“Do you smell that?” Florence Chesson was asked by a fellow JetBlue flight attendant as they prepared for landing in Puerto Rico.

Chesson, as trained, inhaled a lungful of air through her nostrils in a single deep breath. “It smells like dirty feet,” she told her colleague.

Instantly, she started to feel like she had been drugged, Chesson said in an interview.

About an hour later, the aircraft had landed, loaded a fresh group of passengers and was back in the sky returning to Boston. As Chesson wrapped up the drinks service, a colleague rushed past to the back of the plane, her hands around her throat, complaining she was struggling to breathe before starting to vomit. Another was given emergency oxygen.

When the flight landed, the two cabin crew were taken to a hospital in an ambulance, one on a stretcher.

Chesson, her uniform and hair soaked in sweat and with an overpowering metallic taste in her mouth, went to meet her supervisors. “I felt like I was talking gibberish,” she recalled. “I remember being very repetitive, saying ‘What just happened to me? What just happened to me?’”

After months of worsening symptoms, Chesson was diagnosed with a traumatic brain injury and permanent damage to her peripheral nervous system caused by the fumes she inhaled. Her doctor, Robert Kaniecki, a neurologist and consultant to the Pittsburgh Steelers, said in an interview that the effects on her brain were akin to a chemical concussion and “extraordinarily similar” to those of a National Football League linebacker after a brutal hit. “It’s impossible not to draw that conclusion,” he said.

Kaniecki said he has treated about a dozen pilots and over 100 flight attendants for brain injuries after exposure to fumes on aircraft over the last 20 years. Another was a passenger, a frequent flier with Delta’s top-tier rewards status who was injured in 2023.

Chesson’s experience is one dramatic instance among thousands of so-called fume events reported to the Federal Aviation Administration since 2010, in which toxic fumes from a jet’s engines leak unfiltered into the cockpit or cabin. The leaks occur due to a design element in which air you breathe on an aircraft is pulled through the engine. The system, known as “bleed air,” has been featured in almost every modern commercial jetliner except Boeing’s 787.

The rate of incidents is accelerating in recent years, a Wall Street Journal investigation has found, driven in large part by leaks on Airbus’s bestselling A320 family of jets—the aircraft Chesson was flying.

The Journal’s reporting—based on a review of more than one million FAA and National Aeronautics and Space Administration reports, thousands of pages of documents and research papers and more than 100 interviews—shows that aircraft manufacturers and their airline customers have played down health risks, successfully lobbied against safety measures, and made cost-saving changes that increased the risks to crew and passengers.

The fumes—sometimes described as smelling of “wet dog,” “Cheetos” or “nail polish”—have led to emergency landings, sickened passengers and affected pilots’ vision and reaction times midflight, according to official reports. 

Most odors in aircraft aren’t toxic, and neither are all vapors. The effects are often fleeting, mild or present no symptoms.

But they can also be longer-lasting and severe, according to doctors, medical records and affected crew members.

The cause of fume events isn’t a mystery. Airbus and Boeing, the two biggest aircraft manufacturers, have acknowledged that malfunctions can lead to oil and hydraulic fluid leaking into the engines or power units and vaporizing at extreme heat. This results in the release of unknown quantities of neurotoxins, carbon monoxide and other chemicals into the air.

There are various ways bleed air can become contaminated. Here is one of the most common:

About half the air on most flights is pulled from outside, passed through an aircraft’s engines, and then ‘bled’ into the cabin where it mixes with existing recirculated air.​

​First the engine pulls cold, low-density air through its fans​. It is then compressed and heated​, to make it suitable to breathe.

Bearings inside the compressor are lubricated with oils, but seals designed to stop leaks wear and degrade. When that happens, oil enters the air and vaporizes in the heat, releasing unknown quantities of toxic compounds.

The contaminated air is then piped to air conditioning packs in the body of the aircraft before passing into the cockpit or cabin. Passenger oxygen masks, which aren’t sealed, don’t offer protection.

Note: Plane components and vent placements aren’t exact, and are for illustrative purposes.

