Fine particulate matter, or PM2.5, are potentially deadly air particles smaller than 2.5 micrometers in diameter and can be inhaled deeply into the lungs because of their small size. Environmental engineer Josh Apte of the Lawrence Berkeley National Laboratory says that large urban cities are at greater risk because the particles can come from different sources.

"Traffic is a big contributor to PM2.5. Power plants are a big contributor. Any time you burn something, you’re going to get PM2.5. Because of all the different types of sources that you have in a city, you tend to get sort of a regional cloud of PM2.5. So in terms of the environmental risk factors for death, it’s right up there as one of the most important risks that really takes years and years off of people’s lives when you get a condition that leads to you getting as stroke, for instance, all because you have been living in a city with bad air."

Apte studied traffic-related air pollution in New Delhi, India to develop global strategies for PM2.5 and other pollutants.

"It points to the fact that there’s one major source of air pollution among many in the city that could be brought down quite a bit."