An airplane cabin is many things: lounge, snack bar, bedroom, movie theater, duty-free shop. But it is also a chemistry lab of sorts, as a new study published in Environmental Science and Technology shows.

The study, by Charles J. Weschler of the University of Medicine and Dentistry of New Jersey and colleagues, looked at what happens to ozone in a cabin full of passengers. Their finding is it reacts with other chemicals to form aldehydes and other potentially irritating volatile compounds. What’s more, they found, it’s the passengers that help create the problem.

As planes fly at high altitudes, the air entering the cabin contains elevated levels of ozone. Many aircraft have equipment to eliminate the ozone, but many, particularly smaller planes, do not.

The researchers simulated flights of four hours at altitude, using a rebuilt section of a Boeing 767 cabin within a climate chamber. They found that during simulated flights with 16 passengers and ozone levels similar to those encountered in real flights (on the order of 70 parts per billion), the formation of volatile byproducts, including acetone, decanal and acetic acid, increased. The ozone reacts with compounds in the seats and carpeting. But the researchers say more than half the byproducts were a result of reactions with passengers’ clothing and natural oils on their skin and in their hair.

The researchers say their findings may have implications for other indoor environments where ozone buildup can occur and where people are present to create compounds that may affect health.