REHVA published its first COVID-19 guidance mid-March 2020. After 2 updates (April and August), the fourth version is focusing on how to reopen and safely use buildings after the lockdown, providing advice on specific components, buildings/space types, and suggesting mitigation measures. The REHVA guidance summarises advice on the operation and use of building service systems during an epidemic of a coronavirus disease. The fourth version of the guidance document overwrites all previous ones. Updates are expected in the upcoming months as more academic data becomes available.