The 2015 version of ASHRAE’s residential indoor air quality (IAQ) guideline recommends that higher-efficiency filters be considered to help filter out particles below 2.5 microns (PM2.5), which are “more hazardous to human health than originally thought,” according to recent research cited by ASHRAE. Guideline 24-2015, Ventilation and Indoor Air Quality in Low-Rise Residential Buildings, includes explanatory and educational information on achieving good IAQ that goes beyond the requirements of ASHRAE’s Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. The guideline is intended as a companion document to Standard 62.2. “Recent research suggests that mass of particles below 2.5 microns (PM2.5) may be one of the most significant indoor airborne contaminants in terms of chronic health impact in residences of those that have been well studied,” said Rick Karg, a member of the Guideline 24 committee. “PM2.5 is also the most straightforward contaminant to remove from indoor environments through filtration. MERV 10-rated filters and higher are preferred for removing smaller airborne allergens and PM2.5 particles.” The updated guideline explicitly states that higher-efficiency filters with minimum efficiency reporting value (MERV) ratings of 13 or higher should be considered for the removal of ultrafine particles. For more information, see ASHRAE’s press release.