Internationally, there have been similar improvements, said Steven Lott, a spokesman for the International Air Transport Association. Since the 1960s, carbon monoxide, unburned hydrocarbons, smoke and nitrogen-oxide emissions have been substantially reduced, he said. Standards for most of them are set by the International Civil Aviation Organization, a United Nations body.

“It’s a challenge for an industry that continues to grow,” Mr. Lott said. But the industry has invested in developing cleaner aircraft engines and ground-support equipment and vehicles as well as improvements in how equipment is operated. Initiatives like its Green Teams, for example, allow industry consultants to visit airlines to identify and share ways to reduce fuel burn and emissions. More than 105 airlines have participated, he said.

Still, some community groups are not persuaded that air quality at airports has improved.

“It’s way worse than people think,” said Debi Wagner, a board member of Citizens Aviation Watch USA, who lives in Seattle. Some emissions are not adequately sampled and measured, Ms. Wagner said, and other potentially dangerous ones are not monitored at all. She said she was concerned particularly about the health of people living within three miles of commercial airports.

Two recent studies also raise questions about the quality of air at airports. Both focus on small general aviation airports, like the one in Santa Monica, Calif., which was studied in both reports.

“The traditional pollutants did not seem to be a local issue,” said Philip Fine, atmospheric measurements manager for the South Coast Air Quality Management District, an air quality regulatory agency for most of Southern California. “However, there were issues for ultrafine particles and lead.”

Dr. Fine, who oversees a network of air-monitoring stations, was a lead researcher on a study financed by the Environmental Protection Agency that is to be released in the next few weeks.

The lead levels from non-jet aircraft emissions did not exceed federal limits, but were significantly elevated, Dr. Fine said. Elevated levels of ultrafine particles, primarily from jet aircraft, were also a concern. The particles are short-lived, but because they are in high concentration down wind during takeoff, they are particularly worrisome for people who live close to small airports or who are repeatedly exposed, he said.