What percentage of airborne particles (0.3 µm diameter) can HEPA filters remove?

• A. 99.5%
• B. 99%
• C. 90%
• D. 99.97%

Answer: (D)

The key idea is that HEPA filters have a standard performance that is defined at a size that is hardest for them to capture. For particles around 0.3 micrometers, HEPA filters reach their minimum efficiency, known as the most penetrating particle size (MPPS). At this size, the usual capture mechanisms—diffusion, interception, and impaction—combine in a way that makes filtration least efficient, but even there the filters are designed to capture the vast majority of particles. Under standardized testing, a HEPA filter is rated to remove about 99.97% of particles at 0.3 μm. That high value reflects the robustness of the filtration even at the toughest particle size. For smaller particles, diffusion helps trap more particles, and for larger particles, interception and impaction are more effective, so the overall performance is typically at least as good or better than 99.97% for many other sizes. Therefore, the correct figure is 99.97%, which is the standard HEPA rating for 0.3 μm particles.