Yet a Journal review of dozens of recent research papers and interviews with more than 20 medical professionals, including brain and heart specialists, epidemiologists and toxicologists, show an increasing conviction about the link between fume events and potentially fatal diagnoses.

“It’s a pattern. I can’t ignore it,” said Frank van de Goot, a Dutch forensic pathologist who said he has performed autopsies on 18 crew members who showed signs of toxic exposures.

American Airlines wasn’t a party to the Weiland lawsuit. In a statement, a spokesperson said the airline “continues to see a reduction in these types of events” and is investing in training and other procedures to ensure the highest-possible cabin-air quality.

‘Lots of evidence’

Gregory O’Shanick, a specialist in brain-injury medicine from Richmond, Va., has treated flight crew for serious injuries that he says were caused by toxic exposure on commercial aircraft.

He has also identified what he said are clear parallels between crews’ symptoms and those he’s found in soldiers with concussive traumas caused by chemical exposures and explosive blasts on battlefields.

In both groups, the links between severe head injury and life-threatening brain diseases including ALS, dementia, brain tumors, and acute depression are “extremely well-connected and well-associated,” said O’Shanick, who served for 14 years as the medical director for the Brain Injury Association of America.

O’Shanick, Van de Goot and other medical professionals, including Michael Freeman, a professor of forensic epidemiology at Maastricht University, agreed with the industry position that direct causation hasn’t been proven, in part because companies have objected to placing air quality monitors on aircraft. But they also stressed an urgency for that work to be done.

“We have a lot to be concerned about and a lot to be suspicious about, we really do,” said Freeman, who edits the Journal of Forensic and Legal Medicine.

In 2021, researchers at the University of California, Berkeley found that acute exposure to the chemical formaldehyde led to a 78% increase in the risk of developing ALS and a 71% increase in brain cancer. Last year, formaldehyde was identified in an FAA-funded study as repeatedly exceeding occupational exposure guidelines even when low amounts of oil mixes into the air supply.

A separate Harvard-led study in 2024 found pilots had the fourth-highest mortality rate from Alzheimer’s out of 443 occupations in the U.S.

While ALS is one of the more complex neurodegenerative diseases, researchers are increasingly confident that anyone can develop the disease if they accumulate—or are exposed to—enough factors to reach a tipping point.

Having susceptible genes is one. Others include multiple types of exposures associated with fume events: chemicals that appear in both pesticides and engine oils; high levels of ultrafine particles and solvents like formaldehyde; and brain trauma.