**9.1 Interior spaces: air quality**

In the interior spaces, the virus transmission can be reduced by air control through ventilation, humidifying and filtering.

A 2020 research shows that 3 air changes per hour, which is common in most countries legislation, "generated reductions in expected outbreak sizes that would normally only be possible with a substantial vaccination coverage of 50–60%, which is within the range of observed vaccination rates in school settings [63]".

As for filtering, pre-pandemic experiments have been conducted since 1968 on the efficiency of HEPA filters that "showed an average reduction of 99.996% [66]" or in which "aerosol transmission of PRRSV occurred in 0 of the 10 HEPA-filtration replicates [67]". During COVID-19 pandemic, HEPA filters were recommended in hospitals for air filtering in operating rooms or in the breathing circuit [68, 69]. Some papers recommend HEPA for filtering the recirculating air in closed rooms or vehicles [70, 71], although certain studies are reserved concerning the HEPA filters capacity of filtering submicron size particles [70].

Humidifying could play an important role as long as a 2013 research concluded that "maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus [72]".

As in the 19th century, air control becomes a key measure in mitigating pandemics in 2020.
