Barcelona, March 24th, 2026-. According to a study published in the journal Atmospheric Environment: X , the implementation of the low emission zone (LEZ) in the Barcelona metropolitan area significantly reduced concentrations of nitrogen dioxide (NO₂) between 2020 and 2022. However, it had a more limited effect on particulate matter (PM10 and PM2.5) , which is more closely related to pollution sources other than traffic. The results indicate that the LEZ contributes to improving air quality, although additional measures are needed to mitigate air pollution . The research was led by the Barcelona Institute for Global Health (ISGlobal), a centre supported by the "la Caixa" Foundation, in collaboration with the Barcelona Public Health Agency (ASPB), the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) and the Government of Catalonia.
Air pollution poses a major risk to public health , especially in cities, where traffic is one of the main sources. For this reason, many air-quality policies focus on reducing vehicle circulation through temporary restrictions, limitations during high-pollution episodes, or the creation of low emission zones.
Low emission zones (LEZs) aim to improve air quality by restricting the circulation of the most polluting vehicles in specific urban areas. There are currently 363 such zones in Europe and, despite their widespread use, evidence of their effectiveness is still limited, particularly regarding the reduction of fine particles. Barcelona was the first city in Catalonia to implement a permanent LEZ, in 2020. The zone, covering 95 km², banned circulation on weekdays from 07:00 to 20:00 for diesel vehicles EURO 3 or earlier and petrol EURO 2 or earlier, including older motorcycles.
How to assess the effectiveness of the LEZ in a context marked by the pandemic
Assessing the effect of Barcelona's LEZ was not straightforward, as other factors were at play during the same period (beyond the traffic reduction caused by the LEZ) that influenced pollution levels. These included the gradual renewal of the vehicle fleet, the promotion of sustainable mobility and the COVID-19 pandemic , which reduced mobility and encouraged remote working. In addition, meteorological conditions such as temperature and rainfall also affect the concentration of pollutants in the air.
Pollution measurements were taken between 2015 and 2022 at 29 official monitoring stations in Catalonia, including 11 within the LEZ and 18 in control areas (both nearby and more distant), used as a reference to compare pollution levels before and after the intervention. Stations were classified according to their proximity to traffic, distinguishing between traffic stations and background stations.
"We used an innovative technique, the statistical SC-PI method, which allows us to construct a simulated version of the area without the LEZ, in order to compare what would have happened if the measure had not been implemented. This 'synthetic zone' serves as a reference to estimate what levels of NO₂, PM10 and PM2.5 would have been recorded without the measure and to compare them with the actual values," explains Vanessa N. dos Santos , predoctoral researcher at ISGlobal and UPF and first author of the study. The model also controlled for external factors that could alter the results, such as changes in activity during the pandemic , new international regulations or meteorology, with the aim of isolating the real effect of the LEZ as much as possible.
A 15.8% reduction in NO₂ levels
In the study, the introduction of the LEZ was associated with a decrease of up to 7.6 µg/m³ of NO₂ at traffic stations, equivalent to a 15.8% reduction. By contrast, reductions attributable to the LEZ in PM10 and PM2.5 were small (around 1 µg/m³) and became non-significant with some analytical methods. This may be explained by the fact that a substantial proportion of particulate matter is not emitted directly by vehicles but forms in the atmosphere from pollutant gases originating from different sources, including a contribution from natural sources. This so-called secondary origin, together with natural contributions, makes particles more difficult to reduce through local measures such as the LEZ.
"Our results suggest that the LEZ in the Barcelona metropolitan area is an effective tool to mitigate air pollution , although its effects depend on the type of pollutant," explains Xavier Basagaña , ISGlobal researcher and coordinator of the study. "Although it clearly reduced NO₂, its impact on particulate matter was limited. To reach the levels recommended by WHO in 2021, additional measures will probably be required, such as further reducing traffic or acting on other emission sources, including agricultural ones," he notes.
Reference
dos Santos, V. N., Font-Ribera, L., Rico, M., Massagué, J., Nebra, N., Pérez, E., Gómez-Gutiérrez, A., Rivas, I., Querol, X., & Basagaña, X. (2026). Effectiveness of a low emission zone in improving air quality in Barcelona. Atmospheric Environment: X, 29(100428), 100428. https://doi.org/10.1016/j.aeaoa.2026.100428
