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Air Pollution Ozone

3. How are we exposed to Ozone (O3)?

  • 3.1 Critical sources of Ozone responsible for health effects
  • 3.2 Relationship between ambient levels and personal exposure to Ozone
  • 3.3 Short-term exposure to high peak levels or exposure in hot spots of Ozone

3.1 Critical sources of Ozone responsible for health effects

Ozone is a secondary pollutant produced by photochemical activity in the presence of precursors. O3 is also subject to long-range atmospheric transport and may be considered as a trans-boundary problem. More...

The same information on
Particulate MatterNitrogen Dioxide

3.2 Relationship between ambient levels and personal exposure to Ozone

Can the differences influence the results of studies?

Personal ozone (O3) exposure measurements are not well correlated with ambient ozone concentrations measured at fixed sites.

To account for this, additional information (e.g., activity patterns) was used in some studies to improve personal exposure estimates based on fixed site measurements. As O3 is a highly reactive gas, concentrations indoors are generally lower (less than 50%) than those in ambient air. There are very few indoor sources (such as photocopiers or electrostatic air cleaners) in most homes. Outdoor ozone levels vary across city areas because ozone is scavenged as it reacts with NO. Early morning and late night exposures outdoors are lower because of the daily cycle of ambient ozone concentrations. Thus, for ozone, cumulative daily or long-term average exposures are largely determined by exposures occurring outdoors in the afternoon. Exposure underestimations may occur in studies on human populations when outdoor ozone concentration measurements are used to estimate short-term personal ozone exposure. Such misclassifications may cause true effects to appear less strong or be concealed. More...

3.3 Short-term exposure to high peak levels or exposure in hot spots of Ozone

Adverse health effects have been documented after short-term exposure to ozone peaks, as well as following long-term exposure to relatively low concentrations.

Studies have shown that short-term exposure to peak levels of ozone can temporarily affect the lungs, the respiratory tract, and the eyes, and increase susceptibility to inhaled allergens. Long term exposure to ozone has primarily been found to reduce lung function.

Some studies found a clear relationship between variations in peak ozone levels and the intensity of adverse health effects. Because days with very high ozone concentrations are rare, the largest burden on public health is likely to be due to the frequently occurring mildly elevated ozone concentrations.

Being a secondary pollutant, ozone concentrations are usually not significantly higher at specific urban “hot spots ”. On the contrary, levels of ozone tend to be lower in polluted urban atmospheres because traffic-induced NO reacts with ozone, causing ground level ozone concentrations to drop. More...


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