Tuber&

md

Lung

Disense

( 1992)

13,

13-20

The mineral dust diseases M. R. Becklake Respiratory Canada

Epidemiology

Unit, Department

of Epidemiology

and Biostatistics,

McGill University, Montreal, Quebec,

S UM MA R Y. The mineral dust diseases, also called the pneumoconioses, comprise a wide spectrum of conditions ranging from diseases characterized by diffuse collagenous pulmonary reactions to relatively small lung burdens of bioactive dusts (e.g. silicosis, asbestosis) to diseases characterized by largely non-collagenous reactions in the face of heavy lung dust burdens (e.g. coal workers pneumoconiosis). According to information submitted to the International Labour Office, which is however incomplete, substantial numbers of individuals are still at risk for the mineral dust diseases in the workplaces of the world. An overview of their epidemiology in industrialized and industrializing countries reveals more commonalities than contrasts. Commonalities include the major determinants of disease (including exposure level, intensity and particle size distribution), their clinical manifestations and, probably, secular trends towards less clinically severe disease, at least in the larger, better controlled workplaces. Still a risk however, in both industrializing as well as industrialized countries, are the small, uncontrolled workplaces, often the source of mini-epidemics. Contrasts relate to the incidence and/or prevalence rates of tuberculosis amongst workforces at risk for the mineral dust diseases. Rates, which are invariably higher in industrializing than in industrialized economies, usually reflect the background tuberculosis rates in the populations which furnish the industrial workforces and they should be the target for control measures. Research in the industrialized countries should focus on disease mechanisms and on the bioactivity of workplace contaminants, old and new, and in the industrializing countries on the distribution and determinants of mineral dust diseases in their workplaces. All information gathered, whether by governments, unions or industry should be in the public domain, to maximize the potential benefit to be derived from exchanging experiences. R _l?S U M k. Les maladies dues aux poussieres minerales, appelees aussi pneumoconioses, comprennent un large spectre de desordres, allant de maladies caracterisees par des reactions pulmonaires collagenes diffuses en rapport avec des charges pulmonaires de poussiitres bioactives relativement petites (par exemple la silicose et l’asbestose), jusqu’a des maladies caract&i&es par des reactions principalement non-collagenes vis-lvis de charges pulmonaires Clevees de poussieres (par exemple la pneumoconiose des travailleurs du charbon). Selon les donnees recueillies par le Bureau International de Travail, neanmoins incomplbtes, un grand nombre d’individus partout dans le monde demeurent menaces par des maladies dues aux poussieres minerales dans les lieux de travail. Une etude de leur Cpidemiologie dans les pays industrialises et les pays en developpement revele davantage de points communs que de contrastes. Les points en commun comprennent les determinants majeurs de la maladie (niveau d’exposition, intensite et distribution de la taille des particules), leurs manifestations cliniques, et probablement les tendances Cpidemiologiques envers des maladies moins &v&es sur le plan clinique, tout au moins dans des lieux de travail plus vastes et mieux surveillCs. NCanmoins les lieux de travail restreints et non-surveilles exposent toujours au risque, a la fois dans les pays en developpement et dans les pays industrialisb, de devenir des sources de petites CpidCmies. Les contra&es Cpidemiologiques rePvent de l’incidence et/au des taux de prevalence de la tuberculose parmi la main d’oeuvre menacee par les maladies 1iCes aux poussieres mitt&-ales. Ces taux, qui sont constamment plus Beves dans les pays en developpement que dans les pays industrialis&, refletent d’habitude les taux de tuberculose existant en arriereplan, dans les populations qui four&sent la main d’oeuvre industrielle, et ils devraient itre la cible des

Correspondence to: Margaret R. Becklake MD, Department of Epidemiology and Biostatistics, McGill University, 1110Pine Ave West, Montreal, Quebec, Canada H3A lA3. M. R. Becklake is a Career Investigator of the Medical Research Council of Canada. 13

