BIOLOGICAL MONITORING OF CHEMICAL EXPOSURE IN THE WORKPLACE
Franko Alenka
Medical Centre Ljubljana, Institute of Occupational, Traffic and Sports Medicine, Slovenia

Introduction

Measures taken to preserve health and prevent hazardous effects of acute or chronic overexposure to chemical agents should be based on a multidisciplinary approach. Early identification and subsequent reduction of hazardous exposure to chemical agents are of utmost importance in reducing irreversible adverse biological effects and health risk. Monitoring occupational exposure is a procedure used for assessment of environmental and/or biological indicators and the health risks involved. It uses permissible levels of exposure defined in accordance with the current knowledge, and involves regular assessment of health risks based on a comparison of current or integrated exposures with permissible exposure values.

Biological monitoring

Biological monitoring of exposure in the strict sense of the word involves measurement and assessment of the amount of a chemical agent or its metabolites in biological media. It is aimed at assessing internal dose and the health risk involved (e.g. determinations of lead levels in blood and urine, measurements of xylene levels in blood and of methylhippuric acid in urine) (1). In the broad sense of the word biological monitoring includes both assessment of the internal dose, as well as detection of early reversible biological effects. I.e. biological effective dose, e.g. measurement of decreased delta-aminolevulinic acid (ALA), dehydratase levels in blood and of elevated levels of delta-aminolevulinic acid (ALA) in urine and erhythrocyte protoporphyrin in blood following exposure to lead) (1).

Health surveillance

Health surveillance involves periodic medical examinations and physiological testing of occupationally exposed workers to determine the extent of damage to target organ function, and to assess health impairment (1). Health surveillance programmes have a broader scope than biological monitoring (clinical examinations) and are conducted over a longer period of time.

Environmental monitoring

Ambient monitoring of chemical exposure involves measurement and assessment of chemicals present in the work area. It provides assessment of external exposure and health risk based on a comparison to maximum permissible levels or limit values of toxic chemical agents in the workplace (1).

The assessment of the most recent external exposure of individual workers or group of workers is most frequently based on limit values. I.e. average concentrations of hazardous chemicals in the air for an 8-hour daily exposure, or, on more reliable, time-weighted daily concentrations of hazardous chemicals, and takes into account occasional permissible deviations from limit values in the workplace. When assessing simultaneous exposure to several toxic chemical substances, the value of each chemical agent is compared to the established limit values.

The assessment of overall external exposure of individual workers is based on the determination of a cumulative external dose using a formula D = Si ci x ti (D-dose, ci concentration of chemical in the workplace, ti - duration of chemical exposure in the workplace) for each individual worker. The dose is expressed in mg/m3 - years, or no.fibres/cm3 - years or ppm - years. The accuracy of the determined cumulative dose depends on the origin of the data collected. Data provided by personal dosimeter are less variable than those obtained by measurements carried out in the immediate working environment or in the work area (2).

Target organs

Chemicals do not exert a similar degree of toxicity in all organs, but usually cause major toxic effects in one or several organs (e.g. lead is toxic to peripheral and central nervous systems, kidney and gastrointestinal tract). These target organs, however, are not necessarily the sites of highest concentration of the toxic chemical (1,3) (e.g. lead concentrations in bones).

Toxicokinetics

The understanding of absorption, distribution, biotransformation and excretion of chemical agents is essential to accurate evaluation of the results of biological monitoring (1,4). Once absorbed via the respiratory tract, digestive system and skin, and present in the circulation, chemical agents are distributed to various organs of the body. They may be excreted from the organism unabsorbed mostly via urine or expired air. Some of them are metabolised through biotransformation, mostly in the liver, but also in the bowel, kidneys, lungs or skin, which makes them more easily excretble via urine or bile (1).

All these processes may be influenced by endogenous factors (genetic constitution, anthropometric factors, health status) and exogenous factors (work load and simultaneous exposure to several substances, alcohol, drugs and tobacco) (1,5). Recognising these factors is essential for selecting the appropriate biological markers (parent compound or metabolite), the biological medium and the sampling time, and for the interpretation of the results (1,6).

Toxicodynamics

The knowledge of early harmful effects exerted by chemical agents is a prerequisite for the use of screening tests and assessment of health risks, as well as for an accurate estimate of the tolerable level of the biological exposure parameters. Knowing early non-adverse biological effects is useful for the development of biological monitoring methods. For example, the measurement of serum pseudocholinesterase or d-aminolaevulinic acid dehydratase inhibition is employed for estimating the risk of exposure to organophosphorus pesticides and lead.

