A new study has suggested that the significance of low-level environmental lead exposure on cardiovascular disease, in the USA, is greater than previously thought[i].
Lead exposure is a known risk factor for cardiovascular disease, and numerous other acute and chronic illnesses. Historically, the main sources of lead exposure were lead-based paint, leaded petrol and lead pipes. However, lead-based paint and petrol are now banned, and exposure to lead has declined significantly. Still, heavy ground contamination is a persistent source of lead exposure in industrialised countries[ii]. Moreover, it has recently emerged that recycling of computers is a source of lead exposure among workers involved in this process.
It is not clear whether concentrations of lead in blood lower than 5 mg/dL, which is the current action level for adults in the USA, are associated with all causes of death, or specifically cardiovascular disease mortality.
The new study used data from the Third National Health and Nutrition Examination Survey between 1988 and 1994. The participants underwent medical examinations and were interviewed to give information about their health, lifestyle, and exposure to a range of agents. There were 14,289 adults included in the study. In this follow-up, an average of 19.3 years after enrollment, the National Death index was examined for information about the participants.
As part of the medical examination, the concentration of lead in the participants’ blood was measured, and ranged from 0.70 to 56.0 mg/dL. Participants who had the highest concentrations of lead in blood were older, less educated, had high levels of cadmium in urine, earned less (annual income of less than $20,000), were more likely to be male, smoked (including ex-smokers) cigarettes, consumed larger amounts of alcohol, and had unhealthier diets. They were also more likely to have elevated cholesterol levels and higher rates of high blood pressure and diabetes.
The results of testing showed that higher lead concentration in the subjects’ blood was associated with all-cause mortality, cardiovascular disease mortality and ischemic heart disease mortality. The analysis suggested that 28.7% of cardiovascular disease-related deaths and 37.4% of ischemic heart disease-related deaths could be attributed to elevated lead exposure. This corresponds, respectively, to 256,000 and 185,000 deaths in the USA per year.
Overall, the analysis found that about 400,000 deaths per year are attributable to lead exposure, an estimate that is about ten times larger than previously thought. Current estimates had been lower because it had been assumed that blood lead concentrations of less than 5 mg/dL were not high enough to significantly increase health risks. For example, a 2012 report by the National Toxicology Program on the effects of blood lead levels less than 5 mg/dL and 10 mg/dL found ‘limited evidence’ of an association between blood lead level less than 10 mg/dL and cardiovascular-related death[iii].
In the current study, a secondary analysis was performed, in which the only participants included were those with blood lead concentrations of less than 5 mg/dL. Compared to a blood lead concentration of 1.0 mg/dL, blood lead concentrations of 5 mg/dL were more significantly associated with all-cause mortality, cardiovascular disease mortality and ischemic heart disease mortality. It has been proposed that lead exposure-based mortality could be reduced if blood lead levels in the entire US population over the age of 20 were reduced to 1.0 mg/dL or lower.
In conclusion, the researchers found that low-level lead exposure is an important, but largely overlooked, risk factor for cardiovascular disease mortality in the USA.
[i] Lanphear, B. P., Rauch, S., Auinger, P., Allen, R. W. & Hornung, R. W. Low-level lead exposure and mortality in US adults: a population-based cohort study. Lancet Public Health 3, e177–e184 (2018). <https://www.sciencedirect.com/science/article/pii/S2468266718300252> (Accessed 25 April 2018).
[ii] Vaziri, N. D. Mechanisms of lead-induced hypertension and cardiovascular disease. American Journal of Physiology-Heart and Circulatory Physiology 295, H454–H465 (2008). <https://www.physiology.org/doi/full/10.1152/ajpheart.00158.2008> (Accessed 25 April 2018).
[iii] NTP Monograph: health effects of low-level lead. National Toxicology Program. June 2012. <https://ntp.niehs.nih.gov/ntp/ohat/lead/final/monographhealtheffectslowlevellead_newissn_508.pdf> (Accessed 25 April 2018).