14

Tubercle and Lung Disease

mesures de controle. Les recherches dans les pays industrialises devraient viser a determiner les mkanismes de la maladie et la bioactivite des contaminants sur les lieux de travail, anciens et nouveaux. Dans les pays sousdeveloppes elles devraient par contre se concentrer sur la distribution et les determinants des maladies likes aux poussieres minerales dans les lieux de travail. Toute information recueillie par gouvernements, syndicats ou industries devrait dtre versee au domaine public, aux fins de majorer le bCnCficepotentiel issu d’expikiences partagees. R ES U M E N. Las enfermedades provocadas por polvos minerales, tambien lhunadas neumoconiosis, comprenden un vasto espectro de condiciones, variando desde enfermedades caracterizadas por reacciones pulmonares colagenosas difusas a una carga relativamente pequefia de polvos bioactivos (por ejemplo, silicosis, asbestosis) a enfermedades caracterizadas por reacciones esencialmente no colagenosas, frente a una gran dosis inhalada de polvos (por ejemplo, la neumoconiosis de trabajadores de1 carbon). Segtin la information enviada a la Organization International de1 Trabajo, que sin embargo es incompleta, un numero considerable de individuos corre el riesgo de desarrollar enfermedades por polvos minerales en muchos lugares de trabajo a traves de1 mundo. Una vision general de su epidemiologia en 10s paises industrializados y en vias de industrialization revela mL semejanzas que diferencias. Las semejanzas incluyen 10s principales determinantes de la enfermedad (incluyendo el nivel de exposition, la intensidad y la distribution segun el tamafio de las particulas), sus manifestaciones clinicas y, probablemente, las tendencias seculares hacia la presentation de enfermedades clinicamente menos severas, al menos en 10s lugares de trabajo mis amplios y mejor controlados. No obstante, 10s lugares de trabajo restringidos y no controlados representan siempre un riesgo, tanto en 10s pa&es desarrollados coma en desarrollo. Las diferencias e&n relacionadas con la incidencia y/o tasas de prevalencia de tuberculosis en 10s trabajadores con alto riesgo de presentar enfermedades por polvos minerales. Las tasas, que son invariablemente mh elevadas en las economias en vias de industrialization que en las industrializadas, habitualmente reflejan las tasas de tuberculosis existentes en las poblaciones que proporcionan la fuerza de trabajo industrial, las cuales deberian ser el blanco de las medidas de control. La investigation en 10s paises industrializados deberia focalizarse en 10s mecanismos de la enfermedad y en la bioactividad de 10s polutos de 10s lugares de trabajo, antiguos y neuvos; en 10s paises en vias de industrialization deberia focalizarse sobre la distribution y 10s determinantes de las enfermedades provocadas por polvos minerales en sus lugares de trabajo. Toda la informacidn reunida, ya sea por 10s gobiernos, sindicatos o industrias, deberia ser de dominio publico, para optimizar el beneficio potential derivable de las experiencias de intercambio.

HISTORIC PERSPECTIVE AND DEFINITION The mineral dust diseases are ancient diseases which probably date from the Stone Ages when men and women first sought to make tools and exploit the minerals in the Earth’s crust.‘4 Even as late as the 19th century, flints for the arms used to fight the wars of Europe were made by hand, chipped by stone tools.‘S 4 The generic term pneumonokoniosis was introduced by Zenker in 1866 to describe what he considered to be essentially identical forms of disease associated with exposure to mineral dusts; the term was subsequently shortened to pneumoconiosis.2 For a surprisingly long period, pneumoconiotic conditions were confused with tuberculosis, the mycobacteriology of which was only described by Koch in 1882, and this confusion persisted well into the early decades of the 20th century.5 The International Labour Office has contributed to precision in medical discourse concerning the mineral dust diseases in a number of ways. For instance it is responsible for the definition of pneumoconiosis used in most parts of the world as a condition characterized by

‘an accumulation of dust in the lung and the tissue reaction to its presence’.6 Companion terms are used to focus either (1) on the dust believed to be the causative agent; for example, silicosis and asbestosis, referring respectively to the pulmonary fibrosis which results from exposure to crystalline silica and asbestos; and (2) to describe the occupation at risk, (for example, coalworkers’ pneumoconiosis, welder’s lung). The definition also allows that the tissue reaction to the presence of dust may be collagenous (associated with scarring), or non-collagenous (associated with dust accumulation in the lung but minimal scarring). In his classic studies carried out in the 1930% Nagelschmidt7 had noted that the fibrotic reaction may vary from slight in the face of large dust burdens which have a low quartz content (for example in coal workers) to considerable, in the face of a modest total dust burden but with high silica content (for example, in hardrock miners). By contrast, hematite miners may have a large dust burden and quite a high silica content but apparently, from some inter-dust reaction, the level of fibrosis is less than if the exposure had been only to the same level of silica, in the absence of the other dust.

