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Even Low Level Lead Exposure Is Linked to Later Health Problems

New research published in The Lancet estimates that lead exposure results in about 400,000 deaths every year in the USA - which includes about 250,000 deaths from cardiovascular disease. The study looked at "historical exposure" to lead, which means they looked at a person's lead exposure years ago (which could have been from many sources, such as breathing lead contaminated dust, drinking water from leaded pipes, from lead paint, leaded gas, or eating from cans that have been soldered with lead). What was noteworthy in this study following over 14,000 adults was that there were increased death rates even from low lead blood levels (concentrations of lead in blood lower than 5 μg/dL).

Lead exposure has been declining since the 1970s after lead was eliminated (banned) from paint and gasoline, but this study looked at adults born in the years when lead exposure was higher during childhood and adulthood. Baseline data (blood lead levels) was collected between 1988 and 1994 and then individuals were followed for the next 2 decades. The researchers found that there was a dose response curve - the higher the blood lead level, the higher the death rate from any cause (all-cause mortality), from cardiovascular disease, and from ischaemic heart disease in the next two decades.

Lead is a naturally occurring heavy metal, but it's also a poisonous substance for humans, and accumulates in teeth and bones. Lead can pass through the blood-brain barrier in children and fetuses and kill brain cells (and cause neurological damage). Lead can enter blood vessels and harm the endothelial cells that line the blood vessels. This process hardens arteries and causes plaque to form in blood vessels, increasing blood pressure, and the risk of heart disease and stroke. Lead can damage the kidneys, which play an important role in regulating blood pressure, and so increase a person’s risk of heart disease and stroke.

The Centers for Disease Control and Prevention (CDC) have stated that there are no safe blood levels of lead for children (zero is best), and the researchers of this study believe the same is true for adults. From Science Daily:

Historical lead exposure may be linked to 256,000 premature deaths from cardiovascular disease in adults in United States each year

New estimates suggest that 256,000 premature deaths from cardiovascular disease -- including 185,000 deaths from ischemic heart disease -- in the USA may be linked to historical lead exposure in middle-aged and older adults (people currently aged 44 years or over), according to an observational study following 14,300 people for almost 20 years, published in The Lancet Public Health journal. 

Previous estimates, which assumed that low-level lead exposure did not increase the risk of premature death, produced substantially fewer deaths. However, this new study finds that low-level lead exposure (between 1-5 micrograms of lead per decilitre of blood) increases the risk of premature death, especially from cardiovascular disease. Lead exposure is linked to high blood pressure, hardening of the arteries and ischemic (coronary) heart disease.

Exposure occurs from lead that remains in the environment from historic use in fuel, paint and plumbing, as well as ongoing exposures from foods, emissions from industrial sources, and contamination from lead smelting sites and lead batteries.

The study used data from the Third National Health and Nutrition Examination Survey (NHANES-III) for 14289 people in the USA aged 20 years or older between 1988 and 1994, and the end of 2011. All participants had a medical examination, including a blood test for lead -- a measure of past and ongoing exposures to lead -- and a urine test for cadmium at the start of the study. After an average of 19.3 years, 4422 people died including 1801 from cardiovascular disease and 988 from heart disease.

At the outset, the average level of lead found in the participants' blood was 2.7 µg/dL, but ranged from less than 1 to 56 µg/dL. One in five participants (3632 people) had levels of 5 µg/dL or more, and those with the highest levels of lead in their blood were older, less educated, more likely to be male, smoke, consume larger amounts of alcohol, have less healthy diets, have higher cholesterol, and more likely to have hypertension or diabetes. Almost one in 10 participants had lead levels that were undetectable to the blood test, so were given a reference level of 0.7 µg/dL (8%, 1150/14289 participants).

Overall, people who had high lead levels (6.7 µg/dL) were at 37% greater risk of premature death from any cause, 70% times greater risk of cardiovascular death, and double the risk of death from ischemic heart disease, compared with people with lower levels (1 µg/dL).

Overall, they found that up to 18% of all deaths every year in the USA (412,000/2.3 million) would be among people who had levels of lead above 1 µg/dL. They estimated that 28.7% of premature cardiovascular disease deaths (256,000/ 892,000) could be attributable to lead exposure, including a high proportion of ischemic heart disease deaths (lead was linked to 37.4% of all IHD deaths [185,000/495,000]).

The authors note some limitations, including that their results rely on one blood lead test taken at the start of the study and therefore cannot determine any effect of further lead exposure after the study outset. The authors were also unable to control for all potential confounding factors, such as exposure to arsenic or air pollution, which are also risk factors for cardiovascular disease.

Writing in a linked Comment, Professor Philip Landrigan, Icahn School of Medicine at Mount Sinai, USA, says: "A recurrent theme in lead poisoning research has been the realization that lead has toxic effects on multiple organ systems at relatively low levels of exposure previously thought to be safe… A key conclusion to be drawn from this analysis is that lead has a much greater impact on cardiovascular mortality than previously recognized… [The authors] suggest that the time has come to end neglect of pollution's contribution to non-communicable diseases' mortality and to thoroughly re-examine lead's role in changing global patterns of cardiovascular disease."

Note: The authors note that these estimates are comparable to the annual number of deaths in the US in people who currently smoke (483,000 deaths a year). This is because the risk of death from lead exposure is lower (HR = 1.37, 95%CI = but more prevalent across the population (in the study, 90% of participants were exposed to lead). Comparatively, 20% of the US population smoke, but the risk of premature death is higher (HR=1.75 95%CI = 1.50-2.05) than for low-level lead exposure.

 Galena, which is the main ore of lead. Credit: Wikipedia

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