Ozone is a very reactive, colourless gas made up of three atoms of oxygen. Ozone is considered a secondary pollutant as it is created through reactions with other airborne substances. It can react with other pollutants to form photochemical smog.
Ground level ozone refers only to the formation of ‘bad ozone’ near the earth’s surface (Troposphere). A layer of ozone here results in the formation of fog. The ‘Ozone Layer’, or ‘Good Ozone’ is found in the Stratosphere where it acts to protect the earth’s surface from the harmful effect of ultra-violet (UV) rays from the sun.
The largest source of ozone is vehicle exhaust. Other anthropogenic sources include: industry and chemical solvents. Natural sources include: lightning and some vegetation species.
Provincial Guidelines:
| Concentration (ppb) | Exposure Time | Human Symptoms and Other Effects |
| 10,000 | — | Severe pulmonary edema; possible acute bronchiolitis; decreased blood pressure; rapid weak pulse |
| 1,000 | — | Coughing; extreme fatigue; lack of coordination; increased airway resistance; decreased forced expiratory volume |
| 500 | — | Chest constriction; impaired carbon monoxide diffusions capacity; decrease in lung function without exercise |
| 300 | — | Headache; chest discomfort sufficient to prevent completion of exercise; decrease in lung function in exercising subjects |
| 250 | — | Increase in incidence and severity of asthma attacks; moderate eye irritation |
| 150 | — | For sensitive individuals, reduction in pulmonary lung function; chest discomfort; irritation of the respiratory tract, coughing and wheezing |
| 150 | — | Threshold for injury to vegetation |
| 120 | — | US national primary and secondary ambient air quality standard |
| 100 | — | Maximum allowed by OSHA in industrial work areas |
| 82 | 1 hour | Alberta ambient air quality guideline |
| 50 | — | Maximum recommended by ASHRAE in an air conditioned and ventilated space |
| 20–40 | — | Range in which ozone occurs in healthy outdoor environments. Also ozone level produced by some indoor air ionizers when operated according to instructions. |
| 3–10 | — | Low range at which average person can smell ozone |
| 1 | — | Most indoor environments — windows open |