Washington, D.C., awoke Sunday to some of the worst air quality in the world after Independence Day fireworks blanketed the region with pollution, prompting officials to warn residents to limit their time outdoors, CBS News reported.

The Metropolitan Washington Council of Governments issued a "purple" air quality alert for the District and parts of Northern Virginia, classifying conditions as "very unhealthy."

The alert advises people in sensitive groups to avoid all outdoor physical activity, while recommending everyone else avoid prolonged or heavy exertion outside.

Sensitive groups include people with heart or lung disease, older adults, pregnant women, children, teenagers, outdoor workers, and others who may be more vulnerable to air pollution.

Areas surrounding the capital were placed under an "orange" alert, indicating air quality was unhealthy for sensitive groups, while portions of Maryland received a "yellow" designation, meaning pollution levels were considered moderate and primarily affect unusually sensitive individuals.

Researchers attributed the sharp decline in air quality to fireworks displays held across the region Saturday night as part of Independence Day celebrations.

Air quality is expected to improve steadily over the next several days.

According to IQAir, a Swiss company that tracks global air pollution, Washington ranked as the world's sixth-most-polluted city Sunday, with Lahore, Pakistan, topping the list.

The deterioration in air quality extended beyond the nation's capital.

IQAir said major U.S. metropolitan areas experienced elevated pollution levels overnight following July Fourth fireworks displays, with Detroit briefly becoming the world's most polluted city early Sunday morning. Chicago and Minneapolis also ranked among the world's most polluted cities overnight.

Washington's Independence Day celebration was larger than usual this year as part of events marking the nation's 250th anniversary.

The celebrations unfolded amid an intense heat wave affecting much of the country. Temperatures climbed into the triple digits across much of the United States, including the Washington region, forcing organizers to temporarily suspend some scheduled events because of dangerous heat.

The National Weather Service kept a heat advisory in effect for Washington and parts of Maryland and Virginia through 8 p.m. Sunday, warning of hazardous conditions as residents contended with extreme temperatures and poor air quality.