Sources: Airbus, Pratt & Whitney, Wizz Air

Peter Champelli and James Benedict/WSJ

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In a February incident captured on video by several passengers, enough oil entered the bleed air supply of a Boeing 717 that thick plumes of smoke started piling through the vents midflight. “Ladies and gentlemen, please breathe through your clothing, stay low,” the Delta Air Lines flight attendant told passengers, some of whom had noticed a strange smell during takeoff.

Meanwhile pilots had donned oxygen masks, declared an emergency and were turning back to Atlanta, according to a National Transportation Safety Board report. An inspection found the right engine’s oil reservoir was nearly empty, with enough oil leaking into the bleed air supply to fill the aircraft with white smoke. A Delta spokesman said the company was cooperating with a continuing NTSB review.

This incident was somewhat unusual in that most fume events don’t produce smoke.

Manufacturers, regulators and airlines have said these types of incidents are too infrequent, levels of contamination too low and scientific research on lasting health risks too inconclusive to warrant a comprehensive fix. In some cases, they have attributed reported health-effects from fume exposure to factors including hyperventilation, jet lag, psychological stress, mass hysteria and malingering.

Internally, industry staffers have flagged their own fears about the toxic makeup of engine oils. 

In a 2017 internal email, excerpts of which were produced in a lawsuit, Boeing quality inspector Steven Reiman wrote:

Just think what would happen if people realized they are being poisoned by coming to work on our airplanes or as passengers.

He raised concerns that the public might discover that oil leaks could make “aircrew sick to the point of death.”

Airbus, Boeing, the FAA and its European counterpart—the European Union Aviation Safety Agency—declined to make representatives available to interview for this story.

“The cabin air inside Boeing airplanes is safe. There is no indoor environment that is free from ‘contaminants,’” a Boeing spokesman said by email, citing multiple research papers on air quality. He said the research shows “that contaminant levels on aircraft are generally low and that health and safety standards are met.”

Regarding Reiman’s email, Boeing said in its response to the lawsuit that the message was mischaracterized and taken out of context.

“Airbus aircraft are designed and manufactured according to all relevant and applicable airworthiness requirements,” a spokesman for the European planemaker said in a statement.

The individual airlines mentioned in this article noted their commitment to the safety of their passengers and crew, and said they follow the protocols established by the FAA and the manufacturers of their planes. A Delta spokesman said “air transportation remains the safest form of travel in the world.”

“We take nothing more seriously than the safety and health of our crewmembers and customers,” a spokesman for JetBlue said in a statement. “While cabin air quality concerns are not isolated to JetBlue, we continue our work to identify policies and procedures to reduce and manage them.”

‘Sweaty sock’

The FAA on its website says the incidents are “rare” and cites a 2015 review that estimated a rate of “less than 33 events per million aircraft departures.” That rate would suggest a total of about 330 fume events on U.S. airlines last year.

In reality, the FAA received more than double that number of reports of fume events in 2024 from the 15 biggest U.S. airlines alone, according to the Journal’s analysis of service difficulty reports for flights between 2010 and early 2025. The rate has soared in recent years. In 2014, the Journal found about 12 fume events per million departures. By 2024, the rate had jumped to nearly 108. (Read more about how the Journal conducted its analysis.)

In a statement, the FAA attributed the increase in part to a change in its guidance for reporting fume events, although that revision was only implemented in November of last year.

“We recently improved the reporting process with a standardized, centralized system for tracking and reporting fume events,” the statement said, adding that the increase “reflects a healthy safety culture that values critical safety reporting.”

The FAA doesn’t have a formal definition of a fume event and the service reports often don’t indicate the severity. In its review, the Journal mirrored the industry’s practice of relying on crew reports of specific odors and associated maintenance reports. Changes in crew awareness could impact reporting rates.

The actual rate is likely far higher, as crews don’t always report incidents to their airlines, which likewise don’t report all instances to the FAA. A review of internal data by the airline lobby International Air Transport Association, calculated a total rate of 800 per million departures in the U.S., according to an internal document from a member carrier.

