Rimmed by refineries, snared in a spiderweb of highways, the Philadelphia region has some of the smoggiest number days in the northeast, says a report commissioned by the nonprofit advocacy group PennEnvironment Research & Policy Center.

However, that doesn't necessarily mean those smoggy days are unhealthy to all.

The metro area in the study included Philadelphia, Camden and Wilmington. The region, not normally thought of as a smog center, nonetheless saw 97 days with elevated levels of the pollutant -- also known as ground-level ozone -- in 2015, the most recent year available.  On that measure, it ranked second highest in the northeast behind the Washington, D.C. metro area with 99 days.

If there was better news hidden in the data for Philadelphia, most of those 97 smoggy days posed only a moderate health risk, meaning they were elevated - but still at acceptable levels. Twenty-three days were considered unhealthy for sensitive groups, such as the young, old and those with existing health problems. Just two days were considered unhealthy for everyone.

Riverside, Calif., had the worst smog in the nation, with 233 days of elevated levels.

Smog is caused by fossil fuel combustion, such as car exhaust, mixed with heat and sunlight. It is associated with health problems such as respiratory irritation and asthma.

The Philadelphia region ranked high nationally in particulate matter, which are a health risk because they can be inhaled deeply into the lungs.  It was fifth highest in the nation with 212 days of elevated levels of those pollutants.  Pittsburgh ranked third, with 211 days.

Particulate matter can be emitted from nearby sources, such as construction sites and power plants.

"The report shows that air pollution is still a major threat to our health," said Ash Khayami of PennEnvironment. "You can't just chose not to breathe the air."

PennEnvironment presented the report Thursday at City Hall. Gretchen Dahlkemper, one of the speakers, said she watched her then 10-month-old daughter turn blue with her first asthma attack when the family lived in the Pennsport section of the city, in the shadow of I-95.  They have since moved to Point Breeze. Fiona is now 5, and her mother said the child's asthma is "more manageable now."

"No mother should have to watch her daughter turn blue," said Dahlkemper, who went on to become executive director of Moms Clean Air Force.

The study examined EPA data on air pollution across the United States, grouped by metro and smaller areas, as well as by state.  The report was written by the Frontier Group, a think-tank that says it hires across the political spectrum, and the nonprofit Environment America Research & Policy Center.

Also speaking were city council members Derek Green and Blondell Reynolds Brown; Walter Tsou of Philadelphia Physicians for Social Responsibility; and, Jack Groarke from the office of U.S. Sen. Bob Casey.

The group said the data show that President Trump's rollback of environmental programs such as the Obama-era Clean Power Plan, which sought to curb carbon emissions,  could prove harmful.