Collapse of the mean concentrations of nitrogen dioxide (NO2) in Milan, Bergamo and Rome as a result of the mobility restrictions imposed to contain the outbreak of Covid-19. This is confirmed by the european environment Agency, which registers a drop in european cities ‘closed’ for coronavirus emissions of NO2, produced in particular by diesel engines. In Milan, the concentrations of NO2 in the last four weeks have been lower by at least 24% compared to the previous month. The average concentration during the week of 16-22 march was 21% lower compared to the same week of 2019. In Bergamo, the average concentration among the 16 and the 22 of march was 47% lower than the same week of 2019. In Rome, the decline in the last month was 26-35% compared to the same weeks of 2019. The same effect in Barcelona, Madrid and Lisbon, where in the week 16-22 march the NO2 in the air collapsed, according to the cases, from 40 to 51% compared to the previous seven days.