Early results indicate that bicyclists in lanes that are separated from active traffic by a row of parked cars breathe in a lot less pollution than those who use bike lanes adjacent to the traffic. The researchers are also finding perennial pollution hot spots, like the spiraling approach to the Manhattan Bridge that Dr. Jack encounters on his daily ride. “The looping highways on all sides funnel the bad air” he says. “I’m riding uphill, breathing hard, it’s a perfect storm of negative factors.”

Bridges, where traffic bottlenecks are common, and the interior of Manhattan, which is buffeted by fewer refreshing breezes than the island’s periphery, are also prone to higher pollution levels. The city’s roads are generally more polluted during the morning rush hour than during the evening rush hour, when winds tend to be greater.

But just as important as the level of pollution in an area is the effort exerted by a bicyclist to pedal through it. “We know that just walking we are breathing in two to three times the air as we are when we are sitting,” Dr. Chillrud explained. Cycling and other strenuous activities like jogging and playing basketball boost the volume of air — and therefore the particulates — that we are inhaling. Dr. Jack, for example, breathes in roughly eight liters of air per minute when he is resting; when he cycles that volume soars to 70 liters. Biking hard, uphill or fast increases one’s pollution intake still further.

Another consideration is that the impact of air pollution varies a lot from person to person. “If you have a lung disease like asthma, cardiovascular problems or diabetes, or if you are a young child, a teen or elderly, you will likely be more susceptible to harm,” says Janice Nolen, the assistant vice president for national policy at the American Lung Association. “There is also evidence that women — whose lungs are slightly smaller than men’s — are more affected by pollution.”

Ms. Nolen said the Columbia study will provide much-needed information but cautions that people who participate in such research tend to be young, healthy and male, so the results may not accurately represent the population at large. Indeed, Dr. Jack said, the Columbia study’s volunteers do skew young and male. “The good news is we’re getting cleaner vehicles and less pollution,” Ms. Nolen said.

Stephanie Chan, a 42-year-old physical therapist who is volunteering in the study, says that while she finds it exhilarating to bike to work, “I’ve always been aware that you are right in the middle of the traffic breathing in the car exhaust. On a hot humid day, I get home and my scalp is gritty, and I’ve got dirt on my face.” While the study has made her think more about pollution risks, she doubts that it will cause her to change her behavior. “It’s just one of those things where you take your chances,” she says.

Audrey de Nazelle, an expert in risk assessment at the Centre for Environmental Policy at Imperial College London, said that while the benefits from exercise are slightly diminished by pollution, it doesn’t negate them in healthy adults. “Exercise actually lowers the rate of inflammation in the body, which exposure to pollution raises. So the relationship is complicated,” she said.