Kang Sun, an assistant professor at the University at Buffalo studying the chemistry and physics of the Earth’s atmosphere, explained to The New York Times that when biomass like branches, trees, and leaves burn, they spew volatile organic compounds (VOCs) into the air.

When these VOCs are first created, their reaction with the atmosphere causes the campfire smell typically associated with wildfires. As the VOCs linger in the air, however, they interact with ultraviolet (UV) radiation to create pollutants like benzene and formaldehyde, which smell like burning plastic, as explained by The Weather Network.

— If you're in the path of the smoke from the Canadian wildfires, read more in Residents across multiple states notice strange and worrisome new scent in the air: ‘Certainly a health concern’ at Yahoo News.