A hospital engineer is validating the ventilation system of a newly built bone marrow transplant unit fitted with HEPA-filtered air supply. HEPA filters are defined by their ability to retain at least 99.97% of airborne particles of which diameter?
- A 5.0 micrometre
- B 1.0 micrometre
- C 0.3 micrometre ✓
- D 0.1 micrometre
Correct answer: C. 0.3 micrometre
Explanation
D HEPA filter is defined as one that removes at least 99.97% of particles measuring 0.3 micrometre, because this is the most penetrating particle size: larger particles are trapped by impaction and interception, while smaller ones are removed by diffusion (Brownian motion). Efficiency is actually higher than 99.97% both above and below 0.3 micrometre, so the distractors describe sizes that are retained even more effectively.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.