The Journal’s analysis suggests that the growth is driven by the world’s bestselling aircraft: the Airbus A320. In 2024, among the three largest U.S. airlines with mixed fleets, the rate of reports on A320s had increased to more than seven times the rate on their Boeing 737 aircraft.

At JetBlue and Spirit—both majority-Airbus operators—the increase is stark. Together, the airlines saw a 660% surge in the frequency of incidents on their A320s between 2016 and 2024.

A Spirit spokesman said the company follows maintenance protocols as established by Airbus and that safety is a top priority.

The Journal’s analysis shows incidents began climbing in 2016, the year Airbus started delivering its new A320neo, what would become the world’s fastest-selling model. It boasted a new generation of fuel-efficient engines, including one that was plagued by rapidly degrading seals meant to keep oil from leaking into the air supply.

Under pressure from airlines who complained that fume events were keeping aircraft out of service for up to days at a time, Airbus loosened maintenance rules, according to a review of internal documents and people familiar with the changes.

For example, under the old guidelines, Airbus typically required an inspection and deep-clean after a fume event. Under the revised rules, if the smell wasn’t strong and hadn’t occurred in the last 10 days, airlines wouldn’t need to take immediate action.

Airbus was aware the changes would likely lead to an increase in incidents, but approved them anyway for multiple airlines. In one document explaining the change, the manufacturer said:

[JetBlue] have experienced a number of transient oil or “sweaty sock” odours which are a minor comfort issue. When entering the relevant [troubleshooting manual] procedure, the maintenance procedures required for troubleshooting and rectification seem to be out of proportion to a transient odour causing a minor comfort issue.

An Airbus manual from 2016—before the maintenance changes—cautioned that if not properly addressed, an aircraft would encounter “repeated occurrences.”

The new rules exacerbated other causes on both the A320neo and its predecessor, helping transform what had been mostly isolated incidents into a pattern of repeat offenders, the Journal’s analysis suggests. Between 2010 and 2017, for example, the number of aircraft experiencing multiple fume events within a single 30-day window jumped from an average of about six per year to nearly 45 from 2017 through 2024.

In one case, a lone Spirit Airlines A320 had six reported fume events in a single month.

‘The straw that breaks the camel’s back’

Six months before Chesson, the JetBlue flight attendant, became ill in early 2018, an FAA safety inspector aired concern about engine oils containing organophosphates, a family of chemicals that he said had been used “as a nerve agent for warfare.” The inspector said the agency was aware of more than a dozen reports of fumes rendering air crews ill, according to an email to colleagues and JetBlue managers.

The FAA inspector wrote:

These toxic chemicals are present in today’s modern synthetic jet engine oils and are passing into the aircraft cabin/cockpit unfiltered, affecting the air that crew and passengers breathe in.

When Chesson made it home to Richmond, Va. after her exposure, her husband, Patrick Hill, noticed a chemical-like stench radiating from his partner that lasted for weeks.

The next morning, Chesson, who was 58 at the time and 17 years into a second career as a flight attendant, woke up feeling like her brain was on fire. “I felt like someone poured gasoline and lit a match,” she said.

A few months before, Chesson and her husband had undergone a full medical review as part of the process to become foster parents. Hers came back with a near clear bill of health, minus a mild intestinal issue.

A brain scan after the fume event ruled out a stroke. Instead, the specialist who handled her scans found the pattern of damage to her brain matched the symmetrical injuries seen in soldiers who had been exposed to gasses during combat.

In addition to the migraines, Chesson developed heart arrhythmias and severe sensitivity to lights, sound and everyday chemicals that have kept her largely housebound.

Chesson reached out to Kaniecki, the neurologist, who said her case was similar to the others he has seen suffering from the results of a fume event. Most had had previous, less severe incidents—or “micro concussions”—that while not enough to send them to a clinician, predisposed them to a bigger event, he said. “Then they have one that’s the straw that breaks the camel’s back.”

Robert Harrison, an occupational medicine specialist at the University of California, San Francisco, has treated over 100 aircrew for injury from toxic exposure on a flight, including damage to the central nervous system. 

“This is real, this can’t be just all in their heads,” said Harrison, who previously worked on an FAA-funded project to develop medical guidelines for other practitioners.

