DENVER (AP) — Smoke from dozens of raging wildfires in western Canada has drifted south into the United States and prompted the states of Colorado and Montana to issue air quality alerts.

Colorado’s Department of Public Health and Environment put out alerts and advisories for Saturday afternoon through Sunday afternoon for much of the eastern half of the state, including Denver. It warned that air quality may be unhealthy during that period.

“People with heart or lung disease, older adults, and children should avoid prolonged or heavy exertion; everyone else should reduce prolonged or heavy exertion,” the department said.

Particle pollution led the air quality index along parts of the Front Range to reach 168 on Saturday, the department said. A reading between 151 and 200 indicates unhealthy conditions that can affect sensitive groups as well as some members of the general public.

An air quality alert was also in effect Saturday in Montana, with the greatest smoke concentrations in central and eastern parts of the state, according to the Department of Environmental Quality.

Utah’s Department of Environmental Quality said Friday that it was starting to see the smoke on its monitors in northern and eastern parts of the state. It urged residents to avoid outdoor exertion in areas with visible smoke or haze.

The smoke created widespread haze across Idaho earlier in the week, according to its Department of Environmental Quality.

The fires in Canada have been burning mostly in the province of Alberta, where thousands of residents have evacuated and regional officials have issued state of emergency alerts. There have also been fires in British Columbia.

In Calgary and Edmonton, the two biggest cities in Alberta, the health impact was determined to be of “very high risk” on Saturday by the Canadian government’s Air Quality Health Index. Sensitive groups such as children and older people were advised to avoid outdoor physical exertion and the general population was urged to limit outdoor activities.