The mineral dust diseases

DISTRIBUTION DETERMINANTS

WORLDWIDE AND IN THE WORKPLACE

Industries at risk for the mineral dust diseases include hardrock and other mining, and those industries which use silica and/or mineral dust abrasives; the worldwide distribution of the former depends on national resources, and of the latter on the degree of industrialization. Modem mining technology is to a great extent international and in many, though by no means all, large production operations the appropriate methods of dust control are in place. However small, uncontrolled industries with few workers remain at risk and continue to be responsible for sporadic and even epidemics of cases in industrialized as well as industrializing countries. For instance, in a recent paper entitled ‘Does silicosis still occur?’ by physicians working in the state of New Jersey, USA, a state with no hardrock mining industry, a surveillance system for occupational disease identified some 400 cases not registered over the period 1979-1987 and this number was considered to be an underestimate.* The industries involved were quarrying and rockbreaking, sandblasting, construction and the founding of ferrous and non-ferrous metals, Together with the production of ceramics, and the production and use of abrasives and refractories, these constitute the major non-mining industries and/or occupations still at risk for the mineral dust diseases in the 1990~.~-” Thus, in this respect, there is more commonality than contrast between the industrializing and the industrialized countries, and most of these industries are to be found in countries at all stages in the industrialization process. Indeed, even in the industrialized countries where on the whole production is less labour-intensive and more mechanized (see Table l), between 30 and 60% of adult men report exposure to some form of dust with or without exposure to gases and fumes at work.‘2~~‘6

International comparisons are available only for mining, tunnelling, and quarrying through the IL0 who publish 5-yearly reports on the prevention and suppression of dust in these industries.” Countries are invited to submit information on production in these industries within their jurisdiction, on the numbers of cases of pneumoconiosis identified, on dust suppression methods, and on legislative control. Many, but by no means all, countries comply; for instance 42 countries contributed to the 1973-1977 report, 54 countries to the 1968-1972 report. In addition, methods of reporting vary considerably. Nevertheless, incomplete as they are, these reports provide the only information of this sort on a worldwide basis. These data suggest that the number of workers at risk for pneumoconiosis remains high (see Table 2). For instance in Europe in 1977 (with only 14 countries reporting), there were approximately 839 000 men and women who held jobs which involved exposure to dust at work, in Australasia (with only 8 countries reporting) approximately 189 000 men and women, in the Americas (with only 5 countries reporting) approximately 274000 men and women, and in Africa (with 6 countries reporting) approximately 394000 men and women. Note that for all these continents the reporting is incomplete. For instance, for Europe there are no reports to the IL0 from the USSR, for Australasia no reports from China, from the Americas no reports from Bolivia or Chile. Though the numbers are likely to decline as mechanization takes over, nevertheless the men and women who constitute the statistics in Table 2 will be at risk in the coming decades, given the incubation period of the pneumoconioses.2A Information from the most recent IL0 reports” on prevalence (cases per 1000 working miners) which

Table 2. Estimated number at risk for the mineral dust pneumoconioses 1973-l 977 as reported to the IL0 Continent

Table 1. Exposures to dust at work and smoking: prevalence community-based studies in different countries*

in

First author, year

country or city

Numbers of subjects surveyed

Exposure Dust at work?

Current and ex-smokerst

Lebowitz 1977’2 Kom 1986” Krzyzanowski 1986’” Viegi 198715 Gulsvik 19881h

Tuczon, AZ 6 US cities

1132M

60%

35%

3845 M 4667 W 759 M 1055w 1635 M&W 4992 M&W

45% 19% 30% 13% 26%

58% M&W 81% 35% 46%

28%f

59%

Cracow, Poland PO delta, Italy Bergen Co, Norway

* Table adapted from ref.” with permission. i Based on questionnaire information supplied by the subject. ?+ Exposure to dust or gas at work.

