“We should get out of here,” says air pollution chemist Eben Cross. At 7 a.m. on this cold November day the wind blows steadily through the Massachusetts Institute of Technology’s Cambridge campus, cutting through our thin jackets. But Cross isn’t afraid of the cold. He worries about the air we’re breathing — especially considering the six fire trucks directly ahead, idling in the dim morning light.

“We’re getting hammered right now,” Cross says, shouting over the hum of the engines. He’s taken his gloves off to manipulate the display panel on his pollution monitor. The acrid smell of diesel is unmistakable. “Anytime you can smell it, you are in a regime that is very polluted,” he says. “In many ways your nose is a better mass spectrometer than any device on the market.”

Cross’ monitor measures the presence of microscopic particles suspended in the air. Earlier, in his home, the device reported average concentrations of between 10,000 and 100,000 airborne particles per cubic centimeter of air (the latter after he burned some toast). Now it detects millions. The massive size of the fire trucks’ engines, combined with their inefficient combustion in cold weather, means that the air reaching […]