Smells on a plane: Have you been exposed to toxic chemicals while flying?

If you’ve flown on an airplane, chances are you’ve wondered about mystery smells in the cabin. They can be harmless — for example, food in the galley oven.

But you could be smelling toxic chemicals contaminating the plane’s air supply.

A Times investigation found that hundreds of people were sickened after exposure to toxic fumes on airplanes in recent years. Inside the cockpit, pilots have used emergency oxygen to escape fumes and made emergency landings, federal aviation records show.

But because planes don’t have air sensors, pilots, flight attendants and airline mechanics are told to use the smell test to diagnose so-called fume events.

“We have to rely on people’s noses,” one senior Boeing engineer acknowledged in a 2018 deposition.

Want to see if your flight passes the smell test? Here’s a quick guide based on industry sources.

Dirty socks, gym locker — these are among the most common odor descriptions for jet engine oil in the plane’s air supply.

The air you breathe on planes comes directly from the jet engines. If there is a mechanical issue, heated jet engine oil can leak into the air supply, potentially releasing toxic gases into the plane.

Scientists have long warned of potential dangers from breathing heated engine oil, which contains tricresyl phosphate, a highly toxic chemical that can damage the nervous system.

For decades, the airline industry and its regulators have maintained that fume events are rare and that the toxic chemical levels are too low to pose serious health risks.

But no government agency tracks fume events or how often people become sick. And no major research has ever measured the chemicals in fume events as they occur.

A 2015 study conducted by researchers at Kansas State University and funded by the Federal Aviation Administration analyzed aviation records to estimate that fume events occur once every 5,000 flights. That translated to about five fume events a day in the U.S. in the era before the coronavirus.

Toxic gases from many aviation fluids could smell like “chemicals,” according to FAA-funded research and other industry sources.

What‘s a “chemical” smell? Your guess is as good as any. The smell test is an imprecise science based on vague descriptions. But, without air sensors, it’s the industry standard.

You might smell one thing, your neighbor another — or nothing at all. Fume events can be odorless.

Heated jet engine oil can produce carbon monoxide, an acutely incapacitating gas that has no odor. A 2002 study mandated by Congress recommended requiring carbon monoxide sensors on all passenger airplanes. Today, most homes have them; airplanes do not.

A foul, pungent or acrid smell could be cause for alarm. Or it could be something else entirely.

“Odors on aircraft have myriad sources, including food, body odors, electrical equipment, lavatories, and galley/kitchen equipment,” attorneys for Boeing wrote in recent court filings.

With fume events, passengers are often unaware that their air supply has been contaminated. Some of the most common symptoms of exposure — headaches and fatigue — are indistinguishable from jet lag, experts say.

Airlines have no obligation to notify passengers of fume events and sometimes provide misleading information.

On a Southwest Airlines flight in June 2019, for example, passengers watched as both pilots and all four flight attendants were pushed off the plane in wheelchairs. The incident, the captain noted in a report to NASA, was a “fumes event.” But the airline told passengers the smell on the plane was rotten jackfruit.

A musty or moldy smell is a sign that the plane could be having a fume event.

The seriousness of these incidents varies, and many go unreported or cause no apparent health effects.

In recent years, however, pilots and flight attendants have reported an array of health problems, including eye irritation and coughing, as well as more serious long-term conditions: tremors, memory problems, difficulty thinking and speaking, brain damage and other illnesses that have kept them out of work for months and sometimes ended their careers.

Boeing’s lawyers argued in court filings that symptoms such as headaches, difficulty concentrating and anxiety afflict “a wide swath of the population” and there’s no proof that fume events are to blame.

The FAA also maintains that the air on planes is safe. But the agency funded a 2009 medical guide for treating health problems related to fume events that warned that “neurological, psychiatric, respiratory, systemic, and dermal symptoms … may last for years after the exposure.”

Airbus gives airlines a form for reporting fume events. It has boxes to check if pilots, flight attendants or passengers experienced symptoms such as “dizziness/fainting.”

What should you do if your flight fails the smell test? Ring your call button and tell a flight attendant right away, crew members advise. Describe what you smell and any symptoms.

If crew members have been trained to recognize fume events, they’ll know the situation could be serious.

Icon illustrations were designed by Lorena Iñiguez Elebee.