15

Europe (14 countries)* Australasia (7 countries)+ Americas (6 countries)* Africa (6 countries)5

Source:

Coal mines

Metal mines

Quanying/

684000

36000

109000

118000

47000

24 000

133000

141000

3000

‘389000

Not reported 2000

tunnellingl other

International Labour Office” and Becklake’” for data on metal miners in South Africa.

* Austria, Cyprus, Czechoslovakia, France, Germany, Hungary, Holland, Norway, Poland, Portugal, Romania, Sweden, Switzerland, UK. t Australia,

Burma, India, Indonesia, Malaysia,

New Zealand, Turkey.

t Argentina, Canada, El Salvador, Mexico, Peru, USA. § Ghana, Kenya, Mali, Morocco, South Africa, Zambia.

16

Tubercle and Lung Disease

Table 3. Prevalence of mineral dust pneumoconiosis selected countries reporting to the IL0 Continent /country

Europe France

Year(s)

Coal miners Number Cases at risk 11000

35 654

122

Hungary Poland Portugal Romania UK

1977 1977 1977 1977 1976 1977 1977 1977

111992 13 920$ 91000 3671$ 134 094% 252 600

229 129 34? 11 3? 119

Australasia Australia: NSW New Zealand Turkey

1973-1977 1973 1968-1972

15 970 1473 -7OOOO$

29

1972

2646 1975

134

118579

56

Germany

Americas Canada (NS) (Man) El Salvador Mexico (mine A) peru (mineB) USA

1971 1973 1977 1976 1973-1977

Africa Ghana Morocco S.Africa

1968-1972 1968-1972 1984

Source: International South Africa. * t $ $ ** tt

3121

Labour 0ffice,t7

in miners in

Metal & other miners Number Cases atrisk 11000

4954* 3 0807 -7000

22 -4 117

53400

11

4534

10**

% E

0.09

B I._ z

-

Reader 1

--

Reader 2

-._.

Reader 4

----

Reader 5

Reader 3

0.06

Cumulative

Respirable Silica Exposure (mg/ma-years)

::

258tt

5975 709 4815 1421 56819

2 106** 89 16 36

5OOOt 1017 132 765

67 126 14**

and Cowie et a?* for data from

Iron mines. Other metal mines. Refers to 1978. Includes metal mines. Results of a survey. Refers to 1973.

covers the period 1973-1977 is shown in Table 3. The wide variation in rates is undoubtedly in part methodologic, due to differences in the methods used to estimate both numerators (cases) and denominators (men at risk) on which these calculations are based. Nevertheless, in countries reporting on both coal and metal miners within their jurisdiction (presumably using comparable methods), rates on the whole appear to be higher among the former than the latter. Note also that this report, the most recent, was published in 1983 and deals with data from 1973 to 1978. We could all usefully encourage our governments to report more fully to the ILO, and the IL0 to publish their findings more speedily, in order to provide us this type of information on which to base action. As regards the determinants of distribution of the mineral dust diseases within workplaces, there is more commonality than contrast between industrializing and industrialized countries. The main determinants are exposure, its duration and level, the size, distribution and respirability of airborne particles, and their fibrogenic potential. Exposure response curves summarize these relationships. For instance, Figure 1 summarizes the dependence of radiologic silicosis on cumulative

Fig. 1 - The figure shows fitted Weibull models for the relationships between cumulative risk of silicosis and cumulative exposure to respirable silica dust (mg/m, years) lagged 5 years. The results for each of 5 readers are shown separately. Reproduced with permission from Muir et a1.19

