Ozone (O3) is toxic gas and can have a deleterious effect on human health. It is also a powerful oxidising gas, a potent germicide, and is invisible to the human eye at normal concentrations. However, its presence can often be detected due to its sharp pungent odour, often described as that of new mown hay or electrical discharges.
Normal air contains ozone levels of between 0.015 and 0.035 ppm, although during spells of hot weather and in cities with high pollution levels, concentrations of 0.1ppm, or above, have been recorded.
Contrary to some suggestions, ozone is not found at the seaside in any greater quantities than inland. The sea air smell is not that of ozone but of rotting sea life and vegetation.
Ozone is produced by splitting an oxygen molecule into two oxygen atoms, after which each of these free oxygen atoms combine with another oxygen molecule to form ozone
Ozone Production in Welding and Cutting
Due to the high temperature produced by the welding arc plasma, the light generated is very intense and includes radiation in the 130-400nm ultra-violet range. Radiation within the wavelength range 130-170nm is extremely effective at splitting the oxygen molecules in the air to produce free "excited" oxygen atoms that readily combine with other oxygen molecules to form ozone.
As this wavelength spectrum is very effective at producing ozone, it is completely absorbed by the air within a few centimetres of leaving the arc, and results in very high ozone levels, up to 1000ppm in the air layer close to, and surrounding the arc.
Annihilation of Ozone
At concentrations of several ppm, and at room temperature, the half-life of ozone due to decomposition is of the order of 1 day. Only at temperatures above 500oC does the thermal decomposition reaction become important. Thus, in the very hot region adjacent to the welding arc this decomposition reaction can be very important, and the net quantity of ozone emitting from the arc can be greatly affected.
Decomposition can also be accelerated by contact with surfaces and catalysts.
Experiments have shown that the ozone concentration measured during the first minute of welding immediately reaches a peak, which decays after a few seconds, and then remains constant at a background level of between 40% and 60% of the peak value.
Effects of Exposure to Ozone
Ozone is a highly toxic, chemically active gas that is definitely hazardous to human life. Due to its highly oxidising nature, ozone reacts with virtually anything, including the tissue inside the respiratory passages and lungs, producing cell damage and leakage through the cell membranes.
It should be noted that cell damage due to ozone inhalation is permanent and irreversible.
Exposure Limits
Health and Safety regulations in each country set maximum permissible concentrations of ozone to control exposure of people at work. In America these are called Threshold Limit Values (TLV), while in Europe they are known as Occupational Exposure Limits (OEL). These limits are Time Weighted Averages (TWA) and so take into account the amount of time for which people are exposed.
Measurement of Ozone in the Workplace
In order to assess the potential risk, it is necessary to determine whether ozone is present during and after the welding operation, and in what concentration.
Measurement of ozone in breathing zones, and elsewhere, on the shop floor, is normally carried out by means of chemical reaction tubes, detector tubes, that give a single instantaneous reading.
Ozone meters require accurate calibration and also need a suitable filtration system to remove particulate fume.
Once the ozone concentration has been measured, it is compared with the current limit and control measures, if appropriate, put in place. Only suitably qualified and experienced personnel should carry out such measurement, assessment and interpretation.
Control of Exposure to Ozone
Due to the fact that ozone can be formed some distance from the arc as well as close to it, control of exposure and removal of ozone from the workplace can be difficult.
Use of local fume extraction equipment will take some ozone away with the particulate fume but is unlikely to completely solve the problem.
Because of the difficulties with extraction, control of exposure to ozone from welding is often achieved by use of personal respiratory protection for welders, and any other workers at risk, in conjunction with local fume extraction to remove other fume hazards from the area.