ACOEM Position Statement: Workplace Health and Safety Necessitates an Update to Occupational Lead Standard Provisions for Medical Removal Protection, Medical Surveillance Triggers, and the Action Level and Permissible Exposure Level for Lead in Workplace Air: ACOEM Response to OSHA.

In June 2022, the US Occupational Safety and Health Administration (OSHA) issued an “Advance Notice of Proposed Rule Making (ANPRM)—Blood Lead Level for Medical Removal” in which the agency sought input on reducing the current blood lead level (BLL) triggers in the medical surveillance and medical removal protection (MRP) provisions of the general industry and construction standards for lead.1 These lead standards had last been updated in 1978 and 1992, respectively. In response, the American College of Occupational and Environmental Medicine (ACOEM) provided written comments to the OSHA Lead Docket (No. OSHA-2018-0004) on October 28, 2022. The comments, developed by an ACOEM working group, reinforced and expanded on ACOEM's 2016 Workplace Lead Exposure position statement.2 ACOEM believes that multiple, high-quality, prospective cohort studies linking lead exposure to an increased risk of cardiovascular and cerebrovascular disease mortality necessitate that mandatory MRP benefits be instituted for a single BLL ≥30 μg/dL or for two successive BLL ≥20 μg/dL measured at a 4-week interval. The goal of workplace lead exposure management should be to maintain workers' BLL less than 10 μg/dL (or <3.5 μg/dL in the case of women who are or may become pregnant). Because these BLL values provide at best a slim margin of safety, it would be reasonable to enact lead standards whose goal is to maintain all worker blood lead concentrations less than 5 μg/dL, or less than the US Centers for Disease Control and Prevention (CDC) blood lead reference value in the case of workers who are or may become pregnant. ACOEM supports a reduction in the action level (AL) and the permissible exposure level (PEL) for lead in workplace air to 2 and 10 μg/m3, respectively, as 8-hour time-weighted averages (TWAs). Because exclusive reliance on airborne lead monitoring may fail to identify the severity of lead hazards in many workplaces, medical surveillance requirements of a revised lead standard should also apply to employees who perform a trigger amount of lead work based on the duration that they are engaged in altering or disturbing materials that contain lead at a concentration ≥0.5% by weight. ACOEM endorses a prominent role for physicians with requisite training and knowledge to communicate the implications of medical surveillance and blood lead monitoring directly with employees, and to institute discretionary MRP or other workplace restrictions based on the physician's evaluation of an individual worker's health and reproductive status. Excerpts from ACOEM's October 2022 comments and recommendations to OSHA concerning revisions to the occupational lead standards are grouped under the following four subheadings3: I. MRP benefits should be triggered at blood lead concentrations of 20 to 30 μg/dL. The goal should be to maintain all workers' blood lead concentrations less than 10 or 5 μg/dL. ACOEM affirms the recommendation set forth in its 2016 statement on Workplace Lead Exposure that the goal of workplace protections and policies should be to maintain all workers' BLLs <10 μg/dL.2 To the extent that OSHA's question 1 in the ANPRM refers specifically to MRP benefits under the OSHA lead standards, ACOEM affirms its position that mandatory MRP should be promptly instituted for a single BLL ≥30 μg/dL or two successive BLL ≥20 μg/dL measured at a 4-week interval. This same recommendation was issued by a panel of experts in 20074 and by the California Department of Public Health.5 ACOEM emphasizes that the existence of MRP triggers at these BLLs does not indicate that health concerns and the need for lead exposure mitigation should begin at the mandatory MRP BLL. Rather, as discussed later, a series of graded educational, industrial hygiene, and medical surveillance interventions preceding mandatory MRP should be instituted at lower BLL thresholds, including discretionary MRP based on a physician's evaluation of an individual worker's health and reproductive status. Table 2 in the OSHA ANPRM notice of June 28, 2022,1 showed that elevated lead exposure is associated with a myriad of multisystemic adverse health effects. Although it was inexplicably not cited in that table, ACOEM finds particularly compelling the epidemiological studies that establish that long-term (years to decades) exposure to blood lead concentrations in the range of 10 to 25 μg/dL substantially increase the risk of death from cardiovascular and cerebrovascular disease. The general US population experienced years to decades of BLLs that averaged in this range from the 1940s to the early 1980s.6–14 During this time, the widespread use of leaded gasoline and the common presence of lead in residential paint, soldered food, and beverage