Higher-efficiency HVAC filters should be considered as ultrafine particles are more hazardous to human health than originally thought, according to ASHRAE’s updated residential indoor air quality guidelines. 

The new rules from the American Society of Heating, Refrigeration and Air-Conditioning Engineers take a firmer tone than earlier guidance, said the committee’s chairman.

 “In the 2008 version, we indicated that if a lot of ultrafine particles were expected, higher-efficiency filters should be considered. Period,” said Paul Francisco, chairman of the committee that wrote the rules. “Now we say a lot more. We cite research that shows that ultrafine particles are a much more significant concern, and we state explicitly that higher-efficiency filters mean MERV (minimum efficiency reporting value) 13 or higher.”

The update recommends that higher-efficiency filters be considered. The association says when selecting filters, consideration should be given to the effects of the filter’s pressure drop on airflow, fan capacity and energy use. Multistage particle filtration, a relatively coarse filter followed by a high-efficiency filter, can help remove different sized particles without overloading the HVAC system.

“Recent research suggests that mass of particles below 2.5 microns 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 committee who oversaw the revision of the section.