Determining the dose-effect relationship (i.e. the concentration of the chemical at which the effects is expected to occur), as well as the dose-response relationship (the percentage of individuals demonstrating these effects at each dose level) is fundamental for risk estimation when proposing biological limit levels. For example, unspecific tremor is noted in most workers exposed to mercury at a concentration of 35 µg Hg/L in the blood (8,9); at urinary concentrations > 20 µg/L, atypical precoproporphyrins occur in the urine of the majority of these individuals (10).

Elimination half-life

The biological half-life of a chemical in an organ, a tissue or the body, is the time required to excretion half the amount of the substance. Some substances may have several half-lives corresponding the elimination from different organs or tissues (e.g. mercury, lead, and cadmium), yet usually one main half-life predomination (1,4). For chemical agents that have a long biological half-life in various parts of the body, the time of sampling may not be critical (e.g. lead, cadmium). Some chemicals accumulate during the week work- time (e.g. chromium, tricholoroethylene). The sampling time, however, may be of utmost importance for other substances that are rapidly eliminated from the body (e.g. ethylene oxide) (1).

Pharmacokinetic models

Pharmacokinetic models are valuable tools for determining the relationship between external exposure and the internal dose. These mathematical models take into account the variability of the exposure to chemicals and physiological factors, such as the intensity and length of exposure, the work physical loads, the body build and the liver and renal function (5,6).

Internal dose

In addition to external dose or external exposure, occupational medicine uses the term internal dose or internal exposure, commonly defined as the amount of a chemical or its metabolite in biological media, reflecting the integrated external dose for a given period of time (1).

It is very important to determine cumulative biological dose of exposure in chronically exposed individuals. It is expressed either as the average of average annual concentrations of a substance in the biological medium (e.g. lead in blood, mercury in urine), or as a sum of maximum levels of the chemical absorbed, determined for each individual separately. Biological monitoring of exposure estimates the internal dose of a chemical, on the basis of the present knowledge of toxicokinetics of the chemicals in the body (1).

The term internal dose may denote different concepts, depending on the chemical and the biologic parameter analysed (1,3). 1

  1. Internal dose may mean the amount of chemical recently absorbed. A biological parameter may therefore reflect the amount of the chemical absorbed either shortly before sampling. The concentration of a solvent in alveolar air or in blood during the work-shift or during previous day the concentration of a solvent in alveolar air or in blood collected 16 hours after the end of exposure. Or, for chemicals with a long biological half-life - during past months (e.g. the concentration of some metals in blood) or even years (the concentration of Hg in urine) (1).
  2. Internal dose may also mean the amount of chemical stored in one or several body compartments or in the whole body (integrated exposure or specific organ dose). For example, the concentration of polychlorinated biphenyls in blood reflects the amount of chemical accumulated in the main sites of deposition, i.e. fatty tissuess (1).
  3. And finally, with ideal biological monitoring tests, the internal dose means the amount of substance bound to critical organs or tissues (target dose or biological effective dose) (1,3). Such tests can be developed when the critical organs are easily accessible. Haemoglobin exposed to carbon monoxide or other methaemoglobin forming agents, or when the substance interacts with blood constituents in a similar way as with the critical target molecules (haemoglobin alkylation reflecting binding to DNA in the target tissue). In the latter situation, the amount of the chemical bound to the blood constituent is used as a surrogate of the biologically effective dose (1).

Biological tests

The biological tests employed to monitor exposure to industrial chemicals are classified in three type of category (1):

  1. Determination of chemical agents or their metabolites in biological media
    The majority of tests currently used for biological monitoring are based on the determination of the chemical and its metabolites in biological media. The biological media most commonly used include urine, blood and, less frequently, exhaled air. Other biological materials, such as faeces, fat tissue, hair, nail or saliva, can also be analysed (1,4). According to their specificity, the tests can be classified into the following two categories:
    •  a) Selective tests are based on direct determination of the unchanged substance, such as lead, cadmium or xylene, or its metabolites e.g. methylhippuric acid as a xylene metabolite in biological media.
    •  b) Non-selective tests are employed as non-specific indicators of exposure to a group of chemical substances. For example, the determination of diazo-positive metabolites in urine in individuals exposed to aromatic amines, analysis of thioethers in urine to assess exposure to mutagenic and carcinogenic substances and determination of mutagenic activity of urine (1).

