What is the minimum efficiency of a HEPA filter for 0.3-micron particles when properly installed and maintained?

• A. They remove all particles from the air
• B. They require complete airtight seals for any effectiveness
• C. They capture at least 99.97% of particles 0.3 microns
• D. They only work at fixed flow rates

Answer: (C)

HEPA filters are specified to remove at least 99.97% of particles that are 0.3 micrometers in diameter when they’re properly installed and maintained. That 0.3 μm size is the most challenging for filtration, so the 99.97% rating at this size ensures strong performance across a range of particle sizes. The mechanism behind this efficiency comes from a combination of diffusion (helpful for very small particles), interception, and impaction (more effective for larger particles). When installation minimizes bypass—meaning the air actually flows through the filter rather than around it—and maintenance keeps the media clean and the seals intact, the filter can achieve that rated performance. The other statements aren’t accurate: a HEPA filter does not remove every particle, perfect airtight seals are unrealistic although good seals are important, and filters can operate over a range of flow rates within the design limits rather than only at a fixed flow rate.