exposure to silica (mgm/m3 x year) in Ontario hardrock miners” and in Figures 2 and 3, the dependence of radiologic asbestosis on cumulative exposure to asbestos (fibers per m x year) in mining and in various industrial processing.” Factors which can modify exposure response relationships to airborne silica include the presence of other dust. For instance, the mixed dust pneumoconiosis of foundry workers probably represents the modification of the response to silica in the presence of other dusts which contaminate many foundries.’ In the case of asbestosis, both fiber type (Fig. 2) and process (Fig. 3) appear to affect the exposure response relationships. If the between industry differences in Figure 3 were to persist when dose-response (with dose measured as lung dust burden) as opposed to exposure-response relationships are compared (and there is some evidence that they do2’, 2’), then differential fiber retention seems likely, and it becomes less justifiable to generalize experience from one industry and/or workplace to another. Host factors probably also represent a commonality rather than a contrast between the industrializing and the industrialized countries. For instance, the presence of connective tissue diseases such as rheumatoid arthritis may alter the clinical presentation of the mineral dust diseases. In addition, these may an association between exposure to silica dust (and/or silicosis) and the presence of progressive systemic sclerosis. Both associations have been described in workforces from industrialized and industrializing countries.22~23 Genetic factors reflected in histocompatibility antigens may also influence the body’s response to mineral dust exposure though findings are inconsistent between workforces.24*25 The latter association does not appear to have been examined in non-Caucasian workforces. However, even if host factors are important from the

The mineral dust diseases

40

1Australia

Crocidolite miner.s and millers

30 South

20

x

5 -ii

z

ri:

Africa Chrysotile miners and millers

I! I

2;

10 1 I

Quebec

01 500 Cumulative

exposure

1000 fibers/ml.years

Fig. 2 -The figure contrasts idealized exposure response relationships derived from studies of chrysotile miners and millers from Quebec and from Corsica, with those derived from studies of crocidolite miners and millers from South Africa and from Australia. Reproduced with permission from Becklake MR. Occupational lung disease-past record and future trends using the asbestos case as an example. Clin Invest Med 1983; 6: 305-317.

Pipe

coverers Cement and other products

Cement Textile/cement

100 Cumulative

200 exposure

300

400

17

in the past, most countries, whatever their degree of industrialization, have been guilty of allowing epidemics of acute silicosis to occur. 2-4The industries involved are usually small and poorly regulated, e.g. sandblasting,* strip-mining3 and the gemstone industry.‘6’8 Vigilance on the part of all involved in maintaining health in the workplace, whether health, engineering or industrial hygiene professionals, should at least eliminate such episodes in the 1990s. Asbestosis will also probably continue to occur though at a diminishing rate as the environmental controls instituted in the last 20 years take effect. Since their introduction, the use of asbestos substitutes has been much more stringently controlled than asbestos ever was, and it is to be hoped their disease burden will be minimal on exposed workforces. In addition, there is evidence, discussed below, that with each decade, the venerable pneumoconioses, in particular silicosis and asbestosis, are becoming not only less frequent, but also less severe.*,*’ Is it likely that the 1990s will see the recognition of new mineral dust diseases? Based on past experience, this appears to be unlikely; most pneumoconioses first reported as ‘new’ have turned out to be one of the already familiar pneumoconioses, usually silicosis, occurring in an industry or a workplace not previously thought to be at risk.3 Examples include ‘Labrador’ lung, described in the iron miners of Labrador who had not previously been thought to be at risk for silicosis” and ‘Carborundum pneumoconiosis. Caroborundum is an abrasive made from fusing silica and carbon into silicon carbide, and silicon carbide dust was considered to be biologically inert: however two bioactive dusts, namely silica and Carborundum fibers (which have fibrogenic activities comparable to asbestos fibers of similar size), are used in its manufacture, and have been implicated in the pneumoconiosis associated with its production.“’

mpcf.years

Fig. 3 - The figure contrasts idealized exposure response relationships derived from studies of workers engaged in mining and milling asbestos, in asbestos cement and textile production and in pipe covering using asbestos insulation. Reproduced with permission from Becklake MR. Occupational lung disease-past record and future trends using the asbestos case as an example. Clin Invest Med 1983; 6: 305-3 17.

point of view of understanding mechanisms of pathogenesis, they offer less opportunity for public health intervention, and are therefore of less practical importance in disease control than environmental factors such as airborne dust levels in the workplace. What pneumoconioses will persist into the 199Os? Coalworkers’ pneumoconioses will probably continue to occur, despite improvements in environmental controls; however this may be a declining industry. Silicosis and mixed dust pneumoconiosis will almost certainly continue to occur, with an increasing proportion of cases coming from the founding, sand, ceramics and refractories industries and less from mining. In addition,

