MADRID, Jan. 9 (Xinhua) -- Air quality in the Spanish capital of Madrid improved in 2018, according to a report published by the Ecologists in Action (Ecologistas en Accion) pressure group on Wednesday.

Ecologists in Action considers that over the past year air quality in Madrid was "relatively good" with a "notable decrease" in pollution as compared to previous years. However, the group also highlights that 2018 was the ninth consecutive year in which nitrogen dioxide (NO2) levels were above the legal limit of 40 micrograms per cubic meter.

The study was based on air quality samples taken from 24 monitoring stations in the city, with NO2 levels at 7 stations exceeding the legal limit. In 2017, samples from 15 exceeded the limit.

Meanwhile, none of the stations detected higher than permitted levels of particulate matter in the air, although the report points out that the levels registered are still "far higher than World Health Organization recommendations."

The pressure group puts the reduction in air pollution down to two key factors: "favorable" (i.e. wet) weather conditions for much of last year and anti-pollution measures taken by Madrid City Hall. One such measure is the recently introduced 'Madrid Central' scheme, which limits access to parts of the city center to either low-emission vehicles or those belonging to residents.

Ecologists in Action also stresses that weather conditions are susceptible to change. On three occasions in December 2018 and during the first six days of 2019, clear, dry and calm conditions saw NO2 levels rise, causing City Hall to put anti-pollution protocols into operation.

These protocols place further restrictions on traffic, which the ecologists insist is still "by far" the main cause of air pollution in Madrid.