Last week’s fiery explosion at the Philadelphia Energy Solutions refinery drew Philadelphians’ attention to the size and scale of the 1,400-acre complex, the largest on the eastern seaboard.

But those who live near it have long worried about what the PES plant releases into the air, and many wonder if high rates of asthma and other health issues are linked to the facility. Friday’s fire only heightened those fears.

“Why does it take a series of explosions for this to come out?” asked Mollie Michel of the nonprofit Moms Clean Air Force, who lives in South Philadelphia with her husband and two daughters.

City reports show the refinery, which processes 335,000 barrels of crude oil a day, was already Philadelphia’s biggest single air polluter even before last week’s fire sent plumes of black smoke skyward.

In a 2017 “Powering Our Future” report, the Office of Sustainability noted, “For nearly all particulate pollutants, the single-largest source of local air pollution is the Philadelphia Energy Solutions refinery.”

Federal data paint a similar picture. The U.S. Environmental Protection Agency’s Toxics Release Inventory (TRI) tracks toxic chemicals that may pose a health threat to and requires certain industrial facilities to report how much of each chemical is used, combusted, disposed of, or released. According to TRI data, the refinery is by far the biggest releaser of chemicals into the air in Philadelphia.

A PES spokesperson had no comment.

Philadelphia officials, charged with monitoring the air quality at the site, have acknowledged the facility poses a challenge.

The Office of Sustainability report noted that Philadelphia is the 12th most polluted city in U.S. by particulate pollution, a mix of solid and liquid particles that get into the air. It said the refinery “accounts for more than 50 percent of local emissions for each of those pollutants.”

Particulate matter, also referred to as particle pollution, when inhaled can cause serious health problems. Some particles, such as dust, dirt, or soot, are visible to the human eye. Others are not. Smaller particles pose the biggest health risk. They are also the biggest cause of haze in the United States.

The report also found that the refinery is the single largest contributor of emissions, including carbon dioxide.

“While not a particulate pollutant, carbon dioxide (the primary contributor to global climate change) is also emitted at the local level. Again, the PES refinery is the single-largest source of carbon emissions citywide,” the report stated.

Overall, motor vehicle emissions are the largest collective source of pollution in the city, officials said.

The city health department said Friday after the fire was brought under control that the air quality around the facility was not an immediate health threat. The department’s Air Management Services has permanent monitors throughout the city that it uses for air quality checks. The closest monitor to the refinery is at 24th and Ritner Streets.

Additionally, James Garrow, a department spokesperson, said monitor readings from Saturday and Sunday detected no hydrogen sulfide, carbon monoxide or hydrocarbons. The department also looked at an air monitor in Camden and did not detect levels of concern there.

The highest levels of particulate matter occurred Friday at 7 a.m. as the fire was raging. But the readings did not reach a level of concern, officials said.

However, Air Management Services, which also issues violations for air pollutants, has repeatedly flagged the refinery for its emissions in the recent past. It found the refinery had “High Priority Violations” of the Clean Air Act in nine of the last 12 quarters.

Garrow said High Priority Violations of the Clean Air Act “are those which warrant additional scrutiny to ensure local state and federal agencies respond in an appropriate manner.”

Some of the pollutants noted by the city during monitoring included volatile organic compounds such as, ammonia, sulfur dioxide, hydrogen sulfide, carbon monoxide, nitrogen oxides, and carbon. Air Management cited other “visible emissions” and noted odors.

Separately, the EPA cited compliance issues at the facility multiple times during the last five years regarding the Clean Air Act, Clean Water Act and a solid waste disposal issue law, issuing a total of $649,417 in fines.

Another major concern for residents and environmental groups wary of the facility: A unit uses hydrofluoric acid as a catalyst, one of the most toxic materials handled in the refinery. In its gaseous state, hydrogen fluoride (HF), can drift beyond the refinery fence line and imperil the public. City officials said no HF was released during the fire.

Environmental justice advocates have also taken aim at the facility within the past few years.

In 2017, the NAACP and Clean Air Task Force issued a joint report called “Fumes Across the Lines” that singled out refineries as having an outsized impact on African American communities regarding air quality, including ozone smog and high asthma rates. Of all metro areas in the U.S., Philadelphia had the eighth highest amount of asthma attacks on African American children.

The report cited the PES complex as being responsible for 72 percent of the toxic air emissions in Philadelphia and is a factor in the city’s childhood asthma rate, which is more than double the national average. The report said toxic chemicals released from the refinery also are known to cause a range of other health effects including headaches and cancer.

Michel, of Moms Clean Air Force, said she’s frustrated by a “lack of transparency” regarding emissions from the facility and impacts on residents.

“We do not have a lot of information for the surrounding community, which includes some of the city’s poorest residents,” Michel said.