Our results show a persistent decline in BLLs in the US from 1999-2000 to 2015-2016 irrespective of age, gender, ethnicity, and poverty-income ratio. The decline is consistent with a number of previous population-based studies [22–28], including our previous study . This has once again demonstrated the effectiveness of public health efforts to abate environmental exposure to lead in the US. Nevertheless, the rate of decline has been diminishing in recent years, which may indicate that there are other environmental lead sources yet to be identified and controlled.
Despite the substantial decline in BLLs, disparities between different income and ethnic groups remain. From our analysis, we found in NHANES 2015-2016, for the first time, that there was no longer a disparity between high- and low-income families in the proportion of children aged 1 to 5 years with EBLLs. This is a very encouraging finding. Lead affects neurocognitive development in children and BLL may partly lead to a poverty trap in which children from poor families will remain poor when they grow up. The elimination of income disparity in EBLLs is likely to be a result of targeted screening in high-risk children. Until 2014, black children (below age of 18) were twice as likely to live in poverty as white children . Black children in poverty are also more likely to live in substandard housing and hence, at a higher risk of exposure to lead paint at home and lead pollutants from nearby factories [31, 32]. The racial and income disparity has been historically important in blood lead monitoring. In 1982, Mahaffey and colleagues first reported higher BLLs in young children who were black and from low-income families in a national estimate (NHANES II, 1976-1980) . Since then, the observation was confirmed in numerous studies and the disparities persisted over the past four decades [22–4, 27, 33–36]. In 2013-2014, the mean BLLs in black children was still 36% higher than in white children . Children at the age of 1-5 years that were non-Hispanic black, from a low-income family and living in housing built before 1950 were in the highest risk groups for lead poisoning [24, 34, 35]. The black-white disparity in higher BLLs has been found to be independent of income level, housing quality and environmental conditions . Targeted prevention strategies have been implemented such as increased screening on these high-risk groups and identification of high-risk environment . the percentage of non-Hispanic black children with EBLL is still higher compared to other ethnicities, and so more efforts to reduce the environmental exposure to lead, particularly by removing lead paint and lead plumbing in houses, are necessary.
The proportion of EBLL among children aged 1-5 years in NHANES 2015-2016 increased numerically, albeit insignificantly. This should be interpreted with caution. 30.9% of these participants had missing blood lead measurements. The sample size was 790 and only 12 children had EBLL. Caldwell et al.  reported an estimated 35% of lot screening failures due to lead contamination, which could falsely elevate BLL in the NHANES 2015-2016 cycle. However, a study based on a large national clinical laboratory database  reported a slight increase in the rate of children <6 years old with EBLLs in 2014-2015 after consecutive years of decline. Our analysis of NHANES 2015-2016 data does not dispel this disturbing finding. These may just be random fluctuations along an overall decreasing trend, so the next two-year NHANES cycle would clarify the underlying trend better. Continual monitoring of BLL is thus as critical as ever. However, as BLL in the general population and in healthy people becomes lower and lower, advances in analytical methods are needed. This is a reason why the CDC is cautious about lowering the reference level of BLL further to a level where measurements become inaccurate.
Since the well-publicized crisis of lead in drinking water in Flint, Michigan, there have been increased awareness and testing of lead level in drinking water. Recently in Vermont, elevated tap water lead level was found. This is potentially harmful to children. Children absorb 40-50% of water-soluble lead40. Every increase in 1 ppb in water lead level increases 35% in BLL41. The effect of water lead level on BLL is well demonstrated in Flint, Michigan. The switching of water source caused a significant increase in EBLL in children42,43. The switching back for the source of tap water reduced the BLL44, 45.
Although we analyzed the same dataset from which US reference levels of BLL are derived, there are limitations to our analysis. There were high rates of missing blood lead measurements particularly among children aged 1-5 years in NHANES 1999-2016 (ranging from 24-38%), which may affect the representativeness of the survey to estimate the BLL of children in the population. While the sampling in US NHANES was random, not all Americans could be included in the sampling frame. Institutionalized, people without fixed addresses and people who refused were not included. As with any analysis of subgroups, power is reduced and confidence intervals are wider, and multiple comparisons are possible. Our new findings in NHANES 2015-2016 generate a new hypothesis that requires confirmation in future cycles of NHANES and other national estimates of BLL.
In conclusion, our latest analysis of BLL in US NHANES showed that BLL continued to decline overall in the US population. The disparity in BLL in children in high and low- income households has diminished. Black children still have higher BLLs than white children. In young children aged 1 to 5 years in the 2015-2016 survey, BLL did not decline and appeared to increase. Our data suggest that monitoring the trend in BLL in the population is as necessary as ever and that efforts to reduce environmental exposure to lead must not be relaxed.