Blood lead limits

Table I - Blood Lead Limits for Occupational Exposure (Men)...

Lead dust or fiime in the workplace atmosphere should, in so far as possible, be controlled to levels within the legal limits by means of effective exhaust ventilation coupled with suitable arrestment facilities. In situations where exposure cannot be adequately controlled by these means, the use of respiratory protective equipment on the part of the workers becomes necessary. Indeed as blood lead limits become lower, the wearing of respirators has become routine in many plants, not just to protect the worker from airborne lead but because it has the added advantage of preventing hand-mouth contact and consequent ingestion of lead. 

The net result of these various considerations is that neurobehaviour and reproductive ability are the major factors in the health debate at present, with 40 pg/dl looking increasingly likely as the blood lead limit towards which legislation will gravitate. In practice fhis figure has already been anticipated by industry. In North America, the Lead Industries Association and Battery Council International laimched a voluntary commitment in 1997 under which the medical removal level will be reduced to 40 pg/dl (with a return to work below 35 Jig/dl) by 2001. In Europe, several major lead producers have pursued internal programmes to reduce the blood lead levels of all their employees to less than 40 pg/dl by 2000 or thereabouts. These initiatives are not unique, so it is to be hoped that the likely downward trend in legal limits will not cause the industry too many problems. 

The different forms of lead have different bioavailabiUty and this ultimately impacts cleanup levels. Mine tailings, slag, and other such residues have limited impact on blood lead levels because these materials contain lead in the form of lead sulfide, which has limited biological reactivity and uptake. 

In addition to limits on airborne lead, an OSHA regulation provides for biological monitoring and places limits on blood lead levels in workers of... 

Biological limits are also in force for lead and its compounds under the Control of Lead at Work Regulations 1998 different blood lead action, and suspension from work, levels apply to women of reproductive capacity, young persons and other employees. 

OSHA regulations limit the concentration of lead in workroom air to 50 ig/m3 for an 8-hour workday. If a worker has a blood lead level of 50 ig/dL, then OSHA requires that worker be removed from the workroom where lead exposure is occurring. 

An adverse effect of lead on pregnancy rate has been noted in some animal studies (Kennedy et al. 1975). Acute-duration gavage administration of 390 mg lead/kg/day as lead acetate to rats resulted in a sharp decrease in pregnancy rates. This effect was not noted at 39 mg lead/kg/day. The study limitations include a lack of measurement of blood lead levels and lack of statistical analysis of pregnancy incidence. A decrease in the number of implantations was noted in untreated female mice that were mated to males that had been treated with 141 mg/kg/day lead chloride in the drinking water for 3 months... 

Numerous observations of non-linear relationships between PbB concentration and lead intake in humans provide further support for the existence of a saturable absorption mechanism or some other capacity limited process in the distribution of lead in humans (Pocock et al. 1983 Sherlock et al. 1984, 1986). However, in immature swine that received oral doses of lead in soil, lead dose-blood lead relationships were non-linear whereas, dose-tissue lead relationships for bone, kidney and liver were linear. The same pattern (nonlinearity for PbB and linearity for tissues) was observed in swine administered lead acetate intravenously (Casteel et al. 1997). These results suggest that the non-linearity in the lead dose-PbB relationship may derive from an effect of lead dose on some aspect of the biokinetics of lead other than absorption. Evidence from mechanistic studies for capacity-limited processes at the level of the intestinal epithelium is compelling, which would suggest that the intake-uptake relationship for lead is likely to be non-linear these studies are discussed in greater detail in Section 2.4.1. 

Target Organ Toxicity. This section focuses on mechanisms for sensitive health effects of major concern for lead—cardiovascular effects, hematological effects, and neurological effects, particularly in children. Bone is a major sink for lead, and there is some limited information regarding the effects of lead on bone and potential mechanisms of action. Renal effects occur at relatively high blood lead levels and evidence of renal carcinogenicity has been demonstrated only in animals mechanisms for these effects will be discussed briefly. 

The impairment of heme synthesis by lead has a far-ranging impact not limited to the hematopoietic system. EPA (1986a) provided an overview of the known and potential consequences of the reduction of heme synthesis as shown in Figure 2-11. Well documented effects are indicated by solid arrows, and effects considered to be plausible further consequences of the impairment of heme synthesis are indicated by dashed arrows. Additional discussion is provided in the following sections on renal and neurological effects. More detailed information on the exposure levels or blood lead levels at which these impacts may be experienced was provided in Section 2.2 and the relevance to human health is discussed in Section 2.5. 

Needleman et al. 1984). The limitations of these studies include possible bias introduced by use of hospital records and a restricted range of maternal and cord blood lead levels. The sizes of the groups studied were not sufficient for the detection of differences in low frequencies of anomalies. 

Campara P, D Andrea F, Micciolo R, et al. 1984. Psychological performance of workers with blood-lead concentration below the current threshold limit value. Int Arch Occup Environ Health 53 233-246. 

Ong CN, Endo G, Chia KS, et al. 1987. Evaluation of renal function in workers with low blood lead levels. In Fao V, Emmett EA, Maroni M, et al., eds. Occupational and environmental chemical hazards. Chichester Ellis Horwood Limited, 327-333. 

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