DENVER (AP) — Numerous areas of the U.S. West and Midwest were under air quality alerts on Sunday as wildfire smoke lingered over much of the country.

Alerts were in place across much of the northern U.S. Rockies, including portions of Colorado, Wyoming, Washington state and Idaho.

Further to the east, smoke from fires burning into Canada triggered pollution alerts in Wisconsin, Minnesota and Iowa.

Wildfires emit huge volumes of microscopic smoke particles that researchers say can be harmful if breathed in and lead to both immediate and long-term health impacts. Children, the elderly and people with underlying health conditions are particularly at risk.