containers, and plumbing for potable water resulted in ubiquitous lead exposure. Consequently, the health effects experienced by the general population during this period have particular relevance to the risk posed by contemporary occupational exposure at similar BLLs. Five large prospective cohort studies of individuals from the general population who lived a considerable proportion of their lives when BLLs averaged 10 to 25 μg/dL have revealed a significant association between markers of lead exposure or lead burden and cardiovascular mortality. The National Health and Nutrition Evaluation Surveys (NHANES) conducted by the National Center for Health Statistics of the CDC are large stratified, multistage probability surveys designed to select a representative sample of the civilian, noninstitutionalized US population. In 2002, Lustberg and Silbergeld15 examined the mortality experience through 1992 of adult participants in NHANES II who were 30 to 74 years of age between 1976 and 1980 and who had a baseline blood lead concentration of less than 30 μg/dL (N = 4190). After adjustment for multiple potential confounders (age, sex, location, education, race, income, smoking, body mass index, and exercise), BLLs of 20 to 29 μg/dL at baseline were associated with 39% increased mortality from circulatory disease (relative risk [RR], 1.39; 95% confidence interval [CI], 1.01–1.91), and BLLs of 10 to 19 μg/dL were associated with 10% increased mortality from circulatory disease (RR, 1.10; 95% CI, 0.85–1.43), compared with baseline BLLs less than 10 μg/dL. A similar prospective cohort study was conducted in 2006 by Schober et al16 on a subset of participants in NHANES III who were recruited in two successive 3-year phases between 1988 and 1994. Baseline blood lead concentration was available from 9762 subjects 40 years or older. Mortality status and cause were assessed through 2000 (median length of follow-up, 8.55 years). After adjustment for multiple potential confounders including sex, race/ethnicity, education, and smoking status, Cox proportional hazards regression using age as the time scale to examine the relative hazard (RR) found that BLLs >10 μg/dL at baseline were associated with a 55% increase in mortality from cardiovascular disease (RR, 1.55; 95% CI, 1.16–2.07), and BLLs of 5 to 9 μg/dL were associated with a 20% increase in mortality (RR, 1.20; 95% CI, 0.93–1.55), compared with BLLs less than 5 μg/dL. The test for trend by BLL group was also statistically significant (P < 0.01). The group containing a baseline BLL >10 μg/dL had a median BLL of 11.8 μg/dL, with few subjects having BLL >20 μg/dL. Because of the phase out of lead in gasoline beginning in the late 1970s and the decline in BLL with reduction in all exogenous exposure, the blood lead concentrations of individuals older than 40 years at the time of their recruitment into NHANES III were likely lower than the BLLs they experienced earlier in their lives. However, because the subjects were a representative sample of the general US population, their cumulative lead exposure was strongly influenced by BLLs in the 10- to 25-μg/dL range that were typical from the 1940s to 1970s. In 2006, Menke et al17 conducted a different Cox regression analysis of the mortality experience through 2000 of NHANES III participants older than 17 years at baseline whose blood lead concentration was less than 10 μg/dL (N = 13,946). After adjusting for multiple potential confounders, successive terciles of baseline BLLs were associated with an increased risk of death from myocardial infarction and stroke. Compared with subjects in the lowest tercile of BLL (≤1.93 μg/dL), those in the highest tercile BLL (≥3.63 μg/dL) experienced an 89% increased risk of death from myocardial infarction (RR, 1.89; 95% CI, 1.04–3.43) and a 151% increased risk of death from stroke (RR, 2.51; 95% CI, 1.20–5.26). As with the analysis by Schober et al,16 the cumulative lead exposure of the participants and likely their baseline blood lead concentration at the time of recruitment between 1988 and 1994 were strongly influenced by BLLs in the 10- to 25-μg/dL range experienced earlier in life. Lanphear et al18 extended the analysis of Menke et al17 by reporting the cause-specific mortality experience of NHANES III participants through 2011 (N = 14,289; median, 19.3 years of follow-up). In examining the risks associated with an increase in baseline log-transformed BLL from 1.0 to 6.7 μg/dL (10th–90th percentile), after adjustment for multiple confounders, the hazard ratio (HR; RR) for cardiovascular disease mortality increased 70% (HR, 1.70; 95% CI, 1.30–2.20), and that for ischemic heart disease mortality increased by 108% (HR, 2.08; 95% CI, 1.52–2.85). When the relationship between baseline blood lead and mortality was fitted by five-knot restricted cubic splines to visualize the shape of the dose-response curve, the steepness of the relationship between blood lead and both cardiovascular mortality and ischemic heart disease mortality was steeper