With summer approaching, Philadelphia’s air quality will worsen as cars spew pollutants and sunlight kicks off a chemical reaction that causes ozone levels to soar.

And Center City bears the brunt in summer and all year-round, according to the Philadelphia Air Quality Survey, a first-of-its-kind study.

The Philadelphia Department of Public Health’s Air Management Services started collecting data in 2018 from 50 air-monitoring locations set up across the city to gauge air pollution by neighborhood, the first time it tried to drill down for those data.

Overall, the report written last year found that pollution in the city is generally below levels set by the U.S. Environmental Protection Agency, which requires less specific monitoring than what the city is doing. Compared with other metro areas, Philadelphia isn’t particularly high in health-endangering pollutants, according to a recent American Lung Association report.

But when you look at Philly pollution overall, you’re also including leafy and less populated areas. For this newest look, Air Management Services placed monitors throughout the city, concentrating mostly in central Philadelphia because of the density of traffic, buildings, and people. Weather-proof monitors were installed 10 feet up on roadside utility poles to detect fine particulate matter (PM2.5), ozone, nitrogen dioxide, and sulfur dioxide. The monitors also recorded temperature and humidity.

Shannon Capps, an assistant professor at Drexel who specializes in atmospheric modeling, credits the city with trying to gauge pollution at a micro-level and “capture its impact on a variety of neighborhoods.”

Air Management Services issued a report on its findings in October. It found that air pollution varied around the city but that levels rose in the summer.

James Garrow, a spokesperson for the health department, said the report was based on a single year of data, “but we are continuing to use and invest in this system. The current PAQS network is growing and we anticipate being able to place more monitors in the future, in more locations, to better understand what’s happening at a hyper-local level.”

PM2.5

Particulate matter, or PM2.5, defined as concentrations of 2.5 microns or smaller, is of special concern because the particles are so small — far smaller than the width of a human hair — that they are easily inhalable. The World Health Organization says PM2.5 is responsible for the biggest proportion of health effects from air pollution.

Locally, the biggest 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.

Health effects can stem from short or long exposure, and can range from aggravation of asthma and other respiratory illnesses to premature death in people with chronic heart or lung diseases. Children and seniors are most vulnerable.

In general, the monitors detected that PM2.5 concentrations in Philadelphia were highest in summer, followed by winter, then spring and fall.

The air monitors continued collecting data during the pandemic when traffic was lighter, but although pollutants were lower, the seasonal trend was the same. As restrictions have eased, traffic is getting back to its usual congestion.

The EPA standard for annual average concentration of PM2.5 is 12 micrograms per cubic meter. WHO has a more stringent standard of 10.

An air monitor near City Hall recorded a level of 10.1, the highest PM2.5 level of all 50 sites throughout the city.

Parts of Northwest and Far Northeast had lower levels; the citywide average was 7.7.

Not only is Center City clogged during weekdays, but it also is bordered by I-95 to the east, the Schuylkill Expressway to the west, and the Vine Street Expressway traverses its length. PM2.5 concentrations spike as cars are stuck in traffic.

The Inquirer has previously documented the impact of traffic in Center City. Prior to the pandemic, nearly 7,000 vehicles a day traveled Chestnut Street east of Broad Street. Idling delivery trucks add to choking emissions.

Ozone

Ground-level ozone is another pollutant of concern. It forms when nitrogen oxides and volatile organics -- both emitted by vehicles, power plants, and industry -- react in sunlight, especially during hot days.

Ozone can cause airways to constrict, leading to shortness of breath, and so it can especially affect those with asthma, emphysema, and chronic bronchitis.

Nitrogen oxide (NO2) exposure is linked to “increased emergency department visits and hospitalizations for respiratory conditions, particularly asthma,” according to the report.

The EPA standard for NO2 is an annual average not more than 53 parts per billion.

The city took readings for both nitrogen oxides and ozone as key indicators of air pollution at 16 sites around Center City, mostly near I-95, I-76, South Front Street, Columbus Boulevard, as well as Roosevelt Boulevard.

The highest 12-month average for nitrogen oxides, 20.2 ppb, was recorded at City Hall. The average for all sites during the 12-month period was 13 ppb.

Historically, about 40% of nitrogen oxides emitted in Philadelphia came from vehicles. But that is likely higher now that one source of historical nitrogen oxides, South Philly’s PES Refinery, has been shut down since it exploded and caught fire in 2019.

As for ozone, the monitors recorded levels below the EPA standard of 0.070 parts per million averaged over eight hours.

Capps, the Drexel professor, wondered whether the placement of some monitors could have skewed results, and why nitrogen oxides and ozone were sampled at only 16 sites. She said the city should release its raw data so scientists can more closely examine them.

Joseph Otis Minott, executive director at the Clean Air Council, said the city needs to focus on up-and-coming sources of air pollution, such as redevelopment of the city’s industrial areas with warehousing and fulfillment centers that draw heavy truck and car traffic.

“We really focused on whether they are doing enough in environmental justice communities,” he said, referring to the fact that industrial areas often are in communities whose residents frequently can’t afford to move away. “Transportation is what really the city and most of the country needs to focus on.”