A study found that subway air is heavily polluted, exposing transit workers and riders to high concentrations of hazardous metals and harmful pollutants.

Researchers from New York University’s Grossman School of Medicine say subway air could make passengers and transit workers more susceptible to long-term illnesses such as asthma, lung cancer and heart disease.

The study, funded by a National Institute of Environmental Health Sciences grant and published in the Environmental Health Perspectives journal on Wednesday, reviewed underground air at the largest transit systems in the Northeast, located in New York-New Jersey, D.C., Philadelphia and Boston. Each system showed concentrations of hazardous metals and organic toxins that ranged between two and seven times the amount in outdoor fresh air.

The study found that while the New York-New Jersey system contained more air pollutants than others, Metro’s Capitol South station was among the five worst stations that researchers monitored.

Researchers said transit workers who spend most of their shifts below ground are especially susceptible to the long-term consequences of polluted air.

The findings surprised officials at the Port Authority of New York and New Jersey, where some of the worst measurements were taken.

“We’re obviously concerned,” Executive Director Rick Cotton said. “We are totally committed to protecting the health and safety of our workers. We are totally committed to protecting the health and safety of PATH riders, and we will dig into this, come to conclusions, and if necessary, develop an appropriate action plan to address it.”

Other transit agencies responded to the findings by saying projects are underway to improve underground air quality.

Metro said upgrading the filtration in all 308 of its air-conditioning systems is nearly complete. The systems trap small particles, the agency said, including the coronavirus.

“Metro regularly tests air quality in tunnels, stations and station manager kiosks, and the most recent tests conducted last fall showed air quality within OSHA Occupational Exposure Limits,” Metro spokesman Ian Jannetta said in a statement. He noted that work was underway to test more-stringent filtration in rail cars.

Major ventilation improvements and upgrades are underway at Back Bay Station in Boston, said Lisa Battiston, spokeswoman for the Massachusetts Bay Transportation Authority.

“The health and safety of riders and the workforce is a top priority for the MBTA, including issues that involve ventilation and air quality within its stations and on platform areas,” Battiston said.

Similar improvements are underway at the Southeastern Pennsylvania Transportation Authority, officials said.

Tim Minton, communications director for New York’s Metropolitan Transportation Authority, said that NYU researchers sampled fewer than 1 percent of the system’s stations, adding: “We have conducted previous air quality testing on subway trains operating in our system and found no health risks. However, we will thoroughly review this study as the safety of customers and employees is always our highest priority.”

The air quality study started in early 2020 just before the coronavirus pandemic. But the findings support multiple transit systems’ more recent efforts to improve air quality onboard trains as part of efforts to limit the spread of viruses and other pathogens.

Metro is using a $600,000 grant from the Federal Transit Administration to test and evaluate enhanced filters in its subway cars.

The Bay Area Rapid Transit system in the San Francisco area has been trying denser filter panels to trap smaller particles as well as new disinfectant technology that uses ultraviolet light. New York’s MTA is expanding new filtration systems on trains that can trap viruses and cycle air in and out of subway cars every three minutes.

Researchers found that one underground station platform on the Port Authority Trans-Hudson line between New Jersey and Manhattan had up to 77 times the typical concentration of hazardous particles as aboveground city air — equivalent to breathing in a forest fire or near a building demolition, the report said.

“As riders of one of the busiest, and apparently dirtiest, metro systems in the country, New Yorkers in particular should be concerned about the toxins they are inhaling as they wait for trains to arrive,” co-senior study author Terry Gordon said in a statement.

At other transit system stations, researchers found at least twice as many airborne toxins in subway air as in fresh air. Iron and organic carbon made up three-fourths of the pollutants found, researchers said. Organic carbon has been linked to increased risk of asthma, lung cancer and heart disease, according to the study.

“Our findings add to evidence that subways expose millions of commuters and transit employees to air pollutants at levels known to pose serious health risks over time,” David G. Luglio, the study’s lead author and a doctoral student at NYU Grossman, said in a statement.

Researchers took more than 300 air samples during rush hour at nearly 70 stations.

According to the findings, the PATH-New York and New Jersey system measured particle concentrations up to 392 micrograms per cubic meter. Air at MTA stations in New York measured as high as 251 micrograms per cubic meter. Metro had the next-highest level, up to 145 micrograms per cubic meter.

Air in Boston stations measured up to 140 micrograms per cubic meter, while Philadelphia had the cleanest system, measuring up to 39 micrograms per cubic meter, researchers said.

By comparison, aboveground air in the same cities averaged 16 micrograms per cubic meter of pollutants. The Environmental Protection Agency has said daily exposure of fine particle concentrations at levels 35 micrograms per cubic meter or more pose serious health hazards, according to the study.