Blood Lead Concentrations in Marine Mammals Validate Estimates of 102- to 103-fold Increase in Human Blood Lead Concentrations

ENVIRONMENTAL RESEARCH, SECTION A ARTICLE NO.

78, 134—139 (1998)

ER973809

Blood Lead Concentrations in Marine Mammals Validate Estimates of 102- to 103-fold Increase in Human Blood Lead Concentrations1 Benjamin D. Owen* and A. Russell Flegal*Institute of Marine Sciences and -WIGS, Environmental Toxicology, University of California, Santa Cruz, California 95064 Received November 4, 1997

Comparison of lead concentrations in contemporary and preindustrial human bones has shown that lead burdens in contemporary humans are orders of magnitude above natural levels (Flegal and Smith, 1992). However, the extent to which increased environmental exposure has affected human blood lead (PbB) concentrations can only be estimated, as blood samples from preindustrial humans do not exist. Preliminary estimates have placed preindustrial human PbB levels from 0.12 lg/dL (Mushak, 1993) to as low as 0.016 lg/dL (Flegal and Smith, 1992). Those levels are 20 and 200 times lower than the average PbB level (mean$SD"2.8$1.4 lg/dL) in contemporary Americans (Brody et al., 1994). Again, there have not been any previously reported measurements of comparable PbB concentrations in any organisms which would substantiate those estimates. Consequently, blood lead concentrations from Northern elephant seals (Mirounga angustirostris) were analyzed as analogs of the natural PbB levels in preindustrial humans. Northern elephant seals were chosen for this study because it was determined that their environmental exposure to lead would approach that of preindustrial humans (Patterson, 1965; Mushak, 1993; Flegal and Smith, 1992). That determination was based on their unique physiological and behavioral adaptations (Le Boeuf and Laws, 1994), which insulate them from exposure to terrestrial lead contamination while spending several months of the year on land. Their seasonally terrestrial residence also facilitated a test of the hypothesis that the PbB levels of a mammalian population with limited exposure to lead contamination would be markedly lower than PbB levels of modern human populations. Northern elephant seals are exposed to extremely low levels of lead in the environment because their dietary exposure to terrestrial lead contamination is minimal. These animals fast entirely when they

Measurements of ultra-low ambient blood lead (PbB) concentrations (mean6SD50.1360.06 lg/dL) in Northern elephant seals (Mirounga angustirostris) validate previous estimates of ultra-low PbB levels in preindustrial humans. These estimates had been unsubstantiated, since PbB levels in this range had never been measured in any organisms prior to this study. Similarities in PbB levels among these contemporary and preindustrial mammals are consistent with similarities in their measured and estimated lead exposures, respectively. The marginally higher PbB levels and rates of lead exposure in contemporary marine mammals are, also, consistent with lead isotopic composition analyses that indicate their PbB levels have been elevated from exposure to industrial lead. Consequently, these analyses substantiate concerns that current baseline PbB levels in humans, which are estimated to be two to three orders of magnitude above natural levels, may still constitute public health risks. ( 1998 Academic Press

Key Words: lead; blood lead; stable lead isotopes; marine mammals; M. angustirostris.

INTRODUCTION

Over the past 5000 years, the terrestrial environment has been extensively contaminated with lead from anthropogenic sources (Patterson, 1965). 1

This research was supported by a grant from the University of California Toxic Substances Research and Teaching Program and a scholarship from the Friends of the Long Marine Laboratory. We are pleased to acknowledge the advice and assistance of Don Smith, Dan Sampson, and others associated with Environmental Toxicology and the Institute of Marine Sciences at UCSC. Sample collection was conducted in accordance with national and institutional guidelines for the protection of animals. Samples were collected for this project in conjunction with an ongoing physiology study, so that no additional restraint or invasive procedures were required. 134 0013-9351/98 $25.00 Copyright ( 1998 by Academic Press All rights of reproduction in any form reserved.

