Topic: Nitrogen Dioxide (NO2)
Over the course of March, NO2 concentrations measured in more than 300 office buildings located in both Switzerland and Germany varied between 40-75 parts per billion (ppb) each day, on average. This exceeds the permissible short-term limit of 42.5 ppb (based on a 24-hour mean) set by Swiss Ordinance on Air Pollution Control (OAPC).
1. General Description and Health Effects
NO2 in ambient air is mainly a secondary pollutant as only a small fraction of it is directly emitted by sources. Combustion processes emit a mixture of nitrogen oxides (NOx) in the form of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Most of the emitted NOx is NO, from which NO2 is formed through oxidation, mainly by ozone. The actual ambient concentration of NO2 is controlled by various chemical and photochemical reactions leading to formation and destruction of NO2.
Exposure to high levels of NO2 can have adverse health effects, particularly for individuals with respiratory conditions such as asthma. The effects of NO2 exposure can range from shortness of breath and coughing to more severe conditions such as bronchitis and respiratory infections.
2. Sources and Pathways of Exposure
Road traffic is the principal outdoor source of nitrogen dioxide. The most important indoor sources include tobacco smoke and gas-, wood-, oil-, kerosene- and coal-burning appliances such as stoves, ovens, space and water heaters and fireplaces, particularly unflued or poorly maintained appliances. Outdoor nitrogen dioxide from natural and anthropogenic sources also influences indoor levels. In ambient conditions, both outdoors and indoors, NO2 exists in its gaseous form, and inhalation is therefore the major route of exposure at room temperature.
3. Factors Influencing NO2 levels Indoors
Indoor levels of NO2 are influenced by both indoor and outdoor sources, with high outdoor levels from local traffic and combustion sources having a significant impact on indoor levels. Hence, the density of traffic and the distance of a building from the road are important factors. The exchange of air between the indoor and outdoor environments also affects NO2 levels indoors, with the presence of indoor sources, air mixing, and reactive decay on interior surfaces all contributing to variation in indoor levels. Additionally, car exhausts containing NO2 can enter buildings through attached garages.
Seasonal variations in indoor NO2 concentrations are significant and can be attributed to changes in source use, such as heaters and stoves, as well as fluctuations in air exchange rates. During winter months, indoor concentrations of NO2 are typically higher than during summer months, due, mainly, to the increased use of heating, lower ventilation rates, and higher outdoor concentrations. A scientific study in 21 European cities found that concentrations of NO2 were higher in winter than in summer, with an average winter: summer ratio of 1.50.
4. Measures to Mitigate High NO2 Level Indoors
Maintaining healthy indoor air quality requires that NO2 levels during working hours do not exceed an acceptable daily average of 40 ppb. Therefore, it is crucial to take steps to reduce NO2 levels in office spaces and ensure that indoor air quality remains healthy. One effective approach is to ensure proper ventilation by allowing fresh air to circulate, either by opening windows and doors (not facing roadways) or by using air purifiers or HEPA filters to clean the air. Additionally, regular maintenance of HVAC systems can help to improve indoor air quality.
Reducing the use of indoor air pollutants such as smoking can also help to improve indoor air quality and lower NO2 levels. Similarly, decreasing the use of fossil fuels such as oil and gas can be beneficial in reducing NO2 emissions indoors. By implementing these measures, indoor air quality can be maintained at healthy levels, and the risk of negative health impacts from exposure to high levels of NO2 can be minimized.