Int Arch Occup Environ Hlth 39, 59-72 (1977)
International Archives of
Ilealth ( Springer-Verlag 1977
Documents of International Meetings and Activities
Second International Workshop Permissible Levels for Occupational Exposure to Inorganic Lead September 21-23, 1976, Coronel Laboratory, Faculty of Medicine, University of Amsterdam R.L Zielhuis Coronal Laboratory, 1st Const Huygenstraat 20, Amsterdam, The Netherlands
With the support from the Permanent Commission International Association of Occupational Health and World Health Organisation Office for Occupational Health, and on behalf of the Scientific Committee on Permissible Levels and the Scientific Committee on Metals, both of the Permanent Commission, the Coronel Laboratory for Occupational and Environmental Health, Faculty of Medicin, University of Amsterdam, organised a second international workshop, September 21-23, 1976. A number of 38 invited experts participated (Annex 1); 23 papers were submitted for discussion (Annex 2) This report contains the conclusions from this workshop. This publication was prepared by an editorial group (chairman D Malcolm), and was extensively discussed during the workshop The majority of participants agreed unless otherwise stated (section 9) with the conclusions as published, although some of them may not necessarily agree with the precise wordings and exact figures given.
1 Purpose and Objectives of Meeting In 1968 the first workshop was held The conclusions have been published by Zielhuis (1969) and ILO ( 1972) Since that time many new observations have been published and a great deal of research is in active progress throughout the world. Some of this work is relevant for permissible levels of occupational exposure of male and female workers Wider public concern and awareness also was a reason to reconsider previous value judgements. The main topics which are important in view of the recommendations made in 1968 are: early biochemical effects, particulary those related to haemoglobin synthesis early effects on the nervous system, including potential behavioural effects early effects on chromosomes effects on reproduction in male and female workers, and possible effects on the offspring relation between lead in air and lead concentration and biochemical parameters.
Finally because it is the parameter which is most widely understood and accepted, the pitfalls of measurement of lead in biological specimens had to be reviewed again. It was not the function of this workshop to discuss environmental health problems, e.g , effects of lead on children, neither clinically manifest lead poisoning, treatment nor technical control measures Other matters outside the scope of the workshop were standard setting, also based upon economic and legal considerations an enforcement programmes. Standard setting is a two step procedure The first step is recommendation of health based permissible levels, the purpose of this workshop The second stage is the translation of the health based data into regulations or legislation suitable to the needs of each particular nation or supranational body This was outside the scope of the workshop.
2 Previous Recommendations 1968 Workshop The first workshop on guide lines for health protection of lead workers was organized at the Coronel Laboratory, University of Amsterdam, in 1968 under the auspices of the Permanent Committee and International Association for Occupational Health. The working group recommended as guidelines for acceptable exposure of lead workers the following criteria: Lead in blood Lead in urine ALA in urine Coproporphyrin in urine
70 #g/100 ml 130 Mg/l 10 mg/l 300 #g/l
These recommendations were generally accepted internationally with minor variations as criteria which would prevent clinical lead poisoning and any departure from normal wellbeing or any long term effects Recent work on alteration of nerve conduction time together with indications that there may be other effects of lead on the nervous and other systems not detectable by normal clinical methods, has indicated that the recommendations made in 1968 should be reviewed. To take into account factors which are not detectable by normal clinical methods and which are not known to give rise to any long term clinical effects, represents a change in philosophy as to what is or is not acceptable.
3 Definitions The group made reference to the definitions published by the Task Group on Metal Toxicity (Nordberg, 1976) as a result of a meeting in Tokyo The following terms will have the meaning listed below when used in this document A more extensive discussion of the terms is found in the publication of the Task Group on Metal Toxicity 1976.
