IQAir, a company that tracks global air quality, ranked Detroit, Toronto and Minneapolis as the cities with the worst air quality currently in the world.

Smoke from wildfires is dangerous to humans because it contains a mixture of very small polluting particles such as PM2.5 and nitrogen dioxide.

“These particles, when they get into our system, they get into our lungs, into our little breathing tubes, and… they cause inflammation,” Jim McDonald, commissioner of the New York State Department of Health, said in a public video address on social media.

Smoke can aggravate existing respiratory, heart, kidney and eye diseases, according to the World Health Organization., external.

Emergency responders and vulnerable groups, such as those with chronic illnesses and children (who are closer to the ground, where smoke can sink), are most at risk.

Governments will issue warnings based on the level of air quality that will advise people on precautions to take. For example, an orange alert means that people are advised to:

KN95 or N95 masks should be able to filter out 95% of these small particles, although there are still risks.

Fraudulent copies are sold online, so make sure it is certified by the government.

Many cities have activated their emergency air quality protocols and are distributing masks at local centers.

For more information on local air quality alerts, see this site, external if you’re in Canada, and here if you’re in the US., external.