Steering clear of the busiest roads, even just by choosing a parallel route a block away, can halve the pollution you breathe, Dr. Kelly said. The Environmental Defense Fund, an advocacy group, analyzed data from Oakland, Calif., and found pollution levels can vary by as much as eight times in the space of one block.

Route choice is as important for drivers as for pedestrians, since exhaust fumes can become concentrated inside a car. “It’s just like a mini-gas chamber,” Dr. Kelly said.

While it is easy to feel protected inside a sealed car, walking on a traffic-clogged road generally exposes you to less pollution than driving on one, he said.

Testing the air along New York’s bike paths, Dr. Jack — in research conducted with Steven Chillrud, of Columbia’s Lamont-Doherty Earth Observatory — found levels of sooty black carbon were 25 percent lower on those more than 50 meters away from roads designated as truck routes than those on or near truck routes, he said.

Time of Day Matters

Traveling at less polluted times also helps. Rush hours, of course, generally bring pollution peaks. Less obvious is that in many places air quality is worse in the morning, because of a meteorological condition known as temperature inversion, in which a warm layer of air holds down a colder one, trapping pollution in place.

Inversions do not happen everywhere, so researching local conditions is important.

Dr. Jack suggested using one of the new generation of portable or wearable monitors to learn when and where air is most breathable. “You can do some little experiments,” he said, and see what readings are “if I ride at 7 in the morning, if I ride at 9 in the morning, if I ride on this route.” In New York, he said, “you do see pretty significant differences riding the bike path that goes along the river versus riding the bike path that goes in the street canyon” of Midtown.

Avoid Wood Fires

While they may seem more natural than cars and trucks, wood fires are also a pollution source, producing smoke that is thick with the tiny pollution particles that penetrate deep into the body and are dangerous to health. Choosing not to light one is a simple way to reduce the pollution you — and your neighbors — breathe.