Ozone – I thought it was the hole in the ozone layer that we need to worry about?

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Ozone (O3) is a form of oxygen where there are three instead of two oxygen atoms present in the molecule. Ozone is a very reactive gas; it attacks molecules much more vigorously than normal oxygen. Ozone is formed when ultraviolet light strikes molecular oxygen. Ozone formed in this way in the stratosphere (the upper atmosphere, 10–50 km above sea level) is important; it stops ultraviolet rays from penetrating down to the earth’s surface.

There are many compounds which react with stratospheric ozone, but most attention has been focussed on the chlorofluorohydrocarbons (so-called freons). As more and more freon diffused up into the stratosphere, then more ozone was consumed and this led to the creation of the so-called ozone hole which regularly appears over the polar regions in the winter. One consequence of the ozone hole is that more ultraviolet rays penetrate down to the earth’s surface and this has many detrimental effects on many different species from damaging plankton stocks to inducing skin cancer and cataracts in humans.

Local sources of ozone

Ozone can also be produced in the lower atmosphere (the troposphere) due to ultraviolet light striking oxygen molecules. Ozone production tends to be minimal under normal circumstances but ozone synthesis is greatly accelerated by the presence of hydrocarbon emissions from cars and buses. However, cars also emit nitrogen monoxide and this molecule reacts with ozone leading to its instantaneous breakdown. This means that there is virtually no ozone detectable at street level. In fact, ozone concentrations are substantially lower in the vicinity of busy streets than they are in quieter suburbs and the surrounding countryside.

At sites more distant from the source, ozone production may exceed its breakdown and the arrival of traffic-derived ozone on the prevailing winds leads to ozone accumulation. This means that the highest ozone concentrations are encountered in remote unpolluted regions where there are no air contaminants to consume the ozone. Peak levels occur during the spring when the solar UV radiation is at its maximum, especially during the early evening. High daily concentrations (60–70 μg/m3) have been detected e.g. in remote islands in the Baltic Sea as well as in Lapland.

Other sources of ozone were common until the end of the 1980s. Ozone was emitted from photocopying machines and from the first generation of laser printers. Devices which were called electrostatic air cleaners also released ozone into household air. Technical advances have eliminated these air-borne sources of ozone in our offices and homes. Today, any ozone that we breathe at home has drifted in from the outdoor air. The ozone levels in indoor air are only a fraction of those present outdoors, and this is true whether there is mechanical air ventilation or not. Even keeping the windows and doors open does not cause any major elevation in indoor ozone levels.

How does ozone affect our health?

Ozone penetrates deep into the alveolar regions in the lung. In that way it differs from sulphur dioxide, which is a more water-soluble molecule which irritates the nose, throat and mucous membranes of the trachea. Ozone does not cause so much upper respiratory tract irritation; it damages the oxygen-transferring cells of the lung and disturbs the lower respiratory tract.

A multi-centre study conducted in Canada revealed that hospital admissions due to respiratory diseases were linearly related to the prevailing ozone level on the previous day. Other large epidemiological studies have confirmed that the ozone level in urban air is second only to the concentration of small particles in explaining the day-to-day variation in morbidity and mortality.

Some individuals are more sensitive to ozone than others. In general, those most at risk are children, asthmatics and individuals with pulmonary diseases. In the Nordic countries, there is not such a clear relationship between an individual’s exposure to ozone and the ambient ozone level, since most people work and live in air-tight buildings with good insulation and often with mechanical air ventilation. However, the peak ozone concentrations may be sufficiently high to worsen the symptoms of allergic and asthmatic subjects during the pollen season.

What can be done?

Most of the ozone present in the air over the Nordic countries originates from outside the region. Therefore efforts to reduce the ozone levels will require multi-national agreements to reduce emissions, not only of ozone itself but also emissions of nitrogen oxides and hydrocarbons. As old buildings are replaced by new houses with better ventilation, individuals will not be exposed to significant amounts of ozone in their homes irrespective of whether agreements succeed in lowering the outdoor concentrations. However, asthmatics and other individuals with pulmonary diseases could benefit from warnings being broadcast over the radio or on television at times of unusually high peak ozone levels, much as they are provided with pollen counts included in the weather forecast.

Ozone is second only to small particles as a source of morbidity from breathing polluted air. Ozone levels in the indoor air are low and are unlikely to cause any symptoms in countries with well-insulated buildings. Nonetheless, at times of high peak ozone concentrations in the outdoor air, asthmatics could benefit from being alerted to remain indoors since exposure to ozone can worsen their pulmonary symptoms.

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