HEPA filters are not sieves

Yesterday a customer asked this question:

You say your HEPA filters capture nano-size particles, like UFP. Then what’s all that 0.3 micron stuff on your website?

Excellent question

The customer was aware that particles are measured as a fraction of a meter and that nano-size particles, such as ultra fine particles (UFP), are smaller than particles measured in microns.

Sentry Air HEPA filter for Model 300 units

Sentry Air HEPA filter for Model 300 units

The customer was also aware that 3D printers produce UFP, so he was thinking about fume extraction for his 3D printer.

So why the emphasis on 0.3 microns?

Particles that are 0.3 microns in size are the most difficult size for HEPA filters to capture.

Studies have shown that a HEPA filter that effectively captures particles 0.3 microns in size is even more effective at capturing particles of other sizes. (See Resources section below.)

Graphic helps explain HEPA functionality

HEPA filters are so much more than sieves. So we made the graphic below to explain why that is so.

Infographic shows that HEPA filters are not sieves. Particle physics is why HEPA filters are effective.

Three reasons why HEPA filters capture particles

Diffusion: The smallest particles collide with air molecules. The collisions make the particles’ path erratic and more likely to encounter, and adhere to, a fiber.

Interception: Airflow carries the particle into direct contact with a fiber.

Inertial impact: If air flow carrying a particle flows over a fiber, due to inertia — the tendency of an object in motion to stay in motion — the particle continues its straight line movement and contacts the fiber.

Additional factors

Air velocity can have an effect on which particle size is the most penetrating size.

Fiber diameter plays a role in determining the size of captured particles.

Filter thickness supplies the element of time. Thicker filters ensure particles spend more time in the filter, making it more likely the particle adheres to a filter fiber.

Particles already captured by the filter form dendrites, clumps of particles that actually improve the filter’s ability to capture more particles.

NIOSH Current Intelligence Bulletin 65

Health and safety organizations are actively engaged in the identification of worker protection methods for nano particles.

Current understanding of the hazards support the use of HEPA filtration.

HEPA filtration has been shown to be effective in capturing nanoscale particles and should be considered in situations where emissions may be regular, where processes are repeated, and where higher quantities are used in a way that may lead to emissions.”

page 58, NIOSH CIB 65

Particle scanners

We recently took a particle scanner to the Children Museum of Houston, where they use a Model 300 Portable Fume Extractor equipped with both HEPA and carbon filters to capture fumes from their laser cutter and 3D printer.

As you can see in the photos the 3D printer produced over 190,000 particles of varying sizes.

After the Model 300 pulled the particle-laden air through its HEPA filter, the scanner detected 0 particles at the unit’s air output.

Give us a call

If you want to discuss the value of HEPA filtration at your place of work, please give us call at 800.799.4609. You can also fill out the contact form below or email us a sales@sentryair.com. We have applications specialists who’ve got answers.



First MW. Filters, high capacity filters and high efficiency filters: review and production, In-Place Filter Testing Workshop, Boston, Massachusetts, 1971.

Workshop for Certification of Biological Safety Cabinets, The National Institutes of Health (NIH), Office of Biohazards and Environmental Control, Rockville Bio-Engineering Services, Dow Chemical USA, Bethesda, 1974.