Nitrogen Dioxide Air Pollution

The "Nitrogen Dioxide Air Pollution" fact sheet analyses the progress towards the objectives imposed for this pollutant across the country, in order to safeguard the protection of human health.

Nitrogen dioxide (NO2) is one of the main air pollutants, a highly toxic gas that results from the burning of fossil fuels at high temperatures, originating mainly in car traffic and the industrial sector. Exposure to high concentrations can result in serious damage to human health, such as impaired lung function and increased risk of respiratory diseases. When oxidized in the atmosphere, it can even produce nitric acid, one of the components that increases the acidity of rain, and causes various damages to nature and materials, as it is corrosive. It is, however, an occurrence with little significance in our country.

Legislation stipulates air quality objectives for the protection of human health. The indicator used here is the annual average of NO2, which is an aggregated value, based on the hourly concentrations measured at each station, for comparison with the respective limit value (LV). The analysis of the air quality of the zones and agglomerations is carried out considering the worst value obtained in the stations belonging to each of these territorial units.

The type of measuring station, with regard to the environment and the influence of its location (urban, suburban or rural environment and traffic, industrial or background), identifies the nature of the emission sources and the order of magnitude of the levels measured. The trend analysis carried out, based on the aggregation of hourly mean values, depending on the type of station, allows a better clarification on the orientation and direction of the measures to be adopted to achieve the objectives of compliance with the limit values throughout the national territory. For this calculation, all stations with measurement efficiency above 75% are used, and in the case of stations covered by the indicative measurement evaluation strategy, measurements with efficiency above 14% are also used.

This fact sheet concerns mainland Portugal, the Autonomous Regions of Madeira and the Azores and will be updated annually.


Objectives and targets: 
  • Ensure compliance with the EU-wide targets for ambient air quality, which aim to avoid, prevent or limit the harmful effects of air pollutants on human health and the environment;
  • Measure ambient air quality across the country, with a special focus on urban centres;
  • Preserve air quality where it is good and improve it in other cases;
  • As targets, the limit values set forth in the legislation are not to be exceeded  (Decree-Law no. 102/2010):
    • Limit value for the NO2 hourly mean concentration of 200 μg/m3, not to be exceeded more than 18 times a calendar year;
    • Limit value for the NO2 annual mean concentration of 40 μg/m3.
Progress analysis:

The analysis of the concentrations of nitrogen dioxide (NO2), measured at the air quality stations, in the year 2020, shows an atypical year, due to the impact of the COVID-19 pandemic on air quality, resulting from several periods of lockdown which significantly influenced all economic activity and circulation in the national territory. In particular, the measures imposed during the state of alert and the state of emergency resulted in a strong reduction in emissions of air pollutants and led to a significant improvement in air quality, especially in areas with more intense traffic in large urban agglomerations. The reduction in concentrations was mainly felt in the concentrations of NO2, a pollutant whose main source is road traffic and which is responsible for most situations of air pollution in recent years in the cities of Lisbon, Porto and Braga (non-compliance with the annual limit value of 40 µg/m3).

Last update: 
Wednesday, 25 August, 2021