Chronic work exposure to lead and risk of Amyotrophic Lateral Sclerosis

Individuals with chronic occupational exposure to lead have an 80 percent higher odds of developing Amyotrophic Lateral Sclerosis (ALS) than individuals who do not have the exposure. Those are the findings of a recently published meta-analysis of 13 studies of individuals with “Lou Gehrig’s disease" (ALS). The authors, funded by the Public Health Agency of Canada for its National Population Health Study of Neurological Disease in Canada, note:

“Epidemiological studies investigating the association between prior exposure to lead and ALS began about five decades ago, after a series of ALS cases with antecedent exposure to lead were reported as early as 100 years ago.”

The cause(s) of ALS is largely unknown. Only about 10 percent of cases are known to be associated with genetic mutations.

The authors used nine case-control studies, all of which specifically examined work-related lead exposure as a risk factor for ALS, in their meta-analysis. The studies included one based on data from the National Registry of US Veterans with ALS, and community-based studies of ALS patients in New England and from three counties in western Washington State.

Based on their meta-analysis, the authors reported a 1.81 higher odds of developing ALS for those with a history of work-related lead exposure (using a random-effects model: 95% CI 1.39 to 2.35; using a fixed-effects model 95% CI 1.42 to 2.29.) They also conducted sensitivity analyses using four additional high-quality cohort studies which examined the relationship between ALS and occupational exposure to “heavy metals." The authors concluded:

“The available data suggest that about 5% of all sporadic ALS cases may be attributable to occupational exposure to lead, although the actual attributable fraction could be somewhat lower because of the assumptions in this calculation.”

An estimated 5,600 individuals in the US are diagnosed with ALS each year. For a disease for which little is known about its causes, it's refreshing to read a paper (even a meta-analysis) that examined environmental risk factors. Whether the attributable fraction is 5% or half that, more than 100 new cases of ALS each year might be associated with work-related lead exposure. All the more reason to eliminate or control lead exposure in workplaces.

The CDC defines an elevated blood lead level (BLL) for an adult as a BLL ≥ 10 ug/dL. In 2010 (most recent data available) 31,459 adults were identified through laboratory reporting to have blood lead levels (BLL) ≥ 10 ug/dL. Nearly 1,400 of them had BLLs four times higher. From 2002-2011, more than 11,500 adults had BLLs ≥ 40 ug/dL. It's not as if lead poisoning is a thing of the past. OSHA continues to find employers who violate worker protection regulations for lead (e.g., here, here, here.)

Preventing lead poisoning will take some work. Even when worker exposure to lead is found, OSHA's 1978 lead standard hasn't kept up with the science. It certainly doesn't help matters that it only requires medical removal protection for workers with a BLL ≥ 50 ug/dL for construction workers (and ≥ 60 ug/dL for workers in other industries.) Nor does it help that CDC funding for state-based surveillance of adult lead poisoning was eliminated in 2013. TPH’s Kim Krisberg reported on the discontinuation of the National Institute for Occupational Safety and Health's Adult Blood Lead Epidemiology and Surveillance program. The 20-year old program provided a modest, combined $812,500 annually to 40 state health departments to analyze and conduct follow-up on laboratory reports of adult lead poisoning.

"A Meta-Analysis of Observational Studies of the Association Between Chronic Occupational Exposure to Lead and Amyotrophic Lateral Sclerosis" stems from the Canadian government's $15 million effort to better understand neurological health conditions. The National Population Health Study of Neurological Conditions in Canada involves three national surveys and 13 research project, conducted by 125 researchers at 30 institutions across Canada.