Like Kaniecki, all his patients were working aircrew, except one, a passenger. He said it’s an “open question” how many more passengers might have needed medical attention after being exposed to fumes.

‘Am I paranoid?’

The battle over fume events has escalated into one of the fiercest standoffs between the aviation industry and aircrew.

Crews have refused aircraft, abandoned takeoffs or diverted because fumes had filled their cabins. In one case, a captain who had tested positive for elevated carbon monoxide levels in his blood moments earlier refused his airline’s request to continue with his next flight. Another pilot complained of being bullied after wanting to file an official report.

Florentina Tudor, a senior cabin crew at Europe’s Wizz Air—a major Airbus customer—had flagged about 10 separate incidents in less than 12 months when she was suspended, reducing her monthly income to about 40% of her maximum.

The last was in July 2023, when a colleague complained of an acrid or vomit-like smell and was struggling to breathe as their aircraft taxied down the runway. Her captain later told medics that the flight attendant was “just pretending” before they took her away in an ambulance, Tudor said.

“At some point I asked myself, is it just me, am I paranoid?” Tudor said.

Weeks later, Wizz asked her to get an assessment from a company-appointed psychologist. She declined, and instead underwent an independent review at Romania’s aviation medical institute and was ruled fit to fly.

Tudor was fired after refusing a second time. In June this year, a judge ruled that Wizz’s actions were too harsh, reversed her termination, but ordered her salary be docked by 10% for three months. Tudor’s union has filed an appeal.

In an email to crew two months before her suspension, Wizz’s head of flight operations sent a note to crew warning of a recent run of oil leaks. He asked anyone affected to seek medical attention and listed symptoms from fatigue to vertigo, seizures, confusion, tightness in chest and heart palpitations.

A Wizz spokesman said the airline followed all relevant protocols and regulations and that all crew reports are properly investigated.

‘Passed out pilots’

The United Nations has formally recognized fume events as a risk to flight safety since 2015. In addition, more than a dozen accident investigation teams in countries across the world have asked Boeing, Airbus and their regulators to implement measures to mitigate the risk, according to a review of official accident investigation reports. No major changes have been made.

In 2017, Robert Sumwalt, who would become chairman of the NTSB that year, was told by a grounded flight attendant about a fume event she experienced.

Sumwalt said in his reply:

I’m checking with our general counsel to see if this is within NTSB jurisdiction. I really don’t believe it is, and I hate to say that because it sounds so bureaucratic. I’m sure they will tell me this is a health issue and not a safety issue, but honestly, I’m not sure having passed out pilots and flight attendants inflight is a great idea.

After determining the NTSB didn’t have the legal authority to investigate, Sumwalt replied in a letter: “As a pilot and frequent traveler, I am personally concerned about what you experienced and the possibility of an unseen threat in commercial aircraft.”

That year, JetBlue pilot Andrew Myers was asked to help run tests on an aircraft that had had a run of fume events. Sitting in the cockpit, he turned the engines on and was dosed by toxic vapors that came rushing in. The maintenance crew, which hadn’t yet found the source, asked him to try again.

Coughing, he tried to leave the aircraft but collapsed at the exit and fell into the jetway.

His doctors diagnosed him with a “chemical-induced nervous system injury.” He developed chronic headaches, began stuttering, suffered severe tremors and would regularly collapse in public and be helped home by strangers. 

In a medical exam, his FAA-authorized practitioner—who had ruled him perfectly healthy two months prior—refused to reissue his medical certificate stating that “his tragic incident related to toxic fume inhalation…has rendered him incapable of functioning in this capacity.”

Myers’s case turned out to be a watershed event for aircrew in the U.S., recognized as the first time a court has ruled that fume exposure caused a plaintiff’s long-term health conditions.

Over the next few years, Myers was contacted by over 200 pilots, cabin crew, maintenance workers and passengers who sought help after being exposed to fumes, one of whom was Chesson. Experienced pilots would reach out and break down into tears, according to Myers’s wife Wendy, a trained pilot.

