Nitrogen dioxide

Definition

Nitrogen dioxide (NO₂) in its pure form is a brownish, pungent-smelling gas that is formed by natural and anthropogenic combustion processes at high temperatures and released into the atmosphere. It is a component of nitrogen oxides (NOx), the other component of which is usually nitrogen monoxide (NO).

NO₂ is an important indicator of outdoor air quality. Accordingly, the substance is continuously recorded throughout Europe using a dense monitoring network. In addition, nitrogen dioxide in combination with daylight is a precursor substance for tropospheric ozone.

The concentration of nitrogen dioxide depends on the respective source and the state of ventilation. The substance is rapidly degraded on surfaces.

Limit and guide values

Binding EU-wide limit values only exist for outdoor air (annual average not exceeding 40 μg/m³, no more than 18 one-hour average values per year exceeding 200 μg/m³).

The German Committee for Indoor Air Guide Values (AIR) has derived the following short-term guide values for nitrogen dioxide for indoor air: Short-term guide value I (precautionary value) for nitrogen dioxide in indoor air: 0.08 mg/m³, averaged over one hour. Short-term guide value II (hazard value) for nitrogen dioxide in indoor air: 0.25 mg/m³, averaged over one hour. The AIR has not derived a long-term guide value. If necessary, the AIR recommends using the WHO guideline value for indoor air of 0.04 mg/m³ as an alternative assessment standard.

Sources

The main source of nitrogen dioxide outdoors is road traffic. The substance is carried into indoor environments via window ventilation.

Relevant indoor sources are all types of combustion processes. Gas stoves and gas boilers are particularly relevant, as well as all other open flames. These include candles, incense sticks, cigarettes, ethanol fireplaces, aromatherapy burner, camping stove, etc.

Birmili, W., Kolossa-Gehring, M., Valtanen, K., Debiak, M., Salthammer, T., 2018. Schadstoffe im Innenraum - aktuelle Handlungsfelder. Bundesgesundheitsblatt 61, 656-666.

Salthammer, T., Schieweck, A., Gu, J., Ameri, S., Uhde, E., 2018. Future trends in ambient air pollution and climate in Germany – Implications for the indoor environment. Building and Environment 143, 661-670.