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PbB IN MARINE MAMMALS AND HUMANS

haul out on shore to breed and to molt (Costa et al., 1986; Worthy et al., 1992), foraging most of the year in the relatively pristine environment of the Northeast Pacific (Le Boeuf et al., 1993; Flegal and Patterson, 1983). As high trophic level predators, their dietary intake of lead is also depleted relative to their intake of calcium (Michaels and Flegal, 1990). For example, we calculate that adult females consume \6 lg Pb/day (0.02 lg Pb/kg body wt/day) when feeding at sea, based on the lead concentration of pelagic fish from the Northeast Pacific (Settle and Patterson, 1980) and a dietary intake of 20 kg/day for a 320-kg female seal (Le Boeuf et al., 1993). This calculated lead intake is comparable with Mushak’s (1993) estimate of the dietary intake of 0.37 to 0.68 lg Pb/day (0.005 to 0.01 lg Pb/kg body wt/day) for preindustrial humans. Two other major pathways (i.e., the consumption of water and respiration) of lead exposure in humans (NRC, 1993) are considered to be relatively inconsequential in N. elephant seals. Their intake of lead from drinking water is negligible, as N. elephant seals do not consume water (Ortiz et al., 1978). Similarly, their intake of lead aerosols is considered negligible, based on their relatively remote location and calculations using the U.S. EPA Integrated Exposure Uptake Biokinetic Model for Lead in Children (IEUBK), Version 0.99D (NTIS No. PB9450517). There is a possibility that pica may be a relatively important pathway for lead exposure in marine mammals, as it is in humans (NRC, 1993), because of their behavior on land. Since the seals return to the same rookery year after year (Condit and Le Boeuf, 1984; Le Boeuf et al., 1993), the isotopic composition of lead in sediments at the rookery should be comparable to that in their blood if pica were a major source of lead exposure for them. Therefore, acidlabile stable lead isotopic compositions (PbIC) of sediments at the rookery were determined for comparisons with the PbIC of the mammals. Their blood samples were collected from nursing seal pups because it was believed that the lead in the pups’ blood would represent a time-integrated sample of their mothers’ cumulative lead exposure. Lead is cycled through nutrient pathways as a biochemical analog of calcium (Elias et al., 1982). Approximately 90% of the lead in the mammmalian body is sequestered in the skeleton (O’Flaherty, 1993), where it has a relatively long biological halflife: t1/2[5—10 years for cortical bone, [1 year for trabecular bone, and (1 month for blood (Smith et al., 1996). Some of that lead is released from the skeleton during pregnancy and transferred, along

with lead in soft tissues, through the placenta to the fetus (Goyer, 1996). Lead is also transferred to infants via postpartum lactation, which occurs in N. elephant seals while the mother is fasting (Patterson-Buckendahl et al., 1994; Costa et al., 1986). Consequently, the PbB of a seal pup should represent an integration of the mother’s lifetime lead exposure (Patterson-Buckendahl et al., 1994; Smith et al., 1996). MATERIALS AND METHODS

Samples were collected from the N. elephant seal rookery at An8 o Nuevo State Reserve (San Mateo, CA) during the 1994—1995 breeding season. Replicate blood samples (\5 mL each) were collected from four nursing pups. Blood samples were collected in triplicate when possible, with at least two blood samples collected at each sampling point. Sediment samples (n"5) of beach sand were also collected from that site. All samples were digested and analyzed using established trace-metal clean techniques (Patterson and Settle, 1976; NRC, 1993). Blood samples were analyzed for lead concentrations ([Pb]) on a Finnigan MAT ELEMENT magnetic sector inductively coupled plasma source mass spectrometer (ICP-MS) with a CETAC U-5000AT ultrasonic nebulizer. All standards and samples analyzed by ICP-MS were internally standardized with 1.0 ppb bismuth. Procedural accuracy was verified by concurrent digestion and analysis of NIST standard reference materials (SRMs). Recoveries of lead (n"4) from NIST SRM 955a (lead in blood) were all within the certified range of values ($2% RSD). Procedural blank concentrations were below the analytical lead detection limit of 0.015 lg/dL (0.15 ppb). Following the PbB analyses, digested blood samples were processed with an anion-exchange column separation and analyzed for their stable lead isotopic composition (PbIC) on a VG Sector 54/30 thermal ionization mass spectrometer (TIMS) using established techniques (Smith et al., 1996). Sediment samples were leached in 0.5 N HCl at room temperature for 24 h, to simulate the acidic conditions within a seal stomach, and the PbIC of the acid-labile eluate was measured by ICP-MS. Fractionation corrections for all PbIC analyses were derived from concurrent analyses of NIST SRM 981 (common lead). RESULTS

