An important flaw exists in how buildings operate. Current ventilation standards are bare minimums not designed for infection control. Several orgs have called for higher ventilation, but no targets set.
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Can’t really go to a school, university, office and just say ‘bring in more air’. Next question is always, “ok, but how much?” That’s what we tried to address with this commentary.
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The case for higher ventilation/filtration is straightforward: 1) majority of aerosols emitted are small (<10um), stay aloft for a long time, and travel beyond 6 feet 2) many well-described outbreaks indoors, no masks, low/no ventilation, even with physical distancing
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3) basic principles of exposure science and inhalation dose risk reduction (lower intensity exposure = lower risk) 4) 4-6 ACH is consistent with what healthcare settings use
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5) historical evidence shows ventilation plays key role in mitigating disease spread (measles, TB, influenza, SARS-CoV-1...)
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This call for higher ventilation is (finally!) supported by Fauci/NIAID and CDC. Has also been supported by ASHRAE. One problem – when orgs have said ‘higher ventilation’, they failed to set a target! (Whoops!)
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We argue for 4-6 effective air changes per hour. The ‘effective’ part is key because typically ACH means ‘outdoor air changes’. Here we just want ‘clean’ air, either from outdoors or through highly filtered air. (‘clean’ here refers to virus-free, not ‘clean’ in absolute sense…)
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The 4-6 total air changes of clean air is what hospitals use in lower-risk areas of the hospital. But, if you take a look at the table we provide in the supplement, most other places you spend time don’t come close to this. Schools? ~3 ACH Homes? ~0.5 ACH Stores? ~1.7 ACH
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Last, bringing in more outdoor air / using better filters isn’t without challenges/caveats: 1) more outdoor air = more energy. Need energy-efficient systems 2) ventilation/filtration only address far-field airborne, not near-field
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3) ACH approach not the best for large volume spaces (gyms, atria) 4) ACH v. volumetric flow (eg, cfm/p + cfm/sqft) both valid. ACH ensures minimums, even for under-occupied space. cfm/p approach scales up if have high densitites.
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5) 4-6 ACH may not be enough for higher risk locations, and for places where masks are not being used 6) There are many other benefits to more outdoor air beyond infection control (reduced absenteeism, better cognitive function, fewer ‘sick building’ symptoms…).
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I also want to thank
@ShellyMBoulder and@cedenolaurent for early convos on ACH for schools. We built a simple tool over weekend in July to help schools pick portable air cleaner. But again, there were no targets set by any authoritative body. So we added this chart to the toolpic.twitter.com/i0E9kS2fGN
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Big picture. The way we design/operate our buildings has a big impact on our health, including infectious disease. The reality is our ventilation design standards are bare minimums not designed for infection control, or health promotion. That needs to change.
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