CLINCIAL

FEATURES

AND COMPLICATIONS

In respect of the clinical and radiological features of the mineral dust diseases, there is commonality rather than contrast between cases seen in industrialized and in industrializing countries. Thus, chronic, accelerated and acute forms of silicosis have been described from all continents and in most dusty industries from both industrialized and industrializing countries.‘d In the chronic form, exposure is measured in decades rather than years and the course is often compatible with a normal lifespan; this is the form seen in well controlled industries.‘,” An accelerated form in which exposure is measured in years rather than in decades usually causes morbidity; this is seen, in general, in association with higher exposure. Acute silicosis is seen in individuals who have had heavy exposure often measured in months rather than years, and the condition is invariably fatal within years.2.4 Alveolar silica-proteinosis is another manifestation of intense exposure to fine silica dust and,

18

Tubercle and Lung Disease

in contrast to spontaneously developing alveolar lipoproteinosis, carries a poor prognosis.234 The IL0 has also contributed to international comparability in medical diagnoses by providing sets of standard films and a system of reading films which is now widely used for surveillance and research as well as for case diagnosis in most countries and on most continents.3’ Minifilms can also be used for surveillance when full sized films are not available, probably with minimal loss of sensitivity in the hands of experienced readers.32-34 Many countries already have tuberculosis vans equipped to take minifilms and they are less costly than 14 x 17 films. Note, however, that reading minifilms for pneumoconiosis calls for a different expertise than that required for reading tuberculosis3* Secular trends in some of the clinical features of pneumoconiosis have been reported from a number of countries3 and some of this data is summarized in Table 4.‘7,35,36Of note are the trends towards a longer exposure period prior to diagnosis, an increase in the age at diagnosis, and an increase in the age at death. All these changes are consistent with improved environmental controls. Today, in some of the industrialized countries of Europe and North America, pneumoconiosis is invariably limited to its radiological manifestation, and is without impairment or disability.3v37 By contrast, in South Africa, radiological silicosis in black miners is still associated with impairment.38 A major contrast between the industrializing and the industrialized countries relates to background tuberculosis rates in the communities which furnish the labor for the industries still at risk for the mineral dust disease, in particular the mining industries.‘“, I8939A2 Conditions which favour the development of tuberculosis include migrant labor and poor housing conditions, whether in the large urban slums or in labor hostels,@.4’ and currently the prevalence of HIV infection. In the South African goldmines for instance, the majority of the labour has been recruited from rural areas both within South Africa (e.g. Transkei) and outside South Africa (e.g. Lesotho, Botswana, Malawi)

Table 4. Secular trends in mineral dust penumoconioses: selected studies

data from

Country/ exposure

Year(s)

Years exposed prior to diagnosis

Age at diagnosis (years)

Age at death (years)

Ontario: hardrock miners35

1928 1956 1972

-15 -23

-40 -58

42 65 75

Peoples’ Republic of Chitl;: coal miners

1960 1980

-11 -19

41 43

43 65

Switzerland: foundry worky and tunnellers

193&1937 1972-1977

42 67

41 56

42 68

Zambit; miners

1968 1972

-19 -20

n/a nla

n/a n/a

copper

to work on contract, areas in which infection rates in the 1960s as judged by tuberculin skin reactions among mining recruits, ranged from 75 to 99%.42 Until quite recently, miners diagnosed as having tuberculosis were excluded by law from further underground work. This regulation was based mainly on experimental evidence from animal studies that the lung’s resistance to infection by mycobacteria is impaired in direct proportion to its total silica dust load, and the belief that tuberculosis can never be cured in men with silicosis.4,40,43 Not only did workers lose their jobs, but their families usually lost the major wage-earner, treatment facilities were seldom adequate in the rural areas where their homes were located, and their families had the additional burden of an invalid with an infectious disease to care for, as well as being at risk for infection themselves. Indeed the high rates of tuberculosis in the rural areas of South Africa which feed the mine workforce are undoubtedly in no small part attributable to the regulation referred to above. In research carried out in the Anglo-American mining group in the Orange Free State, Cowie and colleagues of the Department of Medicine, Ernest Oppenheimer Hospital, were able to show that once the febrile phase of the disease had subsided on treatment, the return to underground work did not adversely affect the treatment outcome, as reflected by the relapse rate.40x44 This resulted in the change of the mining regulations which now permit men with tuberculosis to continue underground while on treatment. This experience may be pertinent in other jurisdictions where the circumstances of employment are similar.

