Will it hurt me to breathe smoggy air?

It can. People with heart or lung disease, and chronic respiratory disorder are always vulnerable when the quality of the air they breathe decreases. This also applies to infants and the elderly. However, even healthy adults are at some risk when exposed to greater than usual concentrations of the toxins found in smog. Even exposure to low concentrations of ozone for a prolonged time during moderate exercise can reduce lung function. Symptoms may include chest pain, nausea, coughing and pulmonary congestion.

A recent study by the Ontario Medical Association estimates that smog costs Ontario more than $1 billion dollars a year in hospital admissions, emergency room visits and absenteeism. The Association's study, The Illness Costs of Air Pollution, concluded that about 1,900 people die prematurely in Ontario each year because of smog-related respiratory problems.

Ozone exposure can also affect other living and non-living things. It hardens rubber, retards tree growth and damages crops.

The particles defined as "fine" in smog are less than 2.5 µm (micrometres) in size (about one-twentieth the width of human hair) and are the most harmful to human health. These particles, referred to as PM2.5, are small enough to remain suspended in the air. They can be inhaled deeply into the lungs, where they damage cells that transfer oxygen to the blood. By this route, they can also act as a vehicle for other airborne toxins to enter the bloodstream.

Larger, heavier particles (greater than 10 µm) tend to settle to the ground by themselves within a day or so after being emitted. These also tend to be respired more easily by us. However, fine particles can hang around in the lower atmosphere for several weeks before being washed out by rain or snow, or before being carried downwind. This material can be dust, smoke or the products of atmospheric chemical reactions.