HEPA (High Efficiency Particulate Air") filters remove a minimum of 99.97% of all airborne particulate matter down to 0.3 microns in diameter. Because these filters use small grid-like structures to capture dust particles, dirt build-up clogs the airflow pathway sooner than what is expected from S-class filters. However, SEBO's HEPA filters are sold with four-layer "Ultra-bags," which provide highly effective filtration before the airflow goes through the HEPA filter, greatly increasing its life expectancy and reducing the potential for diminished air flow from blockages of the filter material.