  2. Quantification of reversible non-adverse biological effects related to the internal dose
    The second category comprises tests measuring reversible non-adverse biological effects. Most of these tests are non-specific, and are also used for assessment of some other diseases and conditions (1). The development of these tests usually requires some knowledge of the mechanism of action of the chemical substance. An example of these tests is the determination of inhibition of pseudocholinesterase activity in exposure to organophosphorous compounds. Another example is the use of the determination of high- and low-molecular proteins and cytotoxic effects of N-acetyl-ß-D glucosaminidase (NAG) for assessment of impaired glomerular and tubular function in individuals exposed to mercury, cadmium, lead, chrome and other chemical (1).

  3. Measurement of the amount of active chemical interacting with the target and non-target molecules
    These tests directly or indirectly estimate the amount of chemical interacting with the target sites. When these sites are easily accessible, these tests provide a more accurate estimate of the healthy risk than do many others monitoring procedures. A new generation of tests based on immunological and GS-MS techniques are being developed (1).

Comparison of environmental and biological monitoring of exposure

Biological monitoring of chemical exposure offers several advantages over ambient monitoring. It provides the assessment of internal dose and hence the estimate of the health risk involved. Biological monitoring takes into account intra-and inter-individual variability in absorption, distribution, and storage of the chemicals, as well as in susceptibility and characteristics related to work physical load (1,4).

Biological monitoring takes into consideration absorption of chemicals by routes other than the lungs, i.e. through the skin or the gastrointestinal tract (integrated overall exposure) (1,11,12,13). Unlike ambient monitoring, it allows for assessment of integrated overall exposure and takes into consideration inter- and intra-individual variations (1,4).

Environmental monitoring of exposure is more suited than biological monitoring for the assessment of acute exposure to hazardous chemicals. It is particularly useful in case of exposure to substances that exert direct toxic effects to the site of contact (e.g. eye mucous, lung irritants, respiratory tract carcinogens and are poorly absorbed (1). Also, environmental monitoring is superior to biological monitoring as concerns the identification of emission sources, detection of industrial pollutants in exposed groups and in evaluation of the effectiveness of the engineering control measures taken. And finally, the use of biological methods cannot be recommended for assessing exposure to chemical for which data of toxicokinetic and toxicodynamic are too limited.

Biological and ambient monitoring of chemical exposure, therefore represent two complementary approaches for the assessment of health risk in the workplace.

Biological media

The majority of biological tests rely on the analysis of chemical substances in blood, urine or expired air (1,4).

Blood

Blood transports and distributes chemicals in the body. The majority of systemically acting substances and their metabolites are therefore found in blood. Blood, as a biological medium, is used for measuring most inorganic chemicals (e.g. elementary mercury, lead) and for measuring those organic substances that are poorly biotransformed and have a sufficient half-life e.g. methyl mercury, lindane, DDT). The measurement of unchanged substances in blood has higher specificity than the determination of their metabolites in urine. Blood is also a suitable medium for the measurement of substances binding to macromolecules (e.g. aniline haemoglobin). Depending on the substance measured, the analysis is done on whole blood (mercury, cadmium, lead, benzene), plasma (mercury, nickel, cadmium), serum (aluminium, cobalt, lindane) and/or erythrocytes (mercury, chromium).

Measurements of concentrations of many volatile solvents (e.g. benzene, toluene, styrene) in blood and in alveolar air often have the same significance. They reflect either the most recent exposure when blood samples are obtained during exposure or the exposure during the previous day if blood is collected 16 hours after the end of exposure. Blood levels of some cumulative organic chemicals (e.g. polychlorinated biphenyl) may reflect their accumulation in the body, as the concentration of these substances in blood is related to their concentration in the main storage compartment (1).

Urine

Urine is easy to collect, even in large quantities, and the procedure is non-invasive. This biological medium is suitable for determining water-soluble metabolites of organic chemicals. Phenylmercapturic acid in benzene exposure, formic acid in methanol exposure and several inorganic chemicals (e.g. metals, such as mercury, lead and cobalt). In individuals exposed to chemicals with short biological half times, or with fluctuating concentrations in the air, the level of metabolite in urine samples collected at the end of shift. It is commonly a better indicator of the average exposure during the shift compared to the level of the substance in exhaled air or blood. Determination of urinary 2.5 hexanedione levels in hexane exposure, of urinary 2-thiothiazolidine-4-carboxylic acid levels in carbon disulphide exposure, and of phenylglyoxilic acid levels in individuals exposed to styrene. Concentrations of the chemical in exhaled air or blood are more strongly influenced by the recent exposure.

