Philadelphia air quality is ‘unhealthy to breathe’ as wildfires in Canada burn; city pools are closed Thursday

The air quality in Philadelphia dropped to “unhealthy” levels for all residents Thursday as smoke from wildfires in Canada continued to move into the area.

Both Philadelphia and the entire state of Pennsylvania remained under a “code red” air quality alert Thursday, as winds from the west blew smoke-filled air from the Midwest into the region. Delaware and Camden County also issued “code red” alerts, while New Jersey remained under a “code orange” alert.

Late Thursday afternoon, Pennsylvania issued a statewide “code orange” for Friday, meaning the air would still be unhealthy but not as bad. Philadelphia did not issue any late updates but a spokesperson said city officials also expected the air quality to improve.

The air quality index in Philadelphia as of 8 p.m. Thursday was 172, according to government data, enough to place the city in the red “unhealthy” for all category. City pools were closed on Thursday, and summer camps run by the city were moved indoors.

“This alert means that the air is unhealthy to breathe, and some members of the general public may experience health effects,” the Philadelphia Department of Health said in a statement.

The city recommended residents consider canceling outdoor events and gatherings and close all windows and doors to minimize air pollution in the home. If you must go outdoors, the city suggested, wear a high-quality mask and avoid areas of high congestion, such as main streets or highways.

The city said residents who need a KN-95 mask can pick up one for free at the following locations:

Alex Dodd, a meteorologist at the National Weather Service, said forecast air quality levels would remain at red “unhealthy” levels for eastern Pennsylvania and Delaware throughout the day and into the evening Thursday.

“With west winds, it’s going to stay pretty consistent,” Dodd said. “We don’t really see it getting much better [Thursday].”

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Most of New Jersey is expected to have better air quality than Pennsylvania, and Dodd said a sea breeze at the Jersey Shore should offer even more relief.

According to the latest forecasts, the poor air quality is expected to linger into Friday, but winds from the south should help push away some smoke to the north. Dodd said rain and thunderstorms are expected to move in Friday night or Saturday, which should help improve the quality of the air.

“Rain is really efficient at pulling that particulate matter out of the air,” Dodd said.

The U.S. Environmental Protection Agency pulls data from gauges that measure various pollutants in real time. In this case, the EPA uses data collected by Philadelphia’s Air Quality Management Services.

The EPA sets limits for each pollutant for its air quality index (AQI). The level of each pollutant is color-coded based on the highest daily AQI value at selected monitors. The Philadelphia Health Department lists six levels, from lowest to highest: green, yellow, orange, red, purple, and maroon — a level considered “hazardous.”

Earlier this month, Philadelphia reached the maroon “hazardous” level, the worst air quality since at least 2008 if not before, according to historical EPA data. The poor air quality forced people across the city indoors and led to several events, including a Phillies game, to be canceled or postponed.

PM2.5 is fine particulate matter defined as concentrations of 2.5 microns or smaller. It’s of special concern because the particles are so small — far smaller than the width of a human hair — and are easily inhaled. The World Health Organization says PM2.5 is responsible for the biggest proportion of health effects from air pollution.

Locally, the biggest typical sources of PM2.5 are emissions from gas and diesel-powered vehicles. Power plants are also a source. The particles form when emissions react in the air.

But wildfires burning in multiple Canadian provinces and territories contain a mixture of gaseous pollutants and PM2.5 is a chief component.

Particle pollution refers to a mix of solid and liquid droplets in the air. Some particles are so small that they are visible using only an electron microscope. Those particles can penetrate both indoors and outdoors because of their small size. During wildfires, concentrations can rise so high that particles are visible to the naked eye as part of the mix of gases in a haze.

Staff writers Robert Moran and Frank Kummer contributed to this article.