Following a discussion in 2011 at the meeting of the European Joint Enforcement Group of Contained Use & Deliberate release of GMOs (EEP2011) about the current situation on airtightness in highcontainment facilities throughout Europe, the Biosafety and Biotechnology Unit (SBB) of the Belgian Scientific Institute of Public Health (WIV-ISP) decided to survey the biosafety officers of institutions with facilities of biosafety level 31 (BSL-3) in Belgium, and offered the possibility to measure the airtightness of their high-containment area by means of a blower-door test (norm NBN EN 13829:2001). Four institutions, representing different construction types, were selected. These were a newly built and state-of-the-art facility, a box-in-a-box renovation, a thorough renovation with conventional building techniques, and a superficially renovated animal facility. The present report summarises the main fumigation techniques commercially available for decontamination and the techniques that were used for measuring the airtightness, as well as the results of the airtightness tests obtained in the different facilities, and provides some recommendations to achieve optimal airtightness.