The average (mean$SD, n"4) PbB concentration of the N. elephant seal pups was 0.13$0.06

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TABLE 1 Blood Lead (PbB) Concentrations and Isotopic Compositions (Means62 SE) in Blood Samples from N. Elephant Seals (M. angustirostris) from An8 o Nuevo, CA. Lead Isotopic Compositions of Sediments, Sea Otter Teeth, Aerosols, and Upwelling Seawater from that Area are Included for Comparison Isotopic compositions PbB (lg/dL) N. elephat seals 1 2 3 4

206

Pb/ 204Pb

208

Pb/ 206Pb

207

Pb/ 206Pb

0.075 0.21 0.12 0.10

18.836 (0.013) 18.956 (0.037) 18.851 (0.034) 18.832 (0.015)

2.046 (0.0002) 2.036 (0.0026) 2.057 (0.0002) 2.053 (0.0002)

0.830 0.825 0.832 0.830

(0.0001) (0.0008) (0.0001) (0.0001)

An8 o Nuevo sediments (n"5)

—

19.056 (0.013)

2.036 (0.0010)

0.8218 (0.0003)

Sea otter teeth Preindustrial (n"6)a Contemporary (n"6)a,b

— —

19.144 (0.014) 18.668 (0.041)

2.039 (0.0009) 2.056 (0.0014)

0.8204 (0.0008) 0.8360 (0.0012)

Aerosols Southern California (n"5)c Northern California (n"6)a,d

— —

18.887 (0.136) 18.601 (0.012)

2.034 (0.0172) 2.055 (0.0013)

0.8285 (0.0017) 0.8403 (0.0005)

Upwelling seawater (n"6)e

—

18.237 (0.038)

2.089 (0.0035)

0.8547 (0.0015)

a

Smith et al. (1992). One outlier, detailed in Smith et al. (1992), was excluded. c San8 udo—Wilhelmy and Flegal (1994) d Smith et al. (1996). e Flegal et al. (1989). These data include all samples collected at 5100 m depth. b

lg/dL (6.3$2.9 nM), with a range from 0.071 to 0.21 lg/dL (Table 1). This average concentration is more than 20 times lower than the current average PbB level of the US population (Brody et al., 1994). As illustrated in Fig. 1, the PbB levels in these contemporary marine mammals with relatively low environmental lead exposure approach estimates of PbB levels in preindustrial humans with lower environmental lead exposure (Mushak, 1993; Flegal and Smith, 1992). Moreover, complementary PbIC data (Table 1) indicate that N. elephant seals are still contaminated with some industrial lead. This is evidenced by the similarities (Fig. 2) in the PbIC of the seals at An8 o Nuevo and industrial lead aerosols in California (Smith et al., 1992, 1996; San8 udo-Wilhelmy and Flegal, 1994). While a causal relationship cannot be proven with these few initial measurements, the isotopic similarities indicate that U.S. industrial emissions are the primary source of lead exposure for the seals, in spite of their relative freedom from terrestrial lead contamination. DISCUSSION

These measurements of ultra-low PbB levels in N. elephant seals corroborate estimates of the magni-

tude of lead contamination in humans. This is illustrated in Fig. 1, which shows the average PbB in the seals (0.13 lg/dL), preindustrial humans (0.016 lg/dL), the U.S. population (2.8 lg/dL), and the most recent Centers for Disease Control’s (CDC, 1992) action level (10 lg/dL). This comparison shows that while the average PbB level of contemporary Americans is more than 20 times higher than the PbB levels measured in the seals, it is only 4 times lower than the level now considered by the CDC as potentially toxic to children. While PbB levels in American children have been decreasing with the elimination of lead additives in U.S. gasolines and other reductions in environmental lead exposure (Pirkle et al., 1994), the recognized clinical threshold of lead toxicity has also been decreasing (Smith and Flegal, 1992). As several recent studies have shown, the correlation between increased lead exposure and increased risk is continuous at the lowest distinguishable levels of exposure to lead for many chronic symptoms of lead poisoning (Hu et al., 1996; Kim et al., 1996; Schwartz, 1994). Consequently, there still is no established threshold for lead toxicity in humans (NRC, 1993; Silbergeld, 1990; Schwartz, 1994). While the PbB levels measured in the N. elephant seals are within the range predicted by estimates of

