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.