in subjects with a baseline blood lead <5 μg/dL than for those baseline blood lead of 5 to 10 μg/dL. In addition, the relationship between baseline blood lead and both cardiovascular mortality and ischemic heart disease mortality was greater in subjects who were younger than 50 years at baseline compared with those who were older. Because lead accumulates in bone with a half-life of years to decades, measurement of lead in bone by noninvasive K x-ray fluorescence (KXRF) offers advantages over blood lead as a biomarker of long-term cumulative lead exposure.19 The Normative Aging Study (NAS) is a multidisciplinary longitudinal study of aging in men begun in 1963 when 2280 healthy men from the Greater Boston area between 21 and 80 years were enrolled. In the 1990s (mean, 1994 ± 3 years), KXRF measurement of lead in bone and blood lead were collected on a subset of active participants.20,21 In the fully adjusted model confined to subjects who were younger than 45 years at the time of NAS study entry (N = 637), HRs for cause-specific mortality through 2007 were assessed by tercile of patella lead concentration. Analyses were adjusted for age at KXRF measurement, smoking, and education, as well as occupation and salary and parental age and occupation at NAS study entry. Inverse probability weighting based on selected health characteristics (such as diastolic blood pressure and body mass index) were applied as an additional adjustment so that the subpopulation who participated in the KXRF measurements were representative of all NAS subjects alive at the time of the KXRF substudy enrollment. Using individuals in the lowest tercile of patella bone lead (<20 μg Pb per gram of bone mineral) as the reference group, subjects in the highest tercile of bone lead (>31 μg/g) exhibited an RR of 2.47 (95% CI, 1.23–4.96) for all cardiovascular mortality and an RR of 5.20 (95% CI, 1.61–16.8) for ischemic heart disease mortality. Blood lead, which averaged ≈5 μg/dL at the time of the KXRF substudy enrollment, was not a predictor of mortality.21 This prospective cohort study demonstrated that bone lead, a biomarker of cumulative lead exposure, was predictive of cardiovascular mortality in individuals who lived a significant proportion of their lives at a time when BLLs of 10 to 25 μg/dL were common. Other epidemiological, clinical, and experimental studies are coherent in establishing lead exposure as a cause of death from cardiovascular disease. At low to moderate dose, lead has been demonstrated to increase blood pressure, alter cardiac conduction, increase vascular reactivity, induce oxidative stress, increase expression of proinflammatory cytokines, and alter endothelial cell function.22 Occupational cohort mortality studies have observed an increased standardized mortality rate for cardiovascular disease in lead smelter workers,23 and a significant relationship between blood lead and cardiovascular mortality within occupational cohorts assembled from large medical surveillance data sets.24,25 ACOEM considers the weight of the evidence supporting the need for revised standards to reduce occupational lead exposure and the BLLs of workers to be among the most conclusive and compelling that has ever existed for a workplace chemical regulated by OSHA. First, a key health end point of demonstrable concern is death, as compared with subtle or subclinical changes in organ function. Second, the abundant epidemiological data that support the causal relationship arise from multiple large, well-controlled, prospective cohort studies—the most rigorous and persuasive epidemiological study design. Third, causal inference is supported not only through multiple high-quality epidemiological studies that have extensively adjusted for confounders and bias, but also by numerous experimental and clinical observations that have identified plausible modes of action at consistent doses. Fourth, because cardiovascular mortality is the most prevalent cause of death in the US population, the more than 50% increase in mortality risk associated with BLLs in the range of 10 to 25 μg/dL results in a marked increase in the absolute number of deaths. Fifth, the magnitude of the cardiovascular mortality risk arising from lead exposure exceeds that of other prominent risk factors, such as smoking, elevated cholesterol, and hypertension, which have been the subject of extensive public health concern. Finally, BLLs of a magnitude linked to cardiovascular mortality remain prevalent in numerous workplaces. ACOEM recommends that OSHA lead standards require a qualified supervising physician to review, interpret, and respond to all BLLs and other data obtained as part of workplace lead biomonitoring and medical surveillance program. As stated earlier, the goal of such a program should be to maintain all BLLs less than 10 μg/dL. Although MRP should be mandated for a single BLL ≥30 μg/dL or two BLLs at 4-week intervals ≥20 μg/dL, the supervising physician should have the independent discretion and authority to order