Millions across the Midwest and Northeast are being warned to stay indoors as wildfire smoke from Canada blankets large parts of the US. The National Weather Service has issued orange and red air quality alerts for 12 states, urging people to avoid outdoor activities and keep windows closed.

Experts warn that even healthy adults can be at risk. Breathing in smoky air can cause coughing, shortness of breath, and, in severe cases, increase the risk of heart attacks or strokes. “The orange alerts recommend that people limit strenuous outdoor activity,” said NWS lead forecaster Mac Bhenard. Sensitive groups, including children, seniors, and those with lung or heart problems, should be especially careful.

States affected include New York, Michigan, Minnesota, Wisconsin, and more. The poor air quality is due to tiny particles from wildfire smoke, which can penetrate deep into the lungs and cause serious health issues.

Officials say the air quality warnings may stay in place through the week. Staying indoors and closing windows can help keep the air safer.