Ozone (O3)
Environmental Sources
Ozone (O3) is an odourless, colourless gas composed of three oxygen atoms. It is not usually emitted
directly into the air, but at ground level is created by a chemical reaction between oxides of nitrogen
(NOx) and volatile organic compounds (VOC) in the presence of heat and sunlight. VOC’s are emitted
from a variety of sources, including motor vehicles, chemical plants, refineries, and other industrial
source. NOx are emitted from motor vehicles and other sources of combustion. Many urban areas tend
to have high levels of ozone, but even rural area can be subject to increased ozone levels because wind
carries ozone and pollutants that form it hundreds of miles away from their original sources. Peak ozone
levels typically occur during hot, dry, stagnant summertime conditions.
Human Health effects
Repeated exposures to low levels of O3 can make people more susceptible to respiratory infection and
lung inflammation, and can aggravate pre-existing respiratory diseases such as asthma. Health effects
attributed to short term exposure to O3, generally while individuals are engaged in moderate or heavy
exertion, include significant decreases in lung function and increased respiratory symptoms such as
chest pain and cough wheeze and breathing difficulties. Children are more at risk of experiencing such
effects when they are active outdoors during the summer when O3 levels are at their highest.
Other at risk groups include outdoor workers and individuals with pre-existing respiratory disease such
as asthma and chronic obstructive lung disease. Long term exposures to O3 present a possibility of
irreversible changes in the lungs, which could lead to premature aging of the lungs and/or chronic
respiratory illness. Even at very low levels, ground-level ozone triggers a variety of health problems
including aggravated asthma, reduced lung capacity, and increased susceptibility to respiratory illnesses
like pneumonia and bronchitis. These low levels may cause permanent lung damage after long-term
exposure.
Exposure limits
- The South African Department of Environmental Affairs and Tourism’s (DEA&T) guideline values for
ozone are as follows: 250 ppb (instantaneous peak), 120 ppb (hourly average) and 90 ppb (daily
average).
- The United States EPA’s National Ambient Air Quality Standards (US NAAQS) has set an air quality
standard of 120 ppb for a 1-hour (short term) average ozone concentration and 80 ppb for a short term 8
hour average.
- The WHO has a guideline limit of 61 ppb for an 8 hour short term average.
- Canada standards allow an ozone hourly average of 82ppb.
Links and sources:





www.ceroi.net/reports/durban/issues/air/guidelin.htm
http://www.epa.gov/ozone/
http://www.epa.gov/ozone/strathome.html
http://www.airnow.gov/index.cfm?action=smog.page1
www.epa.gov/oar/oaqps/gooduphigh/