MRP at a lower BLL based on an individual worker's medical history or health status. In revising its lead standards, OSHA should explicitly acknowledge that medical conditions, including chronic real dysfunction (serum creatinine >1.5 mg/dL for men, >1.3 mg/dL for women or proteinuria), hypertension, neurologic disorders, and cognitive dysfunction may pose a risk of material impairment of an employee's health at BLLs that chronically are ≥10 μg/dL. Accordingly, such conditions may indicate the need for physician-ordered MRP at BLLs ≥10 μg/dL or at lower BLLs based on physician discretion.4 In view of some evidence of the adverse impact of low-level prenatal lead exposure on neurodevelopment and reproductive outcome, and the absence of any identifiable BLL threshold for the deleterious effects of postnatal lead exposure on neurocognitive development,22,26 it is advisable for women who are or may become pregnant to avoid occupational lead exposure that would elevate BLL concentrations above the CDC reference value for lead (currently 3.5 μg/dL).27 Consistent with our recommendation that a qualified physician be given the discretion to order MRP or other lead exposure reduction for a worker at any BLL based on the worker's health status or condition, ACOEM further recommends that all workers eligible for biological monitoring (including BLL testing) under a revised OSHA lead standard annually complete a confidential health history form that should be forwarded to a qualified supervising physician for review. This health history form should report on health conditions, including but not limited to all of those cited in the paragraph previously, that may increase the worker's risk of material health impairment from lead exposure. In addition to reporting whether the worker is known to have hypertension, the form should also document an annual measurement of the employee's blood pressure. This health information, together with review of all biological monitoring test results, will assist the physician's determination of the need for ordering discretionary MRP or other reductions in an employee's occupational lead exposure. Physicians should be empowered to request confidential consultation, including physical examination, of any employee who completes a health history form. Employees should have the option of supplementing or revising the information provided on the annual health history form at any time for further physician review based on changes in their health status (including reproductive status). ACOEM believes that the workplace lead exposure conditions acceptable for a worker undergoing MRP should be more protective than mere removal from work involving airborne exposure to lead at or above the AL, which is 30 μg/m3 under the current OSHA lead standards. As further explained in ACOEM's responses to OSHA's questions regarding requirements for blood lead testing and revised AL and PEL, ACOEM believes that substantial lead exposure may occur in certain job conditions or activities not subject to exceedance of an airborne AL, and that the PEL and AL should be reduced. Consistent with the recommendations of Kosnett et al,4 ACOEM believes that “removal from occupational lead exposure will usually require transfer of the individual out of any environment or task that might be expected to raise the blood lead concentration of a person not using personal protective equipment above background levels,” currently approximately 3.5 μg/dL at the 97.5th percentile. The suitability of alternative job tasks or locations at a workplace for an employee undergoing MRP could be discerned in part by the pattern of blood lead concentrations documented in other workers who engage in candidate alternative tasks or work in other locations without the use of PPE. In addition, consistent with the Cal/OSHA 2016 discussion draft lead standards pertaining to “temporary removal due to elevated blood lead levels,” MRP should result in a removal of an employee “from work having an exposure to lead at or above the AL, and from work altering or disturbing any material containing lead at a concentration equal to or greater than 0.5% by weight,”28 and additionally from work that constitutes any trigger task defined in the Cal/OSHA discussion draft for Construction.29 The rationale for these stringent criteria pertaining to suitable alternative work during MRP is to facilitate relatively rapid reduction in BLL and to reduce to the greatest extent feasible further hazardous exposure to lead. Although the stated goal to maintain workers' BLL less than 10 μg/dL (or <3.5 μg/dL in the case of women who are or may become pregnant) is supported by extensive scientific evidence, the evidence does not establish that these same BLLs constitute thresholds below which adverse effects are unlikely to occur. Based on contemporary medical knowledge, these BLL values provide at best a slim margin of safety. It is possible that future epidemiological studies of adult populations who have seldom or never