Atmospheric conditions sometimes occur which give rise to high levels of the two most common air pollutants in Barcelona and its metropolitan area.

This is when pollution episodes are declared (either as early warnings or as pollution episodes with or without traffic restrictions) for particulate matter (PM10) or for pollutant gases (NO2).

The declaration of an episode at any of the various stages involves the activation of the corresponding municipal operation, as well as health recommendations for the general public, particularly those most at risk.