Different initiatives have been implemented to improve air quality in large cities, such as encouraging travel by sustainable modes of transport, promoting electro-mobility, or the car-free day. However, to date, we have not found statistics that indicate to what extent the concentration levels of particulate matter PM2.5, PM10 and nitrogen oxides (NOx) pollutants decrease as a result of public policy. We used official data from the Chilean Government's national air quality information system (SINCA) for the Santiago metropolitan region and estimated the impact of the confinement by COVID-19 on the ambient concentration average values of NOx gases and particulate matter PM2.5 and PM10, which are the main air pollutants produced by the transport sector after CO2. We found that in general there are significant differences between the average levels of gas emissions for 2020 compared to 2019. In particular, we found that, for the months of total confinement May-July, the monthly average levels decreased between 7% and 19% for particulate matter PM2.5, between 18% and 50% for PM10 and between 34% and 48% for NOx. With the return to the new normality, these improvements in ambient concentration levels may be affected by the increase in private transport trips, due to the reluctance of citizens to return to mass public transport. Our results, therefore, represent the maximum impact that can be expected in reducing ambient concentration levels in the city of Santiago of Chile when a mobility reduction of gasoline vehicles is implemented. Article Highlights The reduction of PM2.5, PM10 and NOx was no more than 7%, 18% and 34%, respectively. The average concentration of PM2.5 decreased by 7-19% compared to previous years. The average concentration of PM10 decreased by 18% and 50% compared to previous years. Concentrating commuting on public transport would help reduce levels of PM(10 )and PM2.5.