The next time someone denies the use of a portable air cleaner in a classroom on a (false) basis that it might spread infection by moving air, or interfere with the HVAC system, show them this paper. @amandalhu @sameo416 (HT @Poppendieck).
onlinelibrary.wiley.com/doi/epdf/10.11… /1
onlinelibrary.wiley.com/doi/epdf/10.11… /1
When determining the amount of outdoor air provided to a space, ASHRAE Standard 62.1's Zone Air Distribution Effectiveness factors to account for the inefficiency of air delivery and mixing to provide outdoor air to the breathing zone. /2
For example, in heating mode, with a ceiling level supply and return, the effectiveness is only 0.8. So we need to provide 25% more outdoor air. (image: User's manual for 62.1-2010) /3
In this paper by @BrettCSinger1 “When an overhead HVAC system is in heating mode, it's important to add mechanical mixing to avoid stratification.” onlinelibrary.wiley.com/doi/epdf/10.11… /4
“Eliminating stratification is particularly important to ensuring that the benefits of outdoor air ventilation and central filtration are achieved in the room.” /5
“While mixing could be provided by portable fans, portable air cleaners provide substantial additional benefit by filtering as well as mixing the air and their use is recommended.” /6
In fact, without the extra mixing, some people may be exposed to infectious aerosols at 5-6 times what would be expected under fully mixed conditions! /7
In half of the classroom experiments a standing barrier was erected between the students and teacher. The mean exposure relative to perfectly mixed doubled with the barrier in place compared to without. /8
This experiment yet again shows why adding plexiglass into a room increases risk of transmission, confirming what was identified in responses to 500,000 survey respondents:
https://twitter.com/joeyfox85/status/1488590966084841477?s=20&t=aUqOAusEtLOSRoG6bKTLPg/9
Here's a great thread and visual for why plexiglass barriers aren't helpful:
https://twitter.com/smjevnikar/status/1366758624924733446?s=20&t=QdFeIlxUYy_sV0GqAu1CeQ/10
The only case where a portable air cleaner might increase risk is in a specialized situation like an AIIR (Airborne Infection Isolation Room) without a low-wall return.
https://twitter.com/DavidElfstrom/status/1446505744375980032?s=20&t=wgqZYyz-yIH1vhcgSecelA/11
So, just get the #filtration in the room! A #CorsiRosenthalBox can provide substantially faster cleaning than a portable air cleaner because the effectiveness is equal to filtration percent X flow.
https://twitter.com/CappaSnappa/status/1481049117736505351?s=20&t=wgqZYyz-yIH1vhcgSecelA/12
Here's @healthyheating's simplified/idealized illustration of a #CorsiRosenthalBox in a classroom with a HVAC system:
https://twitter.com/healthyheating/status/1481041411776520192?s=20&t=wgqZYyz-yIH1vhcgSecelA/13
Portable air cleaner or #CorsiRosenthalBox, just do it! Select the appropriate number of units to add up to the appropriate noise-limited filtration amount...
https://twitter.com/joeyfox85/status/1486446536884109315?s=20&t=wgqZYyz-yIH1vhcgSecelA/14
... while still being mindful of the cost, and other potential concerns from technology that acts on indoor air chemistry rather than simply removing particles from the air.
https://twitter.com/DavidElfstrom/status/1483219100142866439?s=20&t=wgqZYyz-yIH1vhcgSecelA/15
The practical implications from Singer et al.'s paper “Measured influence of overhead HVAC on exposure to airborne contaminants from simulated speaking in a meeting and a classroom” onlinelibrary.wiley.com/doi/epdf/10.11… /16
Where might you see the effect of stratification, resulting in stagnant air, where effective ventilation is blocked by plexiglass? At an arena. /17
Both of these paragraphs from Health Canada @GovCanHealth need updating. Fans DO improve ventilation effectiveness, and portable air cleaners should be used in any situation indoors, not just when there isn't a ventilation system. canada.ca/en/public-heal… /18
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