Smoke from wildfires in western Canada has led to air quality alerts in four U.S. states, with the Canadian Interagency Forest Fire Centre reporting that Canada has 146 active wildfires burning, including dozens in British Columbia and Alberta that are characterized as “out of control.”

In the United States, Montana, North Dakota, South Dakota and Minnesota have issued alerts as winds carry smoke over these regions.as winds carried plumes of smoke over the region. On Sunday, the Minnesota Pollution Control Agency issued an air quality alert in response to a “band of very heavy smoke from wildfires in northeast British Columbia.” The agency said the northern half of the state has since cleared up, but “smoke will linger over southern Minnesota on Monday as northerly winds become light during the day.”

The air quality index, established by the U.S. Environmental Protection Agency to measure daily air pollution levels and communicate the associated risks, reported on Monday that conditions across those four states ranging from “moderate” to “unhealthy” for the general public.

Authorities issued an evacuation order for thousands of residents in Northern Rockies Regional Municipality and Fort Nelson First Nations in British Columbia because of fast moving blazes. According to the Canadian Interagency Forest Fire Centre, Canada experienced its most devastating wildfire season in recorded history last year, with more than 45 million acres burned; and last summer, smoke from wildfires in Quebec affected wide portions of the U.S., sending air quality down into dangerous levels in cities across the Midwest as well as the Eastern Seaboard. The country is once again bracing for a season of increased fire risk.

Small particles in smoke, less than 2.5 micrometers in diameter, can reach deep inside the lungs, causing inflammation and weakening the immune system. This can exacerbate or increase the risk of asthma, lung cancer, and other chronic lung diseases. Older people, infants, children, and pregnant individuals are most vulnerable when air quality worsens.

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