“There are pilots that we’re both aware of that should not be flying. They should never have gone back, but they did, they had no choice, they had to keep working,” she said.

‘Inappropriately downplayed’

Publicly, the industry has minimized research that has identified hazardous levels of toxins in cabin air linked to fume events and their health risks. It has pointed to research it and aviation regulators have funded to argue there is no evidence that fume-related toxins are released in sufficient quantities to cause significant harm.

But a recent FAA-funded study found two chemicals that can: formaldehyde, a known carcinogen and neurotoxin, and tridecane, which can cause headaches and stupor. Another, an organophosphate called tributyl phosphate, was right at the limit, according to a Navy toxicologist’s review of the findings. The analysis assessed 129 chemicals of which 40 have yet to be given an exposure threshold in the U.S.

Researchers also said that a real-world leak of the same quantity of oil would likely produce significantly higher levels of toxic chemicals than they measured.

“The chemicals exceed the worker threshold, which means they far exceed the threshold to the general public,” said Joseph Allen, an air quality specialist at Harvard University who has worked on previous FAA-funded bleed air studies. “It’s clear to me that there’s concerning data in these studies and it’s inappropriately downplayed.”

How much oil leaks in a fume event can range from small droplets to liters. Researchers could only be confident that the amount of oil tested fell somewhere between those extremes, said Byron Jones, one of the architects of the FAA-funded study.

Still, Jones, a now-retired engineering specialist at Kansas State University, said enough research has now been done. “It’s time to start doing whatever the next step is going to be,” he said.

Last year, FAA investigators formally acknowledged the toxicity of bleed air contaminants in an internal report into two Southwest Airlines flights that collided with large birds in 2023. The strikes exposed a design flaw in the aircraft’s engines that led to liters of oil being dumped into the bleed air supply and risked exposing pilots to potentially lethal concentrations of chemicals “at just 39 seconds.”

In a statement, the FAA said a review board in November decided the design didn’t require an immediate fix. Instead, the regulator alerted airlines and advised them to evaluate procedures and crew training before it mandates a permanent fix.

‘Project Fresh’

A handful of airlines have acted. Allegiant has encouraged flight attendants to use emergency oxygen if they’re exposed to fumes and feeling unwell. Alaska Airlines has asked crew to keep quick-info cards for medics behind their ID badge. In Europe, Portugal’s TAP has been urging its engine-makers and Airbus to investigate safer oils, including one awaiting certification.

Airbus, meanwhile, gave an update to A320 customers last year about an internal program called “Project Fresh” that had found the vast majority of cases were being caused by leaks entering an air vent on the aircraft’s belly. Airbus outlined three fixes, the most radical of which moves the inlet to the top of the jet and which would reduce “smell in cabin” events by 85%, according to an internal presentation.

That fix would be ready from the first quarter of 2026, Airbus said, but available only for newly built aircraft. Airlines had been pushing Airbus to make the change since at least 2019, documents show. 

“We are committed to continuously enhancing our products, working closely with operators and regulators to ensure the best possible cabin environment for passengers and crew,” Airbus said in its statement.

While the Journal’s analysis identifies the A320 as the worst-affected model, engines have been used to prepare cabin air on almost every jet-powered aircraft since jetliners entered service in the 1950s. For almost half a century, smoking on flights helped mask the odors from fumes.

One exception is the 787, Boeing’s first all-new design since smoking on commercial planes was fully banned in the U.S.

An email chain from Boeing’s early marketing meetings for the plane showed managers were grappling with how and whether to promote the new design. In one marketing brief it was referred to as “removing gaseous contaminants” from the air supply. 

One executive was concerned that “if we elaborate on the 787 air purification and say how great and important it is,” he’d be asked why the system wasn’t available on Boeing’s other models, according to a deposition citing internal Boeing documents.

‘If there’s no problem, they should have nothing to fear.’

The wider industry has fought to downgrade or kill new regulations, documents show.

In 2022, airline lobby group, IATA, urged members to press their governments to vote down new toxic exposure standards in the European Union, warning they “may be used in legal proceedings against airlines and by unions.” 

