Posted By Paul Leadbetter Malcolm I do not know if it is still current but there is an HSE method (MDHS73) for measuring air change rates. I have a copy of the method from 1992 which recommends the use of sulphur hexafluoride as the tracer gas but I believe that the use of this material is noe frowned upon as it is a potent greenhouse gas. I have heard that it is possible to measure air change rates using a CO2 fire extinguisher and a portable infra-red spectrophotometer (such as a MIRAN). The MIRAN can be used to monitor the concentration of CO2 by measuring the absorption at infra-red wavelengths. REGARDS Paul Leadbetter