HEPA Filters

How HEPA filters work?

HEPA (high efficiency particulate air) filters like the EVA PAPRFC3 are mechanical type filters that collect particles through several different mechanisms, including impaction, interception, gravity and diffusion.  

  • Impaction – Occurs when particles leave a streamline due to their inertia and are trapped against a filter fiber. Once captured, attractive forces keep the particles against the fiber.
  • Interception – Occurs when particles traveling on air streamlines are carried close enough to the filter fiber to be captured.
  • Gravity – Causes settling of particles which can bring them closer to a fiber for capture.
  • Diffusion – Occurs when very small particles undergo random movements as they are bombarded by air molecules. They are captured as they near a filter fiber.

Can particles fall out of a HEPA filter?

The EVA PAPRFC3 filters are tested against particles sizes of 0.2 – 0.3 micrometers and the minimum acceptable efficiencies of 99.97%. Particles collected on the filter are trapped in the filter matrix and are not easily released from the upstream (outward) side of the filter. Particles are typically only released if the filter is violently agitated, such as being banged on a hard surface. No particles will be able to penetrate to the downstream side after capture.

There are no concerns about the release of particles when changing the filter. No special precautions are needed other than to avoid banging the filter or dropping it on the floor. No particles should “fall” into the lower unit because the particles are trapped in the filter. In addition, the downstream side of the filter, which is the side facing the blower unit, will not have any particles on it.

How often should the filter be changed?

HEPA filters should be replaced when airflow becomes restricted. The EVA Powered Air-Purifying Respirator incorporates a “low flow” alarm that sounds an audible when the airflow falls below 6 CFM.

Please note: All recommendations in this article are subject to revision based upon the latest guidelines from CDC (Centers for Disease Control and Prevention).