Diesel exhaust may raise risk of neurological disease

The risk of amyotrophic lateral sclerosis may be increased for people who are frequently exposed to diesel exhaust, new research suggests.

In a study of more than 1,600 adults, scientists working at the Harvard T.H. Chan School of Public Health in Boston, MA, found that men with higher exposure to diesel exhaust over 5–10 years were at least 20 percent more likely to develop amyotrophic lateral sclerosis (ALS) than men with no exposure.

Study co-author Aisha Dickerson, Ph.D., and her colleagues are due to present their findings at the American Academy of Neurology's (AAN) Annual Meeting, which will be held in Los Angeles, CA, in April.

ALS, or Lou Gehrig's disease, is a progressive neurological condition in which the nerve cells that control voluntary muscle movement are damaged. It is estimated that around 14,000–15,000 people in the United States are living with ALS.

Early symptoms of the disease include cramping, weakness, and stiffness of the muscles, problems chewing and swallowing, and slurred speech. Mobility and breathing problems arise as the disease progresses.

Familial ALS, wherein a genetic mutation for the disease is inherited from a parent, accounts for around 5–10 percent of all ALS cases. The remaining 90–95 percent of cases are sporadic, meaning that no specific risk factors for the condition can be found.

The link between diesel exhaust and ALS

Previous studies have suggested that environmental factors could be involved in the development of ALS. A 2013 study published in PLOS One, for example, uncovered a link between exposure to diesel exhaust and a greater risk of ALS.

Diesel exhaust is a combination of gases and particulate matter that is produced through diesel fuel combustion.

"However," notes Dickerson, "no studies have directly looked at the relation between diesel exhaust exposure during different time points in life and ALS."

With this in mind, she and her team set out to investigate how high exposure to diesel exhaust over 5 and 10 years might impact the risk of ALS.

The scientists included the data of 1,639 adults from the Danish National Patient Registry, who were aged 56, on average. They had all been diagnosed with ALS between 1982 and 2013.

Each of these adults was matched for age and sex with 100 people who had not been diagnosed with ALS.

To estimate cumulative exposure to diesel exhaust for each subject, the researchers looked at their employment history. People with certain occupations, such as construction workers, service station attendants, and bus drivers, have greater exposure to diesel exhaust than the general population.

The team calculated diesel exhaust exposure for up to 5 and 10 years prior to the time period in which subjects with ALS were diagnosed with the condition.

Men's ALS risk increased by up to 45 percent

The researchers divided the study participants into four groups based on their exposure to diesel exhaust.

The risk of ALS was increased by 20 percent for men who held jobs with any exposure to diesel exhaust for up to 10 years before being included in the study, compared with men who had no diesel exhaust exposure during this time period.

For men whose job put them at a 50 percent chance of diesel exhaust exposure or greater for up to 10 years before study inclusion, the risk of ALS was increased by 45 percent, compared with men who had no diesel exhaust exposure.

The scientists found no link between diesel exhaust exposure and the risk of ALS among women. However, they note that women and men may perform different tasks in the same occupation, which may have influenced their level of diesel exhaust exposure.

Since this study is purely observational, it cannot prove cause and effect between diesel exhaust exposure and ALS. Still, the team says that the findings warrant further investigation.

"The overall risk of developing ALS is low, but our findings suggest that the greater the exposure to diesel exhaust, the greater the risk of developing ALS."

Aisha Dickerson, Ph.D.

"This type of exposure," Dickerson continues, "deserves more attention and study as we work to develop a better understanding of what causes ALS."

"Importantly," she concludes, "the general population can be exposed to diesel exhaust from traffic pollution. Understanding whether that exposure increases ALS risk is also an important question to pursue."