Concentrations of mercury, lead, copper and selenium were consistent with recently reported concentrations among non-Arctic North American and European populations (Bjornberg et al., 2003
; Devereux et al., 2007
; Jedrychowski et al., 2007
; Jones et al., 2010
; Koppen et al., 2009
; Lederman et al., 2008; Lorenzo Alonso et al., 2005
; Morrissette et al., 2004
; Osman et al., 2000
; Palkovicova et al., 2008
; Perveen et al., 2002
; Rhainds et al., 1999
; Schell et al., 2003
; Schulpis et al., 2004
; Takser et al., 2005
; Vahter et al., 2000
). However, there were individuals who had cord blood levels of mercury and lead in ranges associated with neurodevelopmental and other adverse health outcomes.
While average mercury levels in this study population are within North American and European population ranges, it is a concern that five infants had total mercury measurements higher than 5.8 μg/L, a concentration corresponding to the United States Environmental Protection Agency’s reference dose for methyl mercury (US Environmental Protection Agency, 2001
). The reference dose is an estimate, including an uncertainty factor, of daily exposure to a population that is likely to be without an appreciable risk of deleterious effects over the course of a lifetime. One recent report suggests that delays in neurocognitive performance may result from even lower mercury concentrations (Jedrychowski et al., 2006
). Studies of populations in the Arctic, Asian and Amazonia have consistently reported mercury cord blood measurements above the level of the methyl mercury reference dose (Butler Walker et al., 2006
; Fok et al., 2007
; Santos et al., 2007
). Although there were relatively few Asians in this study population, we found higher mercury levels among Asian infants. This is consistent with prior observations that people who eat more fish and shellfish generally have higher mercury levels (Tsuchiya et al., 2008
), that Asian American women in the US, particularly those who were foreign-born, have babies with substantially higher mercury levels in umbilical cord blood compared to caucasian women (Lederman, et al., 2005), and that generally there are higher rates of seafood consumption among Asian Americans (Sechena et al., 2003
). Based on one of the author’s (FW) knowledge of women giving birth at this particular hospital, it is possible that several Asian mothers in this population were foreign-born.
Lead concentrations were lower than earlier reports, consistent with previous observations that blood lead levels are generally decreasing over time (Muntner et al., 2005
). However, two infants had cord blood lead levels over five μg/dL, a level that probably confers neurodevelopmental risks (Lanphear et al., 2005
). Not surprisingly, increased blood lead levels were associated with residence in neighborhoods with older housing stock. The higher lead levels observed among Asians are consistent with studies in the United States that have reported higher lead levels among Asian immigrants (Rastogi et al., 2007
). Such lead levels may have resulted from maternal lead exposures that occurred prior to the mother’s immigration to the United States, and subsequent release of long-term lead stores from bone during pregnancy (Gulson et al., 1997
). The higher levels of lead found among infants of mothers with Medicaid are consistent with prior work identifying two factors associated with lower socioeconomic status and lead exposure: poorer nutritional status, including less dietary iron and calcium, and living and/or working in environments with higher lead exposure levels (Bernard and McGeehin, 2003
; Muntner et al., 2005
; Rastogi et al., 2007
Some infants in our study had relatively high cord serum concentrations of selenium and copper. Given that selenium and copper were strongly correlated and had similar patterns of association with demographic characteristics, it is possible that they co-occur in foods and/or environmental sources. Both copper and selenium levels were found to increase over the course of gestation; this was statistically significant for copper and close to statistical significance for selenium. This is consistent with prior studies (Galinier et al., 2005
; Grandjean et al., 1992
; Makhoul et al., 2004
; Perveen et al., 2002
). The relationships with gestational age suggest increased placental transfer over the course of gestation, perhaps to accommodate nutritional and developmental needs of the fetus. Therefore it is possible that the observed association between selenium and copper probably at least in part is related to increased transplacental transfer of copper and selenium with gestational age.
Unexpectedly, we observed a relationship between umbilical cord serum selenium and with older age of housing in neighborhood. Selenium intake is thought to almost exclusively arise from dietary sources. However, selenium is a component of paint, paint in older housing is more likely to be in poor condition, and mothers could theoretically be exposed to paint residues in such circumstances; so that it is possible that selenium in house dust could be contributing to maternal levels. On the other hand, post-hoc analyses indicate that older age of neighborhood housing is significantly related to other proxy measurements of lower socioeconomic status (data not shown), so it is possible that other factors, such as differential dietary levels of selenium, explain this finding.
Fish consumption is a source of mercury, selenium and n-3 fatty acids. If fish consumption was a common and predominant source for all three compounds, one would expect selenium, mercury and DHA+EPA to be strongly correlated with each other. However, while DHA+EPA was significantly correlated with both mercury and selenium, mercury and selenium were not correlated with each other. There are a few possible explanations for this finding. First, it is possible that there are sources of selenium in addition to or instead of fish. Also, Bjornberg and colleagues found that dietary fish intake was related to methyl mercury but not selenium in cord blood samples among Swedes (Bjornberg et al., 2003
). Alternative dietary sources of selenium include wheat (Lyons et al., 2005
) and meat and dairy products (Navarro-Alarcon and Cabrera-Vique, 2008
). Second, it is possible that this relationship is not observable within umbilical cord serum. Chen and colleagues found a correlation of selenium and mercury in urine samples, but not in concurrent serum samples (Chen et al., 2006
One limitation of this study is that we were unable to collect biological specimens from the mothers. Future studies should examine maternal as well as fetal serum levels. Further, it would have been preferable to have additional information about potential exposure sources for these elements e.g., dietary consumption of fish and shellfish, house dust levels of lead and selenium, and additional measures of nutritional status, especially iron, calcium and zinc.
This study provides useful information about umbilical cord blood measures of mercury, lead, selenium and copper among urban newborns. This is particularly useful for selenium and copper, for which few reference values are available. Gestational age and birthweight are related to higher concentrations of the essential elements selenium and copper, perhaps pointing to an additional benefit in reducing preterm birth. These results demonstrate that social characteristics such as maternal race and insurance are correlated with disparate umbilical cord concentrations of mercury, lead, copper and selenium. While mercury and lead concentrations generally were below stated guidelines for exposure, there were several individuals who had higher levels. Despite the progress in reducing lead exposures in relation to deteriorating lead-based paint in the US there still is evidence that older housing is contributing to blood lead levels. In this population, Asian and African American mothers may need to be targeted for future intervention efforts. Infants born to smoking mothers had higher lead levels, yet another reason to quit smoking, especially prior to pregnancy.
No undisputedly safe levels for lead and mercury have been identified and very little is known about toxicity of early life exposures to excess levels of selenium and copper or the impacts of subnormal levels during gestation. As a result, there may still be measurable detrimental health effects even at lower concentration levels, pointing the need for more research on health effects of low-level exposures to mercury, lead, copper and selenium.