Residents of Somerville's Nunnery Grounds neighborhood have long tolerated concrete vistas of Interstate 93 and Mystic Avenue, honking cars, and black grime on their windowsills. Now, they are increasingly worried about an invisible highway nuisance: the tiniest pieces of pollution emitted by passing traffic.

Community members in Somerville, as well as Boston's Chinatown, have joined with Tufts University researchers to determine whether microscopic "ultrafine particles" spewed by combustion engines are harming the health of people who live close to highways. In a study that is the first of its kind in the state and among the first in the nation, the scientists will measure these tiny pollutants in various locations and collect and map heart disease data from residents.

For decades, researchers have studied the dangers of coarse particles emitted by power plants and vehicles, which can aggravate asthma and other health problems and are regulated by the US Environmental Protection Agency. Ultrafine particles are less understood and not subject to direct government limits, even though many scientists believe they could be even more harmful.

"Since we see associations with asthma and cardiovascular disease with people living near highways, you have to ask what's causing that," said Doug Brugge, director of the Tufts Community Research Center and the scientist leading the study, which will begin this summer. "There is a lot of smoke suggesting that there is a fire."

Researchers suspect the health risk from ultrafine particles is greatest downwind and within 300 feet of busy highways. Farther away, the particles tend to dissipate or collide with other particles to become larger. The particles, each thousands of times smaller in diameter than a human hair, are easy to breathe in, and they readily spread throughout people's circulatory systems and enter their brains. On their surfaces, they can carry toxic chemicals and metals.

Eric Barry, whose house in east Somerville overlooks I-93, said he grew concerned recently after he read articles about the health risks of living close to the highway. He has had asthma since he was a child and worries that the pollution could exacerbate it. "This house is what we could afford," he said. "If I could live farther away, I would."

Last year, Brugge and colleagues found high levels of ultrafine particles in some Somerville neighborhoods near I-93 and Mystic Avenue, and deaths from heart and lung disease are more common in Somerville than they are on average in the state. But this does not prove that highway pollution is the culprit. People's genetic predisposition to disease, their habits, and their exposures to other environmental harms may be to blame.