Opsis blog

There are no major man-made sources of ground-level (or tropospheric) ozone. Instead, ozone is formed by chemical reactions involving nitrogen oxides (NO_X), volatile organic compounds (VOCs), and some other substances in the presence of sunlight. Accordingly, where there are emissions of these so-called ozone precursors, there is also a notable risk of finding elevated ozone concentrations. Heat and increased sunlight during the summer months also mean higher levels of ozone during this period of the year.

The ozone precursors are typically generated in urban areas through emissions from road traffic and industrial processes. However, high ozone concentrations do not only occur in urban environments. Ozone can travel long distances with the winds, also making it a major air pollutant in areas where there otherwise are very low levels of air pollution. In fact, ozone can also be consumed by reactions with other air pollutants, so the highest levels of ozone can very well be found in rural areas where no consuming reaction paths are available.

Ozone has several adverse effects on health, not only in high concentrations but also as a result of long-term exposure to moderate levels. By example, it irritates the respiratory system, it may reduce the lung function, it can trigger asthma, and it can cause respiratory infections. It is also an important factor for the formation of photochemical smog. Additionally, ground-level ozone damages crops and forests, causing notable losses to agricultural businesses.

Just as for many other air pollutants, there are both national and international standards for ground-level ozone concentrations. However, to address the long-term exposure aspects, the expressions for the ozone standards can get quite complex. Taking the European Union as an example, the target value for protection of human health is 120 µg/m³ calculated as the “maximum running 8-hours average during the day”, although this value may be exceeded up to 25 days per calendar year averaged over three years. The target value for protection of vegetation is even more complex. Also note that the EU ozone standards speak of “targets” rather than absolute limits. This is because of the complex, indirect formation paths and potential long-distance transportation, making ozone pollution a challenge for the international community rather than something a local or even a national authority can control on its own.

However, for good and for bad, there may also be more straight-forward ozone limits to consider, providing immediate feedback to a concerned public. Continuing the EU example, there is an obligation to inform the public as soon as the hourly ozone concentration exceeds 180 µg/m³. This is of value in particular to people with respiratory deceases like asthma, who then can choose to stay indoors and limit their physical activities. Further, public alerts shall be raised if the hourly average exceeds 240 µg/m³. Such levels are of concern to the public in general, and such alerts may be used for example to trigger emergency plans for the health authorities.

Conclusion: make sure you know what’s in the air, also if you think you are far away from apparent sources of air pollution!