The nitrogen oxides group includes some of the most common and harmful air pollutants. The chemical term NOx covers various compounds that are of relevance to the assessment of air quality, especially nitric oxide (NO, also called nitrogen monoxide) and nitrogen dioxide (NO2).
Nitrogen oxides are toxic and detrimental to health. They reduce lung function and damage the mucous membranes. They lead to asthma and breathing difficulties, coughing and eye irritation. The harmful effects of other pollutants such as particulate matter are increased due to reduced immune activity. They also contribute to the development of allergies. In regions with high levels of nitrogen oxide pollution, an increase in cardiovascular diseases as well as a higher mortality rate have been demonstrated.
Nitrogen oxides also have harmful effects on animals, plants and soil. They inhibit growth and indirectly contribute to acidification of the soil, thus affecting its biological composition.
Much of the nitrogen oxides to which people are exposed comes from internal combustion engines, especially diesel engines. The negative impacts of nitrogen oxides depend heavily on their concentration. This means that vehicle exhaust fumes are particularly harmful as they are not as diluted as exhaust gases from chimneys. The closer you are to the source, the more serious the damage to health.
In regard to Germany, the limit values for nitrogen dioxide levels in the air are regulated in the 39th Ordinance for the Implementation of the Federal Immission Control Act (39. BImSchV). The annual average limit value for NO2 is 40 µg/m³. The value of 200 µg/m³ may not be exceeded for more than 18 hours per year.
Particulate matter (PM) refers to all small particles in the air. It is thus a collective term used to describe a wide variety of materials of different particle sizes. In simple terms, PM10 (particulate matter < 10 µm) refers to particles with a maximum particle diameter of up to 10 µm. PM2.5 refers to particles with a diameter of 2.5 µm or less. A further important size class within particulate matter is ultrafine particles smaller than 0.1 µm (< 100 nm).
Size and chemical composition are decisive in determining how dangerous the particulate matter inhaled on a daily basis is to our health. In order to protect the health of citizens, the EU defined limit values for PM10 concentrations in ambient air, which have been compulsory since 01 January 2005. Since 01 January 2010, a limit value has also applied for particulate matter size PM2.5. Ultrafine particles make up only 10% of the mass but 90% of the particle count in particulate matter and play an important role in air pollution. Nevertheless, they have not been adequately taken into account in the current legislation because this only addresses the particulate mass.