Occupational Exposure to Inorganic Lead
Critical concentrationfor a cell is the "concentration at which adverse functional changes, reversible or irreversible, occur in the cell". Criticalorgan concentration is the "mean concentration in the organ at the time at which the most sensitive type of cells in the organ reach critical concentration". Critical organ is defined as "that particular organ which first attains the critical concentration of metal under specified circumstances of exposure" This definition differs from the one given by the International Commission on Radiological Protection, but has been considered a more accurate one, from the point of view of preventive medicine The present definition implies that if critical effects in critical organs are identified and prevented, no adverse effect from the organism as a whole need appear. Criticaleffect is a defined point in the relationship between dose and effect in the individual, namely the point at which an adverse effect occurs in cellular function of the critical organ At an exposure level lower than that giving a critical concentration of metal in the critical organ, some effects may occur that do not impair cellular function per se, yet are detectable by means of biochemical or other tests Such effects are defined as subcritical effects. The term effect denotes a biological change caused by exposure Sometimes this effect can be measured on a graded scale of severity, although at other times one may only to be able to describe a qualitative effect occurring within some range of exposure levels When data are available for a graded effect, it is apparent that one may establish a relationship between dose (usually an estimate of dose) and the gradation of the effect in the population: this is the dose-effect relationship An example is the relationship between lead concentration in industrial air and the concentration af ALA in urine samples from exposed workers. The term response is used to mean the proportion of a population that demonstrates a specific effect; its correlation with estimations of dose provides the doseresponse relationship For example, a dose-response relationship might compare different concentrations of lead in industrial air (estimates of dose) with the percentage of exposed workers having more than 5 mg of ALA/I of urine Recent reviews of dose-response relationships for inorganic lead have been published by the Subcommittee on the toxicology of Metals (Nordberg, 1976), Hernberg (1975, 1976), Lauwerys (1975), Zielhuis (1975).
4 Summary of Discussions and Conclusions Concerning Early Effects 1 4.1 HaematologicalEffects Erythrocyte Porphyrin Increase of erythrocyte porphyrin (EP) is a sensitive indicator for lead effects on haemsynthesis and of considerable practical significance Moreover, under stable Footnote see page 62
lead exposure conditions, and also following such exposures, having in mind the possible effects of disease and physiological states such as sex difference and iron deficiency the measurement of EP gives a better reflection of chelatable lead than the other available parameters. EP consists to a very large extent of protoporphyrin IX, i e , that type of porphyrin which increases most in Pb-exposure. There is a highly significant positive correlation between EP levels and PbB levels lower than 70 bpg/100 mi The no-effect level of this parameter is considered to be below a PbB level of 35 g/100 ml in adult males (Zielhuis, 1975). There is evidence when lead absorption increases that in women EP levels are higher than in males at the same levels of Pb B; and there is suggestive evidence that with increasing Pb B levels the rate of increase in EP is higher in females than in males, and that the no-effect level is below a PbB-level = 30 g/100 ml. The numerical value of EP depends upon the technique used, that is based upon red blood cells (RBC) or whole blood, chemical or fluorometric methods etc The fluorometric zinc protoporphyrin (ZPP) technique appears to offer a simple and reliable measure. Haemoglobin There is disagreement as to the Pb B level above which Hb starts to decrease; it has been suggested that Hb is already slightly decreased from levels of Pb B = 40-50 g/100 ml onwards in some individuals; other however have not observed such findings This difference may be due to study design: longitudinal versus cross-sectional; a rapid change in Pb B level over previous months should be taken into account, and also possible endogeneous and exogenous host factors independent of lead exposure There is agreement that more than a slight decrease of Hb let alone a clinical anaemia will not usually occur below Pb B = 80 ug/100 ml. 5-Aminolaevulinic Acid Dehydratase The measurement of ALA-D levels is the most sensitive indicator of early lead effect, and the correlation with Pb B in the low level range has been found to be very good. Early inhibition of ALA-D as such is generally been regarded as of no consequence for health; it requires further elucidation The non-linear relation with PbB levels limits the use of this test to low exposure levels (see Sect 7). Reduced Glutathion The endogeneous GSH-concentration in erythrocytes decreases slightly but significantly with PbB below 70 /pg/100 mi The significance of this finding to health is doubtful and the measurement of GSH is not suitable for use as a diagnostic or discriminating
The following abbreviations are used throughout in the report: PbB = lead in whole blood;
Pb U = lead in urine; EP = porphyrin in red blood cells (mainly protoporphyrin); ZPP = zinc protoporphyrin; ALA-U = 8-aminolaevulinic acid in urine; ALA-D = 6-aminolaevulinic acid dehydratase in red blood cells; GSH = reduced glutathione; AT Pase = adenosine triphosphatase
Occupational Exposure to Inorganic Lead
criterion for estimation of the degree of exposure to lead, due to absence of a clear dose-effect/response relationship. Adenosine Triphosphatase With increasing levels of Pb B a decreased activity of Na+/K+-AT Pase has been shown. However, this test has no practical use as a routine test for survey of lead exposed workers because of a complex and time consuming methodology and a relatively poor correlation with PbB-levels.
