The wildfires in Canada have caused another thick layer of smoke to descend upon parts of the United States, prompting air quality alerts across 16 states and Washington, D.C.

According to a Wednesday situation report from the Canadian Interagency Forest Fire Center, at least 30 new wildfires broke out in Canada’s forests Tuesday to bring the number of active wildfires across the country to nearly 500, with about 250 being labeled as out of control. The dense smoke began to cover parts of the northern Plains, the Great Lakes region, Ohio Valley and mid-Atlantic on Tuesday and Wednesday.

According to the National Weather Service (NWS), parts of Minnesota, Iowa, Michigan, Wisconsin, Indiana, Ohio, Kentucky, Tennessee, Georgia, North Carolina, Virginia, Delaware, Maryland, Pennsylvania, New Jersey, New York and Washington, D.C., were all under air quality alerts because of the fires as of early evening Wednesday.

“Air quality advisories and alerts currently stretch from the Upper Mississippi Valley, through the Mid-West, Great Lakes, Ohio Valley and into the Mid-Atlantic,” NWS said in its forecast. “With no end in [sight] to the Canadian wildfires and west to northwesterly winds expected to persist from south central Canada into the north central to northeast U.S., poor air quality conditions are likely to continue.”

According to IQAir, four of the 10 worst major cities in the world for air quality as of Wednesday afternoon are in the United States. After Toronto, Chicago ranks second worst with an air quality rating of 181, which falls within the unhealthy range. Detroit closely follows as the third worst with a rating of 179, while Minneapolis ranks fifth and Washington, D.C., ranks eighth.

Accuweather also reported that Milwaukee measured its worst air quality since 1986 on Wednesday, according to the Environmental Protection Agency.

Parts of Wisconsin, Illinois, Indiana and Ohio are seeing “very unhealthy” levels of air quality, according to AirNow. The service also reported that a part of Ohio south of Cleveland is seeing “hazardous” levels of air quality.

The Kentucky Energy and Environmental Cabinet said in its air quality alert that, “Everyone may experience health effects,” noting that those in sensitive groups, including the elderly, children and people with breathing problems, may be more susceptible to more serious health effects.