Trevor Penning is the director for the Center of Excellence in Environmental Toxicology at the University of Pennsylvania School of Medicine.

Pollutant is any substance that's in the environment that can actually be considered a hazard and at a certain dose will cause a toxic effect.

So air pollution is a complex mixture.

We have to understand that first of all.

And the second of all is it has different phases.

There's a particulate phase and a gaseous phase.

The particular phase consists of a particular matter of different sizes.

So while Dr. Li's research looked solely at particle air pollution, as Dr. Penny explained, there's also the gaseous phase of air pollution.