A HEPA Filter Needs a Sealed Body

A true HEPA filter catches at least 99.97 percent of particles 0.3 micron across. That test size was picked because it is the size that gets through filters most easily, so a filter that stops it does even better on most larger and smaller particles. For comparison, most mold spores are roughly 2 to 10 microns, many times bigger than the test size.

Evidence: Well supported (the US Department of Energy HEPA filter standard and manufacturers' own specifications).

The veteran inspector whose field work this builds on held that a HEPA machine needs two things: a high-grade HEPA filter, and a sealed housing around it. If air can sneak out through seams, gaps, a loose gasket or an unsealed vent before it reaches the filter, the dust goes straight back into the room. The filter never sees it.

One inspector examined a new brand-name canister vacuum that wore a HEPA tag. When she opened the exhaust outlet, she could see that the air only struck the face of the filter and bounced off it rather than passing through. Her laser particle counter read low on one side of the outlet and over 1,500,000 particles per cubic foot on the other. Her lesson: a HEPA filter does not make a HEPA vacuum.

She also wanted the filter to be the final stage the air goes through before leaving the machine. When the filter sits ahead of the motor, everything picked up off the floor is filtered, but dust shed by the motor itself still leaves in the exhaust. Most shop vacuums are not sealed, so she did not count them as true HEPA machines even with a HEPA filter fitted.

As a rough yardstick, she treated an exhaust reading of 0 to 200 particles per cubic foot as a sign of an effective vacuum. Ordinary machines often read in the tens of thousands or higher.

Evidence: Inspector's practice (precautionary).

Do this before the new vacuum ever touches your floor, so you can still return it unused.

1. Switch the vacuum on.
2. Wet one hand.
3. Run it slowly over the body: the seams, the filter housing, the latches, and the spot where the hose joins the body, which is a common leak point.
4. Wet skin feels moving air as a cool draft. Air escaping anywhere other than the intended exhaust is a leak. Think about sending the machine back.

The same idea applies to a room purifier. A filter that air can go around is not doing its job.

• "HEPA-type," "HEPA-like" or "HEPA-style." These words mean the filter is not certified HEPA.
• A claim of 99.97% capture, but only for particles of 5 microns or more. That is about 17 times the true HEPA test size, and much easier to catch.
• "Removes 99.97% of ragweed pollen." Pollen grains are large.
• "Hypoallergenic," "removes allergens" or "for pets." These are marketing words, not a filter test.
• A HEPA filter mounted in a thin, flimsy, all-plastic body, or a design where air hits the filter instead of flowing through it.
• A bagless design. Even a well-filtered cup exposes you to dust when you empty it.

The inspector warned that HEPA could mean quite different things from one manufacturer to the next, and that nobody was checking whether filters lived up to the claims. Ask for the filter's rating, ask whether the whole machine is sealed, and do the wet-hand test yourself.

This page is for education only. It is not medical, legal or engineering advice. The inspector whose work this builds on was an environmental consultant, not a physician. Talk with your doctor about health concerns, and use licensed professionals for electrical work and large mold jobs. Products, prices and labs mentioned reflect the years they were recorded and may have changed.

Source: the field work of environmental inspector May Dooley, MS, MA, CMC

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