Why does the wildfire smoke sometimes smell like burning plastic?

As wildfire smoke continued to cloak parts of the United States and Canada, some residents reported that they smelled a burning campfire. But other residents said they smelled burning plastic and chemicals outside. What was causing the strange smell?

Kang Sun, an assistant professor at the University at Buffalo studying the chemistry and physics of the Earth’s atmosphere, said that when trees, branches and leaves — also known as biomass — are burned in wildfires, they emit gases called volatile organic compounds.

And when these compounds are first emitted, some react quickly and create that recognizable smoke smell typically associated with a campfire.

But some compounds are stable enough to travel thousands of miles downwind to Michigan and upstate New York from the fires in Canada, Dr. Sun said. Other molecular compounds are also formed as byproducts of earlier chemical reactions.

Some of the molecules might be causing the chemical, plastic smell that many people have reported recently. When he opened a window in Buffalo on Wednesday himself, Dr. Sun said, he also smelled the acrid, burning smell of plastics and synthetics, rather than a campfire smell.

These longer-surviving compounds, such as benzene and formaldehyde, are “certainly a health concern,” said Michael Jerrett, a professor of environmental health science at the U.C.L.A. Fielding School of Public Health.

Both benzene and formaldehyde are Group 1 carcinogens, which means a panel of experts at the W.H.O. has found sufficient evidence that these compounds cause cancer. This could indicate the persistent wildfire smoke could potentially get more toxic as time goes on, Dr. Jerrett said, adding that there is limited research on the topic.

In the short term, the best thing people can do to stay safe is to remain inside as much as possible, Dr. Jerrett said. The compounds may not pose a health concern unless the smoke persists over a long period of time, he said.