Canada wildfire season declared worst on record

• Smoke from the fires has triggered multiple air quality alerts in Canada and across the U.S. since the country's worst-ever fire season began in April.
• Chicago, D.C., and Detroit were among the latest cities to endure some of the world's worst air quality on Thursday morning.

Threat level: Inhaling high concentrations of wildfire smoke at ground level can cause serious health complications and is particularly dangerous for elderly populations and those with heart ailments or asthma.

• Wildfire smoke poses a threat to people's health even when hundreds of miles from the fire sites because of its harmful microscopic particles, studies show.

Context: Multiple studies show human-caused climate change is a key driver behind increased wildfire risk and that heat waves are more likely to occur, be more intense and last longer due to this.

Go deeper: Canada's wildfire smoke emissions break records