A multi-step machine learning approach to assess the impact of COVID-19 lockdown on NO(2) attributable deaths in Milan and Rome, Italy

BACKGROUND: Air pollution is one of the main concerns for the health of European citizens, and cities are currently striving to accomplish EU air pollution regulation. The 2020 COVID-19 lockdown measures can be seen as an unintended but effective experiment to assess the impact of traffic restriction policies on air pollution. Our objective was to estimate the impact of the lockdown measures on NO(2) concentrations and health in the two largest Italian cities. METHODS: NO(2) concentration datasets were built using data deriving from a 1-month citizen science monitoring campaign that took place in Milan and Rome just before the Italian lockdown period. Annual mean NO(2) concentrations were estimated for a lockdown scenario (Scenario 1) and a scenario without lockdown (Scenario 2), by applying city-specific annual adjustment factors to the 1-month data. The latter were estimated deriving data from Air Quality Network stations and by applying a machine learning approach. NO(2) spatial distribution was estimated at a neighbourhood scale by applying Land Use Random Forest models for the two scenarios. Finally, the impact of lockdown on health was estimated by subtracting attributable deaths for Scenario 1 and those for Scenario 2, both estimated by applying literature-based dose–response function on the counterfactual concentrations of 10 μg/m(3). RESULTS: The Land Use Random Forest models were able to capture 41–42% of the total NO(2) variability. Passing from Scenario 2 (annual NO(2) without lockdown) to Scenario 1 (annual NO(2) with lockdown), the population-weighted exposure to NO(2) for Milan and Rome decreased by 15.1% and 15.3% on an annual basis. Considering the 10 μg/m(3) counterfactual, prevented deaths were respectively 213 and 604. CONCLUSIONS: Our results show that the lockdown had a beneficial impact on air quality and human health. However, compliance with the current EU legal limit is not enough to avoid a high number of NO(2) attributable deaths. This contribution reaffirms the potentiality of the citizen science approach and calls for more ambitious traffic calming policies and a re-evaluation of the legal annual limit value for NO(2) for the protection of human health. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12940-021-00825-9.