Which statement about HEPA filters is true?

• A. They filter up to 99.97% efficiency down to 0.3 microns
• B. They filter up to 99.0% efficiency down to 1 micron
• C. They are used only for liquids
• D. They are the same as ULPA filters

Answer: (A)

HEPA filters are defined by their very high efficiency in removing particles from the air, with a specific certification for the most challenging particle size. The standard states that a HEPA filter captures up to 99.97% of particles that are 0.3 microns in diameter. That 0.3 µm size is considered the most penetrating particle size, where capture is hardest to achieve; at sizes smaller or larger than 0.3 µm, other capture mechanisms (diffusion for small particles and interception/impaction for larger ones) help increase efficiency. So this rating is the hallmark of a true HEPA filter. The other statements don’t fit: HEPA refers to air filtration, not liquids; ULPA filters have even higher efficiency than HEPA, so they are not the same; and a claim of 99.0% at 1 micron doesn’t match the standard HEPA specification.