Biological monitoring of lead exposure is performed utilizing three types of tests:
1. Direct measurements of lead concentration in tissues (blood, hair, bone)
2. Urine test following the administration of a chelating agent
3. Screening for early biological effects of lead exposure as reflected in changes in hemoglobin, free erythrocyte protoporphyrin, zinc protoporphyrin, basophilic stippling, or measurement of porphyrins in the urine (18)
Periodic measurement of the blood lead level forms the cornerstone of biological monitoring of lead-exposed workers. Measurements are quickly and reliably performed using atomic absorption techniques. Blood lead concentration is the best available indicator of current lead exposure.
It has been estimated that with an exposure level of 1 |g lead/m3 in air, an increase of 1 to 2 |g lead/100 g whole blood will occur. Blood lead does not correlate well with body burden of lead. In experiments with increasing dietary lead in volunteers it has been shown that a plateau of the blood level is reached while body burden continues to increase (18,19).
Lead in urine reflects lead recently absorbed. Urine levels usually average 50 |g/g creatinine for a blood lead level of 40 |g/100 g whole blood.
Because lead excretion varies from one individual to another, and recognizing the low correlation between levels of lead in blood and urine, most authors agree that urine lead levels should not be used for the routine assessment of exposure.
Urine lead levels can be measured following administration of a single dose of chelating agent (usually 1 g of ethylenediaminetetraacetic acid, EDTA). This testing can be used to confirm past exposure in individuals who are not currently exposed to the metal. An excretion of lead exceeding 600 |g/24 h after EDTA administration in currently unexposed individuals suggests increased body burden (18,19).
Zinc protoporphyrin (ZPP) and free erythrocyte protoporphyrin are the most commonly performed assessments of lead-related biochemical effects. Determinations of these levels are recommended for screening purposes. Zinc protoporphyrin levels can be conveniently tested with a hematofluorometer, still found in use in some rural clinics. Most commercial laboratories perform both blood lead and ZPP on all samples submitted for screening.
Zinc protoporphyrin levels are typically less than 40 |g/100 mL whole blood. Biological exposure indices are indicated above 100 |g ZPP/100 mL blood. Elevated ZPP levels must be confirmed and correlated with lead levels, which are a more specific indictor of lead exposure (18-20).
Under the U.S. Occupational Safety and Hazard Administration (OSHA) Lead Standard, medical surveillance is required if workplace lead levels exceed 30 |g/m3 for more than 30 days per year. Many farm environments are exempt from OSHA oversight; however, the strategy for biological monitoring is still broadly applicable though it may not be mandatory (21).
For workers in environments exceeding the action level (30 |g/m3 for 30 days per year) the OSHA standard recommendations are given in Table 9.3. The World Health Organization (WHO) in 1980 specified a time weighted
Table 9.3. Duty action levels in lead monitoring.
For workers in environments exceeding the action level (30 |g/m3 for 30 days per year) the U.S. OSHA Standard recommends:
1. Blood lead levels:
a. Every 6 months if the level is less than 40 |g/dL
b. Every 2 months if the level is less than 40 |g/dL, until two consecutive levels are found to be <40 |g/dL
c. Monthly in workers removed from exposure
2. Medical examinations:
a. Yearly for any exposed worker if the blood lead level has exceeded 40 |g/dL
b. Prior to assignment to a work area in which the action level has been exceeded c. If signs or symptoms of possible lead intoxication develop (see Chapter 17)
3. Removal from exposure of:
a. Workers whose lead levels exceeds 60 |g/dL, unless the last lead level tested was under 40 | g/dL
b. Workers whose last three lead levels exceeded 50 |g/dL
c. Workers judged to be at increased risk of impairment of health from exposure to lead such as during pregnancy and lactation
4. Return to duty:
a. A worker who has been removed from exposure because of elevated lead level may be returned to work if two consecutive lead levels measure <40 | g/dL
Source: Data from National Institute for Occupational Safety and Health (21), Occupational Safety and Health Administration (22).
action blood lead level (TWL) for removal from duty of 40 |g/dL in the general population and a level of 30 |g/dL in women of childbearing age. Baseline blood levels (nonanthropogenic or causing changes in the body) are 10 to 30 |g/dL based on the observation that normal life in an industrialized city will produce levels in that range. Above 30 |g/dL is considered toxic (21-24).
Where required, medical examinations include a detailed work history (with special attention to all toxic exposure potential), medical history, and a thorough medical examination, with special attention to the neurological system, kidneys, teeth, gums, blood, blood pressure, heart, gastrointestinal system, lungs, and fingernails (looking for Mees lines) (21-24).
Germany, Canada, Australia, and Switzerland have each developed lead standards for their general populations. In addition, many states within the United States and provinces within Canada have elected to implement their own guidelines (23,24).
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