4.2 Effects on Nervous System Effects on Conduction Velocity Radial palsy is a classical symptom of lead poisoning Recently it has been recognised that nerve conduction velocities of both motor and sensory fibres are reduced in lead poisoned workers, and in some other lead exposed workers without other evidence of poisoning (Seppllliinen et al , 1972 ; Catton et al , 1970 ; Seppaldinen et al , 1975 ; Ataki et al , 1976). Further evidence from an ongoing study has been presented at the meeting (Seppaliinen et al ) showing the following dose-response relationship: Relationship between abnormally low2 conduction velocity in two or more nerves and the highest blood lead recorded during exposure of 2-20 years Number of persons studied /Ag/100
blood lead ml
No of exposed persons with abnormality
22 15 23 15
< 20 40 49 50 59 60 69
O 1 3 4
It is not known whether the maximum blood lead concentration or the integrated average concentration is the determining factor in the development of changes in nerve conduction velocity However, the Group concluded from the data presented by Seppildinen et al and the data reported in the literature that changes in nerve conduction velocity occur in some lead workers at blood levels exceeding 50 lg/100 ml. It was thought that no conclusion could be drawn from the one case in the blood lead range 40-49 ,g/100 ml. It is not possible to decide what any given measured small deficit means in terms of specific nervous damage However, it is generally recognised that a clear deficit in the nerve conduction velocity of more than one nerve is an early stage in the development of clinically manifest neuropathy There is no evidence that these changes progress Reversibility should be studied Although slight changes may be measured 2
Lower than mean-2 SD in normal population for each specific nerve These SD's were derived from a control group (22) and an additional group of 50 persons employed in railway engineering
in persons experiencing no symptoms, it was the consensus of the Group that such changes should be regarded as a critical effect (as defined in Sect 3). Psychological and Behavioural Effects Changes in the psychological, sensory-perceptual and psychomotor function due to lead are indications of an effect upon the nervous system The nature and site of this action is still unknown The main effect of low-level exposure to lead concerns the sensory-perceptual and psychomotor abilities. From the data presented at this conference it appears that psychological functioning may be impaired at relatively low PbB-levels Hnninen et al reported a significant negative correlation between visual intelligence and dexterity and the highest PbBlevel recorded in the range 16-98 pg Pb/100 ml. Repko reported the results of a partially completed study involving 42 storage battery workers and 18 control workers The range of PbB in these workers was 12 to 79 pg/100 ml Although all workers were asymptomatic, poorer performance in terms of strength of impulse, visual reaction time and auditory threshold level was observed in some lead exposed workers In earlier studies Repko et al ( 1975) reported dimished eye-hand coordination and increased tremor in lead exposed workers with Pb B-levels above 70 g/100O ml They also reported increased tone decay and higher auditory thresholds in workers. Although the psychological and performance tests utilized by Hinninen and Repko have been validated and the results verified within their individual laboratories, the results must be shown to be repeatable in other laboratories before any dose-response conclusions can be drawn. Conclusions Concerning Effects on the Nervous System Changes in nerve conduction velocity occur in some lead workers at blood lead levels above 50 pig/100 ml Such slight changes may be measured in persons experiencing no symptoms It was the consensus of the Group that such changes should be regarded as a critical effect. The Group recognized that impairment of psychological functioning has been reported at blood-lead levels below 100 pg/100 ml Further documentation on these effects are required before any definite dose-response relationships can be set up.
4.3 Chromosome Effects Evidence has been produced that cultured lymphocytes from lead exposed workers show an increased rate of chromosome aberrations As far as is known at present the chromosome aberrations described in lead exposed workers by some (but not all) investigators, are no indication of a detrimental health effect on the individual neither do they signify a potential effect to the offspring There is no evidence so far of any abnormal clone formation important for carcinogenesis or leukaemogenesis in somatic cells of lead exposed subjects.
Occupational Exposure to Inorganic Lead
The cromosome aberrations observed in cultured lymphocytes from lead exposed workers are associated with lead exposure but may be due to a factor induced by such exposure and not necessarily to lead itself In vitro studies exposing cultures of human lymphocytes to lead acetate have shown conflicting results. As new and more sensitive techniques become available (e g , sister chromatid exchanges) further research is needed to clarify the mechanism behind to observed effects on cultured lymphocytes from lead exposed workers and their possible health significance.
