The term "smog" was first coined more than three decades ago to describe a mixture of smoke and fog in the air. Today, "smog" refers to a noxious mixture of air pollutants that can often be seen as a haze in the air.
Smog can make breathing more difficult -- even for healthy people -- and it can make us more susceptible to cardio-respiratory diseases. Even healthy young adults breathe less efficiently on days when the air is heavily polluted, especially if exercising outdoors. Particularly vulnerable to smog are people with heart or lung disease, the elderly and small children.
The two main ingredients in smog that affect our health are ground-level ozone and fine airborne particles.