Meanwhile, after a separate five-year campaign, Boeing won an exemption for aviation oils from new chemical restrictions introduced by the Environmental Protection Agency in November.

For its part, Congress has attempted at least 19 times over more than two decades to tackle bleed air contamination, according to a deposition transcript of Boeing’s 737 chief product engineer, Julie Brightwell. 

Of those efforts, only a handful have been successful. Among the first was a 2003 project that asked crew to carry soup can-sized sensors to measure toxicity levels during a fume event. The plan failed after U.S. carriers argued it would distract flight attendants from safety duties.

“It’s not like Congress hasn’t tried,” said Zoe Littlepage, the lawyer who deposed Brightwell and who has settled seven suits against Boeing on behalf of 30 crew and passengers.

Congress’s attempt last year also drew objections from industry, according to Sen. Richard Blumenthal (D., Conn.), who introduced the bill with Rep. John Garamendi (D., Calif.). Where the original bill asked for sensors and dedicated investigations, the final law requires only new research and better reporting.

“If there’s no problem, they should have nothing to fear installing detectors and monitors,” Blumenthal said.

The FAA in a statement said it had made progress addressing those new requirements.

Congress is trying again. A bipartisan bill reintroduced last month would phase out the use of bleed air and require specialized filters on aircraft within seven years.

‘I don’t have time for bitterness.’

In early June, Susan Michaelis, a 62-year-old former pilot, sat in her garden at her home an hour south of London wearing blue-and-white striped shorts that exposed a tattoo of a heart on each ankle. She was connected to an oxygen supply via a translucent green cable she referred to as her “dog leash.”

A few days earlier, Michaelis was told by her oncologist that her last round of chemotherapy had failed. As she left his office, she was called aside by two nurses who suggested she think about getting her affairs in order.

Michaelis had wanted to be a pilot since she was a child, when she’d insist that her parents drive to Melbourne Airport, near their home, to watch aircraft take off and land from the family car.

After getting her license years later, she’d spend her first few years flying small turboprops before being offered a dream role flying regional routes in Australia on a BAe 146.

She noticed the fumes on her first day, a near-constant smell that permeated the flight deck. After the first few weeks, she started to develop headaches, which she passed off as stress. After a few months, the dizziness and nausea began.

On July 23, 1997, she raced home from the airport and collapsed on her couch. It was the last time she would pilot an aircraft.

“I thought I was having a stroke, like an elephant was on my head,” Michaelis said from her garden, while she slowly made her way through a tomato and mozzarella salad her husband had prepared.

Documents uncovered later show that the year before Michaelis got the call-up, the manufacturers of the BAe 146 and its power units agreed to pay compensation to two Australian airlines for persistent oil leaks. The deal included a confidentiality clause.

In 2004, the FAA issued a directive ordering all BAe 146 operators to conduct ongoing checks and cleaning to “prevent impairment of the operational skills and abilities of the flightcrew caused by the inhalation of agents released from oil.” BAE Systems didn’t respond to requests for comment.

Michaelis was diagnosed with a slow-growing form of lobular breast cancer in 2013. She was surprised. She’d never flown at high altitudes or latitudes that can expose crews to high levels of radiation, and she’d also only flown regionally, limiting disruption to her circadian rhythm that has also been linked to the disease.

She underwent two DNA tests at the University of Washington to see if she had any genes that might have predisposed her. She didn’t. Her doctors told her they were confident her cancer was related to chemical exposure on the aircraft.

At the time, Michaelis was already testifying at government hearings about fume events. She had earned a Ph.D. in occupational safety and later qualified as an aircraft accident investigator.

In 2017, she was the lead author on a paper in the academic journal Public Health Panorama, which is published by the World Health Organization, that proposed a new fumes-related disease called “aerotoxic syndrome.” Five aviation medical groups together subsequently challenged the paper’s findings and methodology and accused the researchers of bias.

In 2021, Michaelis was told her cancer had metastasized.

“I know now I might have a year. I might have less, I might have way less,” Michaelis said in June, discussing plans to wrap up a research paper exploring links between oil fumes and breast cancer. “I don’t have time for bitterness.”

Michaelis died a week later.