studies may be largely affected by issues of endogeneity in which several unmeasured determinants may have lead to overestimation (or underestimation) of the regression coefficients in cross-sectional analyses. Future studies with randomized controlled designs or fixed-


effect modeling for longitudinal studies can provide more robust predictions regarding the effect of SHSe on smoking behavior variables. Finally, the majority of studies were epidemiological with few clinical studies testing the direct effects of SHSe on the main outcomes of interest. 6 . Conclusions In summary, despite inherent limitations in the designs of studies, clinical and epidemiological studies have demonstrated a relationship between SHSe and smoking behaviors. Specifically, longitudinal studies from our review suggest that SHSe is predictive of current smoking status among children and adults, (Glover et al., 2011; Homish et al., 2011; Hopenhayn et al., 2013) smoking initiation among children, (Becklake et al., 2005; Glover et al., 2011; Wang et al., 2011) and poor success in smoking cessation among adults (Helgason et al., 2004; Kashigar et al., 2013). It is important to place the findings of our review within an international scope among countries with varying levels of policies addressing SHSe. Our findings lend support to the notion that apart from reducing the well-known adverse physical health outcomes associated with SHSe, policies restricting SHSe may also protect against the initiation of smoking behaviors and provide supportive environments for smokers in their efforts at cessation. Nonetheless, future prospective studies utilizing more robust measures of SHSe (i.e., air monitors, biological markers) and clinical studies testing particular effects of such exposure are needed. Policy studies are also needed to examine the effect of both voluntary and non-voluntary smoke-free policies on smoking behavior outcomes. Such studies will be important to strengthen existing and future policies to reduce SHSe as well as provide recommendations for treating tobacco use and dependence. Role of funding sources This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors Contributors Chizimuzo Okoli conceived, planned, conducted, and drafted the report of the manuscript. Jonathan Kodet conducted the data retrieval and review of articles for the study. Jonathan Kodet also assisted in drafting sections of the report. Conflict of interest All authors declare that they have no conflicts of interest.

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