OTHER WORK-RELATED FOUND IN WORKPLACES PNEUMOCONIOSIS

LUNG DISEASE AT RISK FOR

Evidence reviewed elsewhere is now persuasive that including chronic airflow chronic airway disease, limitation, may also be the consequence of exposure to environments heavily contaminated by dusts and/or pathology studies have fumes.3, 1’,45 In addition, confirmed that small airway disease is associated with exposure to mineral dusts, apparently independently of any concurrent pneumoconiotic process.4w9 Nonspecific bronchial reactivity may also be a risk factor for the development of chronic airflow limitation in dust exposed individuals.4g In the case of asbestos exposure, animal studies show that the pathologic changes of small airway disease appear to be evoked by a lower dust burden than the peribronchiolar fibrosis which marks the onset of asbestosis.4’ Documentation of these ill health effects in man of chronic asbestos exposure comes from industrialized as well as industrializing countries.432035~52 An excess of cancer in relation to occupational exposures in workplaces contaminated by asbestos dust has been well documented4.’ and more recently the issue

The mineral dust diseases

has been raised in relation to workplaces contaminated of by silica dust.53 Evidence for the carcinogenecity asbestos dust is considered to be complete by the International Agency for Research into Cancer,54 while that for the carcinogenecity of silica dust is considered to be limited.53,55 Possible mechanisms for both are the development of scar cancers, long believed to complicate fibrotic lung disease of occupational as well as non-occupational origin. Nevertheless, for both asbestos and silica exposures, lung cancer may occur in cases with only minimal pulmonary fibrosis. In addition, many mining environments at risk for silicosis are also contaminated by other potential carcinogens, for radon in uranium mines,56,57 low-level example, radiation in certain goldmines, and diesel fumes in many hardrock mines.59

SYNTHESIS This overview of the epidemiology of mineral dust diseases in the industrializing and the industrialized countries of the world reveals more commonalities than contrasts. Commonalities include: (1) the major determinants of disease (in particular exposure level and intensity); (2) the clinical features; and (3) the routine diagnostic tools in regular use (see Christiani, this symposium). Also in common is the key role of the physician and/or other health personnel in identifying new variants of these ancient diseases, or their manifestation in classic form in workplaces that have not previously been thought to be at risk. Particularly at risk for the mineral dust diseases, in both industrialized and industrializing economies, are the small uncontrolled workplaces. The contrasts relate to (1) the background tuberculosis rates of the populations which furnish the industrial workforces, and (2) special diagnostic tools such as bronchoalveolar lavage59 and the analysis of lung dust burden in pathology specimens,m the availability of which may be limited in industrializing countries. As a result of this, the research thrusts in countries at different stages of industrialization may be different. It has been said that ‘epidemiologists in developed countries can carry out a good deal of research without ever encountering patients: in contrast, the primary activity of an epidemiologist in a developing country is that of reliable data collection’.61 Research contributions of both types are needed to provide the necessary information base for adequate protection of the health of working men and women, whatever the level of industrialization of the country in which they reside. In the case of the industrialized countries, research into disease mechanisms and factors which influence the biological activity of dusts are pertinent to the current and future use of new products. In the case of the industrializing world, fact finding is often the priority,62

19

and epidemiological research, to document the extent of the mineral dust and other work-related diseases and their determinants, in whatever industries are at risk, would be a first step in the process of control. Nor is it possible to overemphasize the mutual dependence of all in this field of endeavour on each other’s experiences. Internal reports to institutes or governments, or to unions or companies virtually guarantee that the information they contain will be buried, or at the very least, be limited in access. In order to capitalize on our interdependence, we must assure that information we as health professionals gather is published in indexed journals which make the available universally. experience To this end, governments should also all be encouraged to contribute information on an international scale, for example to the ILO’s 5-year report on the prevention and suppression of dust in mining, tunnelling and quarrying.17 There is little excuse for not exploiting the modem communication technology in the service of improved disease control in the world’s workplaces.

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