With recent research showing that ultrafine particles are more hazardous to human health than originally thought, higher-efficiency filters should be used, according to ASHRAE's Guideline 24-2015, Ventilation and Indoor Air Quality in Low-Rise Residential Buildings. MERV 10 rated filters and higher are preferred for removing smaller airborne allergens and particles below 2.5 microns, which may be one of the most significant indoor airborne contaminants in terms of chronic health impact.