Thread by @DavidElfstrom

Air purifier manufacturers say HEPA should always be the filter of choice, and their product's proprietary filter delivers. Which HEPA? ISO 35H at 99.95% or ISO 40H at 99.99%? Why not ISO 50U? That's 10x better at 99.999%. Why stop there? Go for ISO 70U at 99.99999%! /1

The answer is, single-pass filtration efficiency DOESN'T MATTER except in specific cases like Powered Air Purifying Respirators (PAPR), clean rooms, operating theaters, or nuclear laboratory exhaust—HEPA's original purpose. /2

For portable/in-room air cleaners, all that matters is the Clean Air Delivery Rate (CADR) for a target particle size and type, within acceptable for sound power and frequency characteristics for the people in the room. /3

HEPA-class filters aren't the optimal solution, unless that's all you are offering as a product. Instead, design for a system using a ISO 16890 or ASHRAE 52.2 rated filter, say ePM1 > 60% or MERV-13. These filters have far less restriction to airflow than HEPA filters. /4

A portable air cleaner designed to use a les expensive general ventilation filter can move substantially more air through it to achieve a greater CADR at low noise, and with lower electrical power consumption than a unit based on a HEPA filter. /5

And in case you are wondering, good quality general ventilation filters will remove health-damaging ultrafine submicron particles too, that optical particle counters can't even detect. /6

To recap, for in-room air cleaners, HEPA *IS NOT NEEDED*. What matters is the combination of filtration, fit, and airflow, to maximize the Clean Air Delivery Rate for a target particle type and size, verified by an independent laboratory test to an industry standard. /7

Addendum: This is why it doesn't matter if some brand of air purifiers aren't "HEPA" by a particular rating system, other than for faithful labeling & consumer trust (looking at you, Levoit). All that matters is the independent CADR test (looking at you, Austin Air). /8

FAQ: What about a vacuum cleaner? For that, you want high single pass filtration of a HEPA filter. That's one of the exceptions. A vacuum cleaner is horrible air cleaner (and can make the air worse with a weak filter), while an air cleaner is a horrible vacuum cleaner. /9

@joeyfox85 has written a post on this same topic. /10

@lovingtaeonmain Also, look for units that are Energy Star. energystar.gov/productfinder/…

@RolandSB13 If you crank up air speed to 52.2 levels, yes, you will see a much lower filtration level at the MPPS.
HEPA type filters are lab tested at a much lower velocity (~one fifth!). They would show lower filtration at MPPS if pushed to 52.2 speeds.

@daveberkeleyuk Hmmmm Dyson says the washable filter is HEPA. It's more likely a leak in the collection cylinder door, which is frequently opened and closed. Sometimes I'm not careful and it doesn't fully latch closed, then there's a REAL mess.