HEPA (High-Efficiency Particulate Air) is a filter standard requiring at least 99.97% capture of particles at 0.3 micron (the most-penetrating particle size). True HEPA filters are used in clean rooms, hospitals, and biosafety labs.
In residential HVAC, true HEPA cannot be installed inline as your normal filter — the static pressure drop across a true HEPA filter is far too high for any standard air handler blower. Instead, "whole-home HEPA" systems use a bypass configuration: a side loop with its own dedicated fan pulls a fraction of system airflow through a HEPA filter and returns it to the supply duct.
For homes with severe allergy or asthma sufferers, a whole-home HEPA bypass is the best filtration available. For most homes, a high-MERV media cabinet (MERV 13-16) delivers most of the benefit at much lower cost.
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