Air Pollution Exposure Tied to Greater ALS Risk in Women in Study

Long-term exposure linked to increased risk in postmenopausal women

Marisa Wexler, MS avatar

by Marisa Wexler, MS |

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A person puts one hand to the chest and covers the mouth with the other amid a cloud of air pollution.

Higher long-term exposure to certain forms of air pollution — specifically coarse particulate matter, such as that from traffic pollutants — significantly increases the risk of amyotrophic lateral sclerosis (ALS) in postmenopausal women, a study showed.

“Although the effects noted within the current study are limited by sample size due to the rarity of ALS, it represents the largest group of women ever followed prospectively in order to consider the relationship of [air pollution] and risk of death from ALS,” the researchers wrote.

“Our study found a relationship between the risk of ALS death and [particulate matter] exposure,” notably “significant” for one time period investigated, the team wrote.

While further studies are needed to confirm these findings, the results support regulations to improve air quality by limiting air pollution, according to the team.

The study, “Long-term air pollution and risk of amyotrophic lateral sclerosis mortality in the Women’s Health Initiative cohort,” was published in Environmental Research. It was funded by the ALS Association.

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Investigating risk factors for ALS

The causes of ALS, which affects an estimated 15,000–21,000 people in the U.S., are incompletely understood. But it’s thought that both genetics and environmental exposures may play a role in prompting the disease’s development.

Prior research has suggested a link between exposure to air pollution and the development of other neurological disorders, such as Alzheimer’s and Parkinson’s diseases. However, there have been few investigations into any potential connections between air pollution and ALS risk.

To learn more, a team of researchers in the U.S. analyzed data from the Women’s Health Initiative (WHI) study, which enrolled 161,808 postmenopausal women from 1993 to 1995 and tracked their health outcomes over time. The women were ages 50–79 at enrollment.

“Our goal was to improve understanding of potential environmental causes and modifiable risk factors for ALS by studying long-term residential exposure to [various air pollutants],” the researchers wrote.

“This study is one of the first to examine long-term ambient [air pollution] in relation to ALS risk in a large [group] of US women,” the team wrote.

A total of 256 women died of ALS between 1993 and 2017. For each of these ALS cases, the researchers identified 10 controls — a total of 2,486 women who were alive and without ALS at the time of ALS case death, and who were similar to the ALS patients in terms of demographics and other factors.

Among all the women included in the analysis, 90.5% were non-Hispanic white females. Their mean age was 64 years, and about two-thirds were college-educated. Most participants resided in the Northeast and West U.S. regions.

Using previously published information about pollution in different locations and participants’ residence data, the researchers determined each woman’s level of exposure to several air pollutants.

These included inhalable particulate matter — those with diameters up to 2.5 micrometers (PM2.5) and those up to 10 micrometers (PM10) — and gaseous pollutants, such as ozone, nitrogen oxides, nitrogen dioxide, and sulfur dioxide (SO2).

The team then used statistical models to assess whether long-term exposure to these pollutants — five, 7.5, and 10 years before death — was associated with an increased risk of death from ALS.

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Air pollution, ALS and women

For the majority of air pollutants, the models showed no difference in ALS risk based on levels of exposure.

This was true even after adjusting for factors previously suggested to contribute to ALS risk, including education level (as a proxi for employment and occupational exposures), smoking, and total energy expenditure from recreational physical activity.

There were two noteworthy exceptions to these findings: coarse particulate matter — those between 2.5 and 10 micrometers in diameter, referred to as PM10-2.5 — and SO2. PM10-2.5 results from traffic, road dust, agriculture, construction and industry, while SO2 is a chemical released when fossil fuels are burned. SO2 contributes to acid rain.

Higher exposure to PM10-2.5 was consistently associated with a greater risk of ALS in the five, 7.5, and 10-year periods.

In one of the statistical models, greater exposure to SO2 also was associated with a higher ALS risk.

However, these risk differences only reached statistical significance for the PM10-2.5 exposure during the 7.5-year window, which was associated with an increased risk of ALS death by about 7%.

“This is the first investigation of associations between long-term exposure to PM constituents and gaseous air pollutants and the risk of ALS mortality among a group of postmenopausal women in the US.,” the team wrote.

“We found that the estimated risk of ALS mortality associated with one-unit increases in PM10-2.5 and SO2 were consistently elevated … [but] only PM10-2.5 at 7.5-years was significantly associated with ALS death,” the researchers wrote.

“Moreover, the latter PM10-2.5 risk estimates were relatively stable, even after adjusting for education, smoking, and physical activity — three possible ALS risk factors,” they added.

These findings suggest “that even low-level exposures to airborne, coarse particulates in the US may be associated with modest increases in the risk of ALS mortality among older women,” the team wrote.

The fact that air pollution exposure was estimated based on residency was a noted limitation of this study; the researchers acknowledged that other sources of pollution exposure, such as in the household or workplace, were likely not taken into consideration. The analysis also was limited in that all participants were women and nearly all were white, they noted.

“This research should be replicated in other studies with a large enough [group of people] to capture this rare but rapidly fatal disease outcome and with consideration of earlier [air pollution] measures given the approximately 2–4 years mean/median survival time from onset of ALS symptoms to death,” the team concluded.