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FIG. 1. Comparison of PbB levels. A comparison of the average blood lead (PbB) concentration measured in N. elephant seals (M. angustirostris) with the most recent CDC action level (CDC, 1992), or the PbB level defined by the Centers for Disease Control as potentially toxic to children, the average PbB level of the US population (Brody et al., 1994), and Flegal and Smith’s (1992) estimate of natural PbB levels in preindustrial humans.

natural PbB levels in preindustrial humans, PbIC analyses indicate that the natural PbB levels in the seals are even lower (Fig. 2). In addition to the previously noted similarities in the PbIC of the seals and industrial aerosols, the range in PbIC of contemporary seals is intermediate to those of contemporary and preindustrial California sea otters (Enhydra lutris) whose ranges overlap (Condit and Le Boeuf, 1984). Differences between the latter are attributed to industrial lead in contemporary otters, based on the increase in the atomic ratios of lead/ calcium atomic ratios in their teeth relative to those of preindustrial otters (Smith et al., 1992). The apparent increase in lead in contemporary seals is also supported by similarities in the PbIC of preindustrial otters and An8 o Nuevo sediments, which contrast with those of contemporary otters and seals at the site. While there are no comparable PbIC measurements of preindustrial seals, these comparisons indicate that contemporary seals are contaminated with industrial lead, albeit to a lesser degree than the contemporary otters. Finally, the apparent variation in contamination between the two species is consistent with differences in their feeding habits, since sea otters forage benthically in

relatively contaminated coastal habitats (Smith et al., 1992) and female N. elephant seals forage in relatively uncontaminated pelagic waters (Le Boeuf et al., 1993).

CONCLUSIONS

Measurements of PbB concentration and PbIC in blood samples collected from N. elephant seals corroborate previous estimates of natural PbB concentrations in humans. The measurements demonstrate that marine mammals with relatively low environmental lead exposure have PbB levels within the predicted range of preindustrial humans with similarly low environmental lead exposure. These estimates, which range from \20 to 200 times lower than current baseline levels, in the U.S. popuation indicate that PbB levels in contemporary humans have been elevated two to three orders of magnitude above natural levels. Validation of these estimates substantiates concerns that hundreds of thousands of children in the U.S. may be at risk due to subclinical lead toxicity (Brody et al., 1994), despite recent declines in children’s PbB levels (Pirkle et al., 1994).

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FIG. 2. Comparison of stable lead isotopic compositions. (Top) A 204Pb-normalized plot of 206Pb vs 208Pb. (Bottom) A 206Pb-normalized plot of 207Pb vs 208Pb. Both plots compare the isotopic composition of lead (PbIC) in blood samples collected from N. elephant seals (M. angustirostris) with sediments (n"5) from the N. elephant seal rookery at An8 o Nuevo, California, preindustrial (n"6) and contemporary (n"6) California sea otter (E. lutris) teeth (Smith et al., 1992), aerosols collected in southern California (San8 udo-Wilhelmy and Flegal, 1994) and northern California (Smith et al., 1992, Smith et al., 1996), and upwelling seawater (n"6; 5100 m depth) collected off the coast of northern California (Flegal et al., 1989). Both plots show that lead in blood collected from elephant seals is isotopically similar to industrial lead aerosols collected throughout California and isotopically intermediate between lead in contemporary and preindustrial sea otters from the same area, suggesting that these seals are exposed to U.S. industrial lead in the pelagic environment. Lead leached from An8 o Nuevo sediments is isotopically similar to the lead to which preindustrial otters were exposed, suggesting that natural sources of lead are the same for the two species.

Based on the pronounced increase in human PbB levels above natural levels and on the absence of an established threshold for lead toxicity, it may be that current PbB regulatory levels are still inadequate.

Therefore, identifying and treating the effects of subclinical lead toxicity should remain priorities of public health policy and research into the next century.

PbB IN MARINE MAMMALS AND HUMANS

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