5 Biological Tests to be Performed with Different Objectives There are three main objectives for application of biological tests in occupational health practice, namely: screening of work populations where exposure levels are not known prevention of the development of adverse effects supporting clinical diagnosis of poisoning, or documentation of functional impairment in the pre-clinical area. For screeningpurposes measurement of EP or ZPP is recommended Elevated values should be verified by subsequent measurement of lead in blood Alternatively the ALA-D test can be used, noting that ALA-D activity is almost completely suppressed at blood lead levels about 70 /ig/100 ml. For the prevention of the development of adverse effects, regular measurements of the concentration of lead (Pb B) in blood are recommended Although this variable measures current exposure and not effects, there are reasons supporting this view, namely: the blood level is specific for absorption up to the present it is the best understood indicator of current dose control opposed to any single measure of effect it is superior to Pb U from the methodological point of view. In addition the use of at least one biochemical test is recommended e g , EP or ZPP, ALA-U, because in evaluation of health significance one should also take into account the biological response. For documentation of functional impairment in individual workers in the clinical or pre-clinical stage, it is necessary to get a more complete picture of the two critical organic systems involved, namely the haematopoetic bone marrow and the nervous system This requires a set of tests including ALA-U, EP (ZPP), Hb and Ht When available, nerve conduction velocity tests should be applied The interpretation in regard to health significance should be a matter of clinical judgement by experienced physicians.
6 Relationship between Environmental and Biological Tests According to data presented by King et al (1976) at the Workshop it appears that any relationship between lead in air and lead in blood is not sufficiently precise for a lead
in air standard to be derived The Group did therefore not propose a permissible level for lead in air. All occupational hygiene control procedures for toxic materials rely upon an evaluation of the overall environment, including uptake from food and drinks, and not merely on air measurements Such an evaluation is best made by biological monitoring of lead exposed individuals in accordance with guidelines given in Sec 5 In cases where respirators are worn or chelating agents are being used, blood lead measurements are obviously not a measure of overall environmental exposure. Lead in air measurements do have a very definite role in the engineering control necessary for the protection of individual workers For the purpose of "engineering" lead in air, quantitative guidelines are necessary, but these should be interpreted in the light of the overall impact of the workman's total environment as determined by his blood lead levels However, lead in air standards will need to be used where facilities for biological monitoring are not available.
7 Analytical Aspects The Group restricted its discussion to blood analysis and considered only ALA-D, EP (ZPP) and to a more detailed extent Pb B. ALA-D It was agreed that ALA-D was a useful parameter for study However, although the methodology of analysis is simple and standardized, it may be of limited use in assessing occupational group exposure This stems from the facts that the activity of the enzyme decrease to only 15 European Units 3/1 rbc in the Pb B range of 40-50 ug/100 ml the relationship between PbB and ALA-D is curvilinear and almost completely suppressed when Pb B concentrations are in the region of 70 jig/100 ml the sample stability is regarded to be poor by some research workers; others however found a stable level of ALA-D for at least 24 h, if kept at 40 C. When reliable blood lead estimations are not practicable, ALA-D estimations are useful for screening groups or individuals So long as ALA-D activity exceeds 30 European units/l rbc, it is most unlikely that PbB-concentrations will exceed 40 pg/100 1. EP/ZPP Measurement of EP (or ZPP) was generally felt to be the best secondary parameter to be used but was not yet in a position to replace Pb B except for screening purposes (see Sec 5) Although initial comparisons of EP(ZPP) with Pb B have been most encouraging, further validation, correlation and standardization will be needed before any more definite recommendations can be made.
European U = /mol/min at 370C
Occupational Expusure to Inorganic Lead
PbB PbB was considered still to be the reference measurement to which other parameters should be related The following guidelines in the analytical procedures for the measurement of Pb B from the occupationally exposed, were agreed upon: the need is emphasized to reduce any possible sample contamination by the use of lead free syringes, ampoules etc whether made of glass or plastic venous samples are recommended of the methods currently in general use, there is no reason to emphasize or recommend any individual technique All are dependent upon the man and not upon the technique operator error can be reducted by those techniques in which a smaller number of chemical/mechanical steps are involved prior to actual measurement for internal control and validation of all laboratories it is recommended that at least two methods based on different principles be available for Pb B measurement Ideally these should be in the same laboratory but failing this close collaboration between pairs or groups of laboratories is encouraged method comparison and replicate analysis is essential in order to check calibration graphs it is recommended that routine control samples be analysed at a frequency of at least 10 % of all analyses it is felt that individual national authorities should decide the extent of quality control and double blind analyses and the example set by the UK (Whitehead) and Scandinavian (see Nordman 1975) schemes can be used as a guideline.
8 Summary of Dose-Response Relationships In the following table a semiquantitative dose-response relationship is given for changes induced by lead, in which Pb B levels are taken as estimate of dose.
Changes induced by lead related to given blood lead concentrations PbB levels in Mg/100 ml
ALA-D inhibition in rbc EP elevation in rbc ALA excretion in creased in urine CP excretion in creased in urine Changes in peri pheral nerve conduction velocity