Wildfire smoke from Canada hovered over large portions of the United States on Wednesday, creating hazy skies and hazardous air quality from the Northeast to the Southeast.

The National Weather Service issued air quality alerts on Wednesday for New York City and many surrounding areas, including New Jersey and most of Connecticut. Officials said that those more sensitive to air pollution should limit their time outdoors and watch for respiratory symptoms from 11 a.m. to 11 p.m.

The most concentrated area of smoke on Wednesday was over a strip spread from Iowa and Illinois into Wisconsin, with Milwaukee experiencing some of the worst air quality in the country.

The U.S. Environmental Protection Agency warns that wildfire smoke contains dangerous pollutants in the form of fine particles known as PM 2.5 that can penetrate deep into the lungs and enter the bloodstream. Short-term exposure can lead to bronchitis, worsen asthma and create other health issues.

Despite those risks, people flocked to Madison Square Park in Manhattan on Wednesday afternoon, sprawling on the lawn in swimsuits and having lunch at picnic tables. More than a few business meetings seemed to have migrated to picnic benches in the shade.