True HEPA vs 'HEPA-Type': How to Tell a Real Filter From a Marketing Word

HEPA ratings are quoted at 0.3 microns because that is the Most Penetrating Particle Size — the hardest size for a fibre filter to catch. Larger particles are intercepted and impacted; smaller ones move erratically by Brownian diffusion and collide with fibres more readily.

This means a HEPA filter is more efficient above and below 0.3 microns, not less. The occasional claim that 'HEPA cannot catch particles smaller than 0.3 microns' has the physics backwards. A true H13 filter's efficiency at 0.1 microns is higher than its rated figure, not lower.

• Find the grade printed on the filter frame — H11, H12, H13 or H14. If the only place the word HEPA appears is the carton, that is your answer.
• Read the exact wording. 'True HEPA H13' is a claim. 'HEPA-type filtration technology' is a sentence constructed to avoid making one.
• Check the replacement filter listing as well as the machine. A handful of units ship with a real H13 and sell replacements that are quietly not.
• Ask whether the machine is sealed. Dyson certifies whole-machine sealing to H13; most brands certify the filter only. On a poorly sealed body, some of the air goes around the filter.
• Be suspicious of very cheap replacement filters from third-party sellers. An unbranded 'H13-compatible' filter at a third of the price is usually not H13.

Higher grades cost airflow. A denser mat has higher pressure drop, so the same fan pushes less air through it, and clean-air delivery rate — the number that actually determines how fast your room gets clean — falls.

For a bedroom, an H13 filter moving 300 m³/h of air will out-clean an H14 filter moving 180 m³/h, comfortably. Buy the grade that is standard for the category and spend the difference on CADR, carbon mass or a second unit for another room.