People in and around Washington, D.C., experienced a substantial dip in air quality on Sunday, with officials warning that any amount of exposure to the outdoors could potentially come with health consequences. Researchers connected the shift in pollution levels to massive fireworks displays held to celebrate July 4th on Saturday night.

Air quality in the region began to improve Monday and was expected to steadily return to normal throughout the week.

But on Sunday, a “purple” air quality alert was issued for the nation’s capital and parts of northern Virginia by the Metropolitan Washington Council of Governments, a nonprofit organization that brings together local leaders in the region. 

That distinction corresponded with “very unhealthy” air, accompanying warnings against all outdoor physical activity for people in sensitive groups and prolonged outdoor physical activity for everyone else. Sensitive groups include people with heart or lung diseases, older adults, pregnant people, children and teenagers, outdoor workers, and others who may suffer undue health impacts from the currently harsh environment. 

Suburban areas around D.C. received an “orange” air quality alert Sunday, meaning conditions were unhealthy for sensitive groups, who were advised to “make outdoor activities shorter and less intense,” according to a breakdown of the alert index. Air quality was better farther from the city, with parts of Maryland assigned “yellow” alerts for Sunday, meaning pollution was “moderate” and may only have been consequential for unusually sensitive people.

Washington, D.C., was ranked sixth among the world’s most polluted cities on Sunday, according to IQAir, a Swiss air quality technology company that monitors global pollution levels. Its position trailed Pakistan’s Lahore and Congo’s Kinshasa in first and second place, respectively, followed Jakarta, Jerusalem, and Tel Aviv, in that order.

In the United States, major metropolitan areas have seen declining air quality in the wake of fireworks celebrations held on July 4th, according to IQAir, which said its air quality maps showed increasing concentrations of air pollution across the country overnight Saturday into Sunday. The company said Detroit was briefly ranked as the world’s most polluted city early Sunday morning, while Chicago and Minneapolis neared the top of its global list, in addition to D.C.

The nation’s capital put on an especially elaborate Fourth of July spectacle this year, to commemorate the annual holiday and the 250th anniversary of American independence. Its packed event roster included what organizers had said would be the largest fireworks display ever seen in skies over D.C.

Saturday’s festivities across the U.S. were hampered to varying degrees by a treacherous heat wave that fueled triple-digit temperatures across large swaths of the country, including the capital, where event organizers temporarily paused scheduled programs because of the searing warmth. A heat advisory was in place for D.C., and nearby sections of Maryland and Virginia, until 8 p.m. ET on Sunday, according to the National Weather Service.