Thread by @Poppendieck

#AAAR2022. Plenary: @IAQinGWN

- A story about filters....

- How to improve indoor air cleaner
1- Keep it dry
2- Source control
3- Ventilation
4- Air cleaning

- Filtration (air cleaning) has the shortest history. Only 50-100 years use in buildings.
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- ASHRAE standard 52.2 for filtration is only 23 years old.

- Measured CO2 and PM2.5 decay in a classroom that was supposed to be operated at 6 air changes per hour with ventilation and filtration. But it was actually at ~2 air change per hour.
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- Why does filtration fail to meet goals?

- Issue #1. Air has to get to the filter. For central heating homes the recirculating rate can range from 2-8 when the fan is on. BUT the fan can often only 20% of the time, with HUGE variation on the run time.
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- Issue #2. Face velocity (flow rate through the filter). Face velocity matters and has wide ranges. For high face velocity the larger particles are removed better, but smaller particles are removed to lesser degree.
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- Issue #3. Filter bypass. If not installed correctly means real world performance can be much worse than tested efficiency (10-50%).

- Issue #4. Filter aging. Charge effects on charged filters decay with time in a manner that is very dependent on the type of dust.
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- Every filter tested after 3 months of loading in real house had different efficiency curves. It was not a function of the amount of dust on the filter. Possibly a function of size distribution, relative humidity, conductive and particle morphology.
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- Issue #5. Filter effectiveness. Effectiveness is not just a function of filter efficiency. Many other factors (fit, run time, etc.). If you have low run time, your filter efficiency doesn't matter much as it doesn't have time to work.
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- Portable filters. CADR declined in performance as they age (1-10 m3/h over 100 h).
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- Filtration DOES WORK. 69 papers demonstrating positive health effect. Some variation. Benefit is highest in locations with high ambient PM.

- Cost-to-benefit ratio for filtration ranges from 10-100 (mainly due to avoid health care costs).
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Other Secrets of Filtration (oh boy! here comes the good stuff).

- Filters remove ozone (5-40%). Lots of reactive things accumulated on the filter, resulting in byproducts that can emit.
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- Some filters can emit formaldehyde, especially at high relative humidity.

- There is a whole world of other (non-media) filter technologies. In general, "They don't work". No independent testing. Gaming testing.
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- "Filters don't use energy." Conventional wisdom higher filtration rating equals higher pressure drop which means more energy.
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- However, not completely/always true.
--Large variation in actual pressure drop for given filter rating.
--Fan type matters.
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"I like dirty filters.",,,I cannot lie...

-Filters can be used to tell something about what is in the air (virus, radioactivity, dirt, phthalates, fungi, flame retardants).
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- Homes in the same building can have different things on the filter (over an order of magnitude for chemicals).
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- We should be thinking about filtration in the context of broader equity goals. (YES)

- Multi family homes can have an order of magnitude higher concentrations for phthalates on filters than single family homes.
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- RSV outbreaks in Inuit communities. Costs => medical transport, treatment, family housing, diminished income, schooling. "We are foolish for not trying filtration."
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Thanks for the story about filtration @IAQinGWN!
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