Ozone is a powerful pollutant affecting lung health which occurs both indoors and outdoors. Very low levels which occur naturally are unlikely to cause any problems, but occupational exposures in a number of injuries may cause acute effects on the eyes and respiratory tract. Ozone is valued for its reactivity as an oxidising agent which means it is a powerful disinfectant. Thus it is used in many applications from aquaculture and the food industry, to wastewater treatment and the chemical industry. Ozone is also produced as a by-product by processes using ultraviolet lamps or electrical discharges (i.e. ionisation).
Ozone is an elemental form of oxygen known as an allotrope. Where oxygen molecules consist of two atoms of oxygen, ozone molecules have three oxygen atoms. Ozone is produced naturally from oxygen in the atmosphere by ultraviolet radiation or from electrical discharge (lightning). Ozone in the stratosphere prevents too much radiation from the sun from reaching the earth's surface and, as such, has a protective function, hence environmental legislation to protect the ozone layer from destruction. Ground level ozone is produced naturally and also from the reaction of components of vehicle exhausts in the presence of sunlight.
Ozone is a colourless gas with a characteristic odour that is detectable at concentrations above 0.01 parts per million (ppm). It is a powerful oxidising agent and when it reacts it breaks down into oxygen.
Ozone used for commercial applications is produced on-site by an ozone generator (it cannot be stored or transported, because it is very unstable). The process involves passage of a high voltage alternating current through a stream of gas containing oxygen. This breaks down the oxygen molecules into oxygen atoms, some of which then react with further oxygen molecules to form ozone. This produces ozone at a level of 1-2 per cent by weight (from the air) or 2-8 per cent by weight from pure oxygen.
Ozone is highly reactive and has the potential to damage the cells, tissues and organs of the body. Occupational exposure to ozone occurs through inhalation of the gas and it is very irritating to the upper and lower respiratory tract. Acute exposures lead to:
Level, intensity and duration of the symptoms depend upon the extent of exposure. Some of the above symptoms, occurring together, may be classed as 'Sick Building Syndrome', a poorly understood condition which occurs with exposure to various forms of indoor pollution, including ozone. Very high exposures may even be fatal. In the occupational context, acute symptoms are more common but the possibility of long-term impact on the lungs cannot be discounted. People with asthma and other lung diseases are likely to be more sensitive to the adverse effects of ozone exposure. The Control of Substances Hazardous to Health regulations set an occupational exposure standard for ozone of 0.2 ppm averaged over a 15 minute period.
Ozone is valued as a disinfectant because of its powerful oxidising action which destroys pollutants, bacteria and viruses very rapidly without generating any harmful by-products. Its on-site generation is also useful in many applications because there are no transport or storage issues. The first drinking water plant to use ozone for purification was actually built in Holland as long ago as 1893. Increasingly, ozone is being used as an alternative to chlorine in disinfection applications because of concerns about the environmental impact of chlorine and the by-products its use generates.
The industrial applications of ozone include:
Ozone is also produced as a by-product in many industries where processes involve ultraviolet radiation or electrical discharge. These include:
As with all occupational exposures, the approach to protecting employee exposure to ozone should be based upon prevention. Thus, office equipment should be situated in a well-ventilated area and should be regularly serviced and maintained to prevent ozone build up in confined spaces. Local exhaust ventilation could be used where there is a known source of ozone emission for example with ultraviolet lamps used in ink curing.
Of course, in some situations, these control measures will not be adequate to prevent ozone exposure, particularly where this is a central part of a process, rather than a by-product. In such cases, the use of respiratory protective equipment (RPE) may be the more appropriate approach. Employees need to be trained to use RPE properly and it needs to be regularly checked.
Depending upon the situation, it might be necessary to monitor levels of ozone in the workplace, to ensure standards are being met. There are detector tubes which can measure ozone levels in workplace air and these can be useful to detect leak situations which may be increasing exposures. There is also a method that has been developed by the United States Occupational and Health Administration which involves drawing an air sample through a sample tube containing nitrite. Ozone present in the sample will oxidise nitrite to nitrate which is detectable by chromatography.
Finally, employees should be encouraged to report any respiratory symptoms which may be caused by ozone exposure. Those with asthma and hay fever should be especially vigilant as they are more likely to be sensitive to ozone.