HEPA Filtration

Definition

HEPA (High-Efficiency Particulate Air) filtration refers to filters rated to remove at least 99.97% of particles at 0.3 microns. ANSI/IICRC S520-2024 defines a HEPA filter that way (Section B) and explains the test point: 0.3 microns is the particle size that gets through a filter most easily, so particles both larger and smaller are captured at better than 99.97%. The 0.3 figure is the filter's performance rating, not the size of a mold spore. During remediation, HEPA filtration is used to capture mold spores, fragments and other airborne particulate. A HEPA filter captures particles through three processes working at once: interception, impaction and diffusion (Section B).

Why It Matters

Disturbing mold-contaminated material during remediation can release spores and fragments into the air. As the extent and density of the growth and the amount of disturbance increase, the potential for airborne release and for contamination spreading beyond the work area generally increases too. Controlling that is the job of engineering controls on a mold project, and S520 gives air filtration devices two roles in it (§9.1.3): set up as negative air machines, they create a pressure difference so air flows into the work area rather than out of it, and run as air scrubbers, they remove airborne particles. The containment itself is the barrier, usually polyethylene sheeting, that the pressure difference works with. For homeowners, the useful check is the wording on the machine or vacuum: terms like "HEPA-type" or "HEPA-like" don't mean the filter meets the HEPA rating, so ask whether it does.

How It's Used in Mold Work

Professional remediators use HEPA-filtered negative air machines to keep the air pressure inside containment lower than in the rooms around it, so air flows in through any gap rather than out. The aim is to keep contamination from spreading into clean parts of the house, and the pressure difference limits how far it can travel. Under S520, containment should hold a minimum of 4 air changes per hour, with air drawn from the entry across the work area and exhausted outside where possible (§9.3.5), and wherever the exhaust goes, it should not put people at risk of exposure (§9.1.3). HEPA air scrubbers can also clean the air inside the work area. How long the equipment runs follows the job's completion checks rather than a set number of hours: when an independent post-remediation verification is planned, the controls stay on until the verifier directs otherwise or reports a successful verification (§12), and when it isn't, until the remediator's own post-remediation evaluation is complete (§9.3.12). ANSI/IICRC S520-2024 treats HEPA use as a recommendation rather than a blanket requirement: remediators should use HEPA vacuums (§9.1.5), and air filtration devices, whether run as negative air machines or air scrubbers, should be HEPA-filtered and leak tested so all incoming air passes through the filter (§9.1.3). Equipment rental costs typically run $50-150 per day. S520 has HEPA filters changed away from the jobsite, in an area a release of contaminants would not affect (§9.1.3), and says mold-contaminated waste generally needs no special provisions for disposal beyond the laws that apply (§9.3.11).