Air-source heat pumps (ASHP) have a significant potential for decarbonizing the heating sector. In this article, we compare the environmental impacts (climate change, particulate matter formation, human toxicity, and ozone depletion) of an ASHP and a natural gas boiler (NGB). The main originality is that we perform the life-cycle analysis (LCA) of the ASHP and the NGB for 18 European countries while sizing the ASHP according to the dwelling thermal demand. We highlight that using refrigerant R290 instead of R32 decreases the ASHP impact on climate change and ozone depletion. Moreover, the building stock is found to greatly influence the potential benefits of ASHP in several countries (e.g. the Czech Republic, Greece). In recent dwellings, ASHP reduces climate change in 17 out of 18 countries, with a 54 % average reduction. However, it often increases particulate matter formation mainly due to the electricity mix, and the use of copper for ASHP manufacturing. Our results can be helpful to European policy makers since they assess in which country ASHP should be installed to yield the highest reduction of environmental impacts. Countrywide, our results can help to deploy ASHP as they indicate which dwelling type should be given priority for ASHP installation.