sustained BLLs greater than 10 μg/dL will detect a lead-related risk of cardiovascular morbidity and mortality, as well as other significant health effects associated with long-term exposure. The emergence of such findings will require further protective policies and regulations. A principle of regulatory toxicology and public health holds that standards governing hazardous exposures should offer a margin of safety below levels that pose a significant adverse risk to health. In issuing its 2016 final rule on occupational exposure to respirable crystalline silica, OSHA explicitly noted that it “may incorporate a margin of safety even if it theoretically regulates below the lower limit of significant risk.”30 In view of this, it would be reasonable for OSHA to enact occupational lead standards whose goal is to maintain all worker blood lead concentrations less than 5 μg/dL, or less than the CDC reference value in the case of workers who are or may become pregnant. ACOEM believes that after MRP for an elevated blood lead concentration, return to work should be considered, based on a physician's review of the worker's health and work status, when BLL has declined to less than 15 μg/dL. BLLs assessed for the purpose of return-to-work decisions after MRP should be measured at monthly intervals. ACOEM believes that considering intraindividual and laboratory variability, a return-to-work BLL of 15 μg/dL will usually reflect a true reduction the worker's BLL. As noted in ACOEM's response to OSHA's question on MRP trigger BLLs, the physician who supervises workplace biological monitoring and medical surveillance for the worker should have the discretion, on a case-by-case basis, to continue MRP until a worker's BLL has declined to a value less than 15 μg/dL as necessary to avoid material impairment to a worker's health from lead exposure. This will include consideration of a worker's reproductive status and their ability to procreate a healthy child. In addition, ACOEM believes that return of a worker after MRP should be predicated on the supervising physician's assessment that the conditions that resulted in MRP, including but not limited to lead exposure resulting in a BLL that may have triggered MRP, are unlikely to resume. Consistent with provisions proposed in the draft lead standards of Cal/OSHA (and a similar requirement proposed by the Washington state Division of Occupational Health and Safety),31 employers should be required to issue a “written elevated blood lead level response plan with a description of specific means that will be employed to reduce and maintain employee blood lead levels below 10 μg/dL.”28 The physician's decision to authorize return to work after MRP may be based, in part, on a review of this written plan and communication with the employer, the employer's industrial hygiene, safety, or engineering consultants, and/or the employee to assess the suitability of the response plan and its implementation. II. The AL and PEL for lead in workplace air should be reduced from 30 and 50 μg/m3 (as an 8-hour TWA average) to 2 and 10 μg/m3, respectively. ACOEM believes that effective worker health protection requires a reduction of the current lead standards' PEL and AL level for lead. Numerous epidemiological studies have demonstrated a significant positive relationship between airborne lead concentration and workers' blood lead concentrations. Biokinetic models based on the toxicokinetics of lead have used epidemiological data as a means of calibrating and confirming the models.32 Recently, two independently developed biokinetic models, the Leggett+ model and the DoD-O'Flaherty model, have been published to assist regulators in the identification of airborne occupational exposure limits (ie, permissible exposure limits) for lead that would maintain workers' blood lead concentration below various thresholds. The Leggett+ model,33,34 published by the California Office of Environmental Health Hazard Evaluation at the request of the California Department of Health, estimated various concentrations of lead in workplace air inhaled by workers without respiratory protection that could result in specified lead concentrations in workers' blood. Assuming 40 years of working life exposure beginning at age 25 years and a background blood lead concentration of 1.5 μg/dL, the model predicted that an 8-hour TWA PEL of 2.1 μg/m3 would result in a 95th percentile BLL of 10 μg/dL. A PEL of 10.4 μg/m3 would yield a 95th percentile BLL of 30 μg/dL. The DoD-O'Flaherty biokinetic model,35,36 based on a model first developed by O'Flaherty, differed from the Leggett+ model by relying on somewhat different physiological assumptions and by considering the impact of a worker's birth year and accumulated lifetime lead exposure to assess the impact of adult workplace lead exposure. It predicted the blood lead distribution that would have existed in the DoD workforce on January 1, 2018, had the current DoD workforce, composed of men and women of various ages, been