"Where it is not possible to obtain details of the fan performance from the Fan nameplate or Building Logbook, O & M manual etc during the Site Survey, an approximation of the air flow rate in l s-1 m-2 (often written as l/s/m2 in SBEM) can be established by using an appropriate ventilation rate from the attached table (Local Mechanical Exhaust and Ventilation Rates) and converting this to l s-1 m-2. It is not acceptable to use an arbitrary value such as 5 l s-1 m-2 or a software applied default for extraction rates where the mechanical extraction rate is unknown."