Air quality is arguably really important in reducing indoor spread of coronaviruses. During the original SARS epidemic, hospital wards with poor air-handling saw far more new infections than those with good air-handling. More than 16 air changes/hour seemed to be the key number. I haven't seen anything specific published for Covid but we took this as our target. In the older part of our hospital (where the main ICU and Covid wards lived) the air-conditioning system is very antiquated and could only manage about 4 changes per hour, so we put in huge extractor fans linked to hepafilters in all the Covid units. This seemed to work really well - ICU was probably the safest place in the whole hospital. The only downsides were that the howling gale made it hard to keep the doors closed and we couldn't condition such volumes of air, so it got pretty unpleasant on hot days.