historically exposed full time to specified levels of airborne lead since they were 18 years of age. The DoD-O'Flaherty model related various candidate occupational exposure levels to corresponding predictions of the 95th percentile BLL. It predicted that a PEL of 3.6 μg/m3 would yield a 95th percentile blood lead concentration of 10 μg/dL.35 In a formal review of the DoD-O'Flaherty model, the National Academies of Science, Engineering, and Medicine found that the Leggett+ model and DoD-O'Flaherty model “described available BLLs with similar accuracy…. The consistency of simulated BLLs between the DoD-O'Flaherty model and Leggett+ model provided additional evidence of the reasonableness of the model inputs and assumptions.”32 The California Department of Public Health has recommended that Cal/OSHA adopt a PEL of either 0.5 or 2.1 μg/m3 to maintain workers' BLL less than 10 μg/dL.5 The Cal/OSHA discussion draft of 2016 proposed a PEL of 10 μg/m3 and an AL of 2 μg/m3. In establishing requirements for (1) basic hygiene requirements for all workers with occupational lead exposure; (2) medical surveillance requirements (including blood lead measurements) that would be required if an AL of 2 μg/m3 were exceeded for ≥10 days per year, or irrespective of air measurements if a “trigger amount of lead work” were performed; (3) exposure monitoring at least every 12 months if the ALs were exceeded, with more frequent monitoring dependent on the magnitude exposure; and (4) a written plan for investigation and deficiency correction in the case of a blood lead ≥10 μg/dL, Cal/OSHA's discussion draft standard evinced a commitment to the goal of keeping blood lead concentrations less than 10 μg/dL for all lead workers. However, by establishing a PEL of 10 μg/m3, Cal/OSHA sought to provide employers with flexibility in the approach to maintaining all workers' BLLs below this value. As noted in discussions held before the Cal/OSHA Advisory Meetings for Revision of the General Industry and Construction Lead Standards,28 it was recognized that the selection of PEL carries with it an implication for work that can be performed with various types of respirators based on their respective protection factors. For example, abrasive blasting of lead-coated surfaces (such as steel bridges covered with lead paint) is customarily performed by workers inside enclosures equipped with supplied air respirators with protection factors of 1000. If the PEL were 2 μg/m3, this work could be performed with a supplied air respirator only if the exposure within the enclosure were less than 2000 μg/m3. According to some stakeholder comments at meetings of the Cal/OSHA Advisory Meetings, air levels inside enclosures during abrasive blasting may sometimes be in the range of 5000 μg/m3. The selection of a PEL of 10 would allow supplied air respirators to be used inside an enclosure provided the exposure were less than 10,000 μg/m3. In similar manner, full-face respirators have a protection factor of 50. A PEL of 10 μg/m3 would allow employers to use full-face respirators in environments where air levels extended up to 500 μg/m3, instead of up to 100 μg/m3 as would be the case with a PEL of 2 μg/m3. A Standardized Regulatory Impact Assessment (SRIA) issued by the California Department of Industrial Relations in 2020 found that Cal/OSHA's proposed revisions to the occupational lead standards were feasible and were associated with a large benefit-cost ratio.37 CDIR concluded, “As the full, long-term benefits of the proposed regulatory revisions are realized, the annual benefit-cost ratios for this regulation are quite high and sustained, with benefits expected be substantially larger than compliance costs. However, compliance costs begin to accrue immediately while the health benefits manifest themselves over time.37 The estimated aggregate breakeven point under the assumptions of this assessment would occur approximately within the first 7 years after the proposed revisions come into effect. It should also be recalled that the benefit estimates used in this study are not comprehensive and that total benefits are expected to be substantially higher.”37 According to the SRIA, if the revised lead standards were introduced in 2020, the net financial benefit by 2040 would be approximately $3,800,000,000.00 (3.8 billion dollars) unadjusted for inflation.37 ACOEM considers the Cal/OSHA proposal for a PEL of 10 μg/m3 and an AL of 2 μg/m3 to be feasible and health protective when combined with the additional provisions set forth in proposed Cal/OSHA discussion draft standards. ACOEM urges OSHA to revise its federal OSHA lead standards to include a PEL and AL at least as stringent (ie, health protective) as the Cal/OSHA proposal. III. Instead of exclusive reliance on measurement of lead in workplace air, performance of a “trigger amount of lead work” based on the duration of time spent altering or disturbing materials that contain lead at a concentration ≥0.5% by weight should be a criterion for initiation of blood lead