Still, experts agree that nitrogen oxide (pdf) is a nasty pollutant. Once released into the air it quickly converts into nitrogen dioxide—a reddish-brown gas with a pungent odor—and then absorbs sunlight to transform into the yellow-brown haze that blankets cities. It is this smog that can exacerbate dozens of health problems, including asthma, bronchitis and emphysema. Alternatively, it can be washed into the ground in the form of acid rain, which can kill plants and animals. Once the damage is done “there is no antidote,” says Yiannis Levendis, an engineering professor at Northeastern University who focuses on diesel emissions.

Scientific American