Determinations done on 24-hour urine specimens are more representatives than those done in spot samples, except in the case of exposure to chemicals with long half times. As for rapidly excreted substances, such as solvents, measurements at the end of the shift are more appropriate. It should be pointed out that urinary concentration of a metabolite largely depends on the rate of urine production. Determining it in either over-diluted (large beverage intake) or over-concentrated (inadequate liquid intake, sweating) urine samples may lead to misinterpretation. For some substances, such as mercury and lead, the use of urinary creatinine and specific density is recommended to correct chemical concentrations in urine (14). The renal excretion is regulated by three mechanisms: glomerular filtration, tubular secretion and tubular reabsorption. Changes of any of these mechanisms may have a significant influence on the elimination of a chemical.

Expired air

Alveolar air analysis (the end-exhaled air method) is used to assess exposure to volatile organic solvents, such as benzene, toluene and styrene. The method is non-invasive, yet it carries a risk of external contamination during sampling. The concentration of the solvent in alveolar air may be subject to a rapid fluctuation due to changes in the intensity of exposure (1).

The time of sampling is of utmost importance. It determines either the recent exposure level (sample collected during or immediately after the end of the shift, e.g. hexane, toluene, styrene) or the level of exposure during the previous day (the sample collected 16 hours after the shift, e.g. benzene, styrene, trichlorethylene) (1).

Other biological media

Maternal milk or fat tissue is sometimes analysed to assess the body burden of lipophilic substances (e.g. organochlorine pesticides) or the risk of transfer of toxic substances to the new-born. The excretion of a chemical in faeces is the reflection of the level of oral intake and has no practical value in occupational monitoring. Hair or nail specimens are sometimes used to assess exposure to lead, methyl mercury and arsenic, yet external contamination easily leads to misinterpretation of the results. Various methodological issues and the influence of external contamination restrict the use of sweat, sputum and saliva as biological media. Methods for determining the concentration of lead in bones and that of cadmium in the liver and kidney have been developed, yet their use is still restricted to research (1).

Biological limit values

Biological limit (permissible) value is defined as a maximum permissible amount of a chemical or its metabolites in biological media that urges appropriate control action to be taken in the occupational setting (1,5). According to current knowledge exposure to concentrations lower than biological limit values generally does not produce irreversible biological damage and does not increase the risk for health impairment in employees (subclinical and clinical impairment), even if exposure is repeated or lasts for several years. Biological limit values constitute upper permissible limits for healthy adults, yet they do not exclude potential health risks for susceptible individuals. Women of child-bear women age and pregnant women (e.g. limit lead value for women in blood is 300 µg/L, for men 400 µg/L and for children 100 µg/L).

Interpretation of the results of biological monitoring

The interpretation of the biological monitoring results must be based on our current knowledge of the relationship between external exposure, internal dose, and the risk of adverse health effects, on which basis the biological limit values have been established (1). If the quantitative relationship between external exposure and the internal dose is known, the biological parameter can be used as an index of exposure and may provide fairly reliable information on the health risk (e.g. the relationship between airborne Hg levels and Hg levels in blood and urine) (8). Under some conditions biological monitoring is much more an estimate of the intensity of exposure than of the potential health risk.

Sometimes, when the relationship between the internal dose and adverse health effects has been established (e.g. reduced production of haemoglobin as a result of lead concentrations in blood > 500 µg/l), the biological parameter can serve as an indicator of health risk. When the internal dose is quantitatively related to both adverse effects and external exposure, the biological indicator provides information on both exposure and health risk. The results of biological monitoring are usually compared to adequate reference values. Because of differences in individual susceptibility, the threshold values above which adverse effects will occur vary from one individual to another. The established biological reference values therefore give no assurance that they will protect all occupationally exposed individuals from adverse health effect. In some susceptible individuals, a biological response occurs even with exposure below the reference values (1).

