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

www.epa.gov/oar/aqtrnd99/chapter2.pdf

www.epa.gov/air/urbanair/o3/what1.html

www.who.int/environmental_information/Air/Guidelines/Chapter3.htm#3.1

www.epa.gov/children/

www.modcon.co.il/exposure.htm

www.epa.gov/oar/oaqps/gooduphigh/

www.epa.gov/airprogm/oar/oaqps/airnow/ozone.html

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