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Air Pollution Levels in Barcelona Rise During Public Transport Strikes

Increases of up to 48% observed in some traffic-related pollutants during metro strike

Photo: Andrew Nash

Air quality in Barcelona deteriorates during public transport strikes, clearly indicating the important role played by public transport services in curbing urban air pollution in the city. This was the main conclusion of a study undertaken by ISGlobal, a research centre supported by the ”la Caixa” Banking Foundation, together with the Institute of Environmental Assessment and Water Research (IDAEA-CSIC).

Alterations in public transport services can trigger a rise in the number of private vehicle trips, which in turn increases air pollution levels. The study, which has been published in the journal Science of the Total Environment, studied changes in air pollution concentrations during strikes affecting public transport in the city of Barcelona. 

The researchers measured daily levels of various pollutants—including nitrogen monoxide (NO), nitrogen dioxide (NO2), particulate matter (PM10, PM2.5 and PM1), black carbon (BC) and carbon monoxide (CO)—on the 208 days between 2005 and 2016 with stoppages caused by strikes affecting the city’s underground, rail and bus system.

On days when the public transport system in Barcelona was affected by strikes, the results show a mean daily and citywide rise in air pollution levels ranging from 4% in the case of NO2 to 8% in the case of NO compared to days when the system was running normally. Stoppages affecting the underground metro system had the greatest influence on air quality, probably because it is the most used mode of public transport in the city. On days when the metro service was interrupted, increases in mean daily levels ranged from 8% (PM 10) to 48% (NO).

These findings highlight the essential role played by public transport in curbing high levels of urban air pollution” explains Xavier Basagaña, ISGlobal researcher and first author of the study.  “Almost half of all inter-urban trips to and from Barcelona are still made in a private vehicle, so there is ample margin for increasing public transport use and improving air quality”, he adds.

The increases were detected above all in pollutants emitted mainly by cars (black carbon, nitrogen monoxide, carbon monoxide) and less in those produced by other sources as well as road traffic” adds Xavier Querol, a researcher with IDAEA-CSIC and second author of the study.

According to data published by Barcelona’s city council, 73% of daily trips in the city are internal. Of these, more than half are made on foot or by bicycle, 29% on public transport and 14% in private vehicles. Of the other 27%, which are trips made to and from other towns, 3% are made on foot or by bicycle, 50% on public transport and 47% in a private vehicle. 


Basagaña X., Triguero-Mas M., Agis D., Pérez N., Reche C., Alastuey A., Querol X. Effect of public transport strikes on air pollution levels in Barcelona (Spain). Sci Total Environ. 2017 Aug 23;610-611:1076-1082. doi: 10.1016/j.scitotenv. 2017.07.263.