With parameters showing large inter-individual variability, the exposure level may be better interpreted by comparing it to the individual pre-exposure (baseline) value. For example, the cholinesterase activity of erythrocytes as a result of exposure to organophosphates or carbamates should preferably be expressed as a percentage of the individual baseline activity. The results can also be interpreted on a group basis. The working conditions are considered satisfactory when all the observed values are below the biological permissible level. If the majority of results exceed the biological permissible level, adequate measures must be taken to reduce exposure. Sometimes the majority of workers exhibit values lower than the biological permissible level, but a few of them have abnormally high values. Several interpretations are possible in this situation: either the individuals with the high values perform activities exposed to increased chemical pollution, or the higher levels observed in some workers are due to inadequate adherence to personal protection policy, poor hygiene, non-occupational exposure, genetic polymorphism or laboratory errors (1).

Conclusion

Two complementary approaches for health risk assessment, i.e. the assessment of external exposure and biological monitoring of specific contaminants, should be used to ensure safer working conditions and overcome the drawbacks of ambient monitoring, except in the case of acute exposure to toxicants. Biological monitoring provides a direct estimate of internal dose, i.e. internal chemical exposure, and in some cases also the assessment of early reversible biological effects. It affords early detection of hazardous exposure that at the same internal dose is likely to cause early biological effects in a given population group. Unless this hazardous chemical exposure is identified and eliminated, the internal dose will increase and so will the number of workers exhibiting early biological effects that may become irreversible. Timely biological monitoring, followed by appropriate remedial action taken in the occupational setting, can decrease the level of exposure and reduce its harmful health effects. Yet, the above mentioned limitations of biological monitoring and the non-specificity of some tests, should be taken into account when interpreting the results and estimating the health risk.

The physician involved with biological monitoring needs a sound knowledge of toxicokinetics and toxicodynamics of chemical hazards and routes of exposure. He/she should be competent in the interpretation of the results of biological monitoring by comparison to adequate biological reference values, done in collaboration with other experts in the field.

Sažetak

BIOLOŠKI MONITORING EKSPOZICIJE HEMIKALIJAMA NA RADNOM MJESTU

Medicina rada zahtijeva multidisciolinarnu saradnju u prevenciji zdravstvenih oštećenja koja mogu biti rezultat prekomjerne akutne ili kronične ekspozicije kemijskim noksama. Strogo govoreći biološki monitoring ekspozicije znači mjerenje i procjenu kemijskih noksi ili njihovih metabolita u biološkom mediju u cilju otkrivanja aktualne ekspozicije- interne doze. Postojan koncept biološkog monitoringa uključuje ocjenu interne doze, kao i otkrivanje reverzibilnih bioloških efekata. S druge strane ambijentalni monitoring izloženosti kemijskoj materiji znači mjerenje i procjenu hemijskog agensa u radnoj okolini.

Biološki testovi izloženosti industrijskim kemikalijama može se klasificirati u tri kategorije: određivanje kemijske materije ili njenog metabolita u biološkom mediju, zbrajanje reverzibilnih bioloških efekata, mjerenje količine aktivnih kemijskih interakcija s ciljanim i neciljanim molekulama. Biološki monitoring ekspozicije ima različite prednosti u odnosu na monitoring okoline. To znači ako procijenimo internu dozu (internu ekspoziciju), u mogućnosti smo odrediti zdravstveni rizik. Naravno tu postoje inter-individualne and intra-individualne varijacije u absorpciji, distribuciji, retenciji kemijske supstance u organizmu, individualne preosjetljivosti i druge karakteristike koje se odnose na fizičku aktivnost. Ambijentni monitoring je više promjenjiv u otkrivanju akutne ekspozicije i ekspozicije kemijskim supstancama za koje je uobičajeno da izazivaju toksični efekat u kontaktu. Za izbor biološkog monitoringa važno je razumjeti toksikokinetičke, tolsikodynamičke osobine kemijskih supstanci, a isto tako poznavati nivoe bioloških aktivnosti.

Glavni biološki testovi su analiza kemikalije ili metabolita u krvi, urinu ili disanju.
Tačnom detekcijom opasnosti rizika ekspozicije možemo značajno dovesti do pada incidencije neželjenih efekata ekspozicije, a za smanjenje nivoa ekspozicije možemo zahvaliti preventivnim mjerenjima. Glavni cilj biološkog monitoringa ekspozicije je odrediti ne čini li ta izloženost neprihvatljiv zdravstveni rizik. Preventivne aktivnosti su nužne.

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