To remove the obligation, in Sentences 4.1.7.4.(1) and 5.4.5.2.(1), for exhaust air to be discharged outside the building and to permit its recirculation inside the building if a fail-safe system is provided. This is to limit the probability that vapour will accumulate, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.