NIOSH Finds Modified EHMRs Meet Exhalation Resistance Performance Requirement

Elastomeric half-mask respirators (EHMRs) whose exhalation valves were either modified or covered to improve source control met NIOSH’s performance requirement for exhalation resistance, according to an agency report released earlier this month. The report describes a NIOSH study that measured exhalation resistance and levels of inspired carbon dioxide in nine different EHMR configurations mounted on a headform.

Interest in the use of EHMRs for source control stems from the early days of the COVID-19 pandemic when healthcare workplaces experienced shortages of N95 filtering facepiece respirators (FFRs). Because EHMRs—unlike FFRs—are designed to be cleaned and reused, they are an appealing choice for healthcare workers, especially when FFR supplies are low. But most EHMRs feature an exhalation valve that allows exhaled breath from users who are infected with a respiratory virus to potentially spread the infection, compromising the devices’ usefulness for source control.

The study discussed in the new NIOSH report was designed to test whether modifying or covering the exhalation valve increases breathing resistance and concentrations of CO₂ inside the respirator. In the modified EHMRs, investigators removed the membranes around the inhalation valves and covered the exhalation valves with duct tape. For covered respirators, investigators used either a Level 1 or Level 3 surgical mask positioned over the exhalation valve. Both N95 and P100 filters were assessed.

While users who modify a respirator void the NIOSH certification of the device, manufacturers may find the study useful when considering new EHMR designs that eliminate the exhalation valve, according to the NIOSH report. The study did not attempt to determine whether modifying or covering EHMRs improved source control or affected the level of respiratory protection provided to users.

Exhalation resistance was measured at a continuous airflow rate of 85 L/min, which is comparable to the mean expiration rate of healthy adult males exercising at high work rates, according to research cited in the report. NIOSH requires respirators’ exhalation resistance not to exceed 25 millimeters of water column (mmH₂O). Modified and covered respirators were compared to a control group of respirators purchased off the shelf.

The modified respirators allowed both inhalations and exhalations to pass through the inhalation valve port. While eight of the nine modified EHMR configurations exhibited increased exhalation resistance, all nine passed the NIOSH test. According to the report, removing the membranes from the EHMRs increased the “dead space” inside the respirator and resulted in increases in mean inspired CO₂ levels ranging from 0.33 to 0.98 percent. At the high end of this range, users may experience discomfort, the report states.

For the covered respirators, the investigators centered the surgical masks over the exhalation valve. Exhalations passed through the valve and were either deflected by the mask or flowed through it. With both Level 1 and Level 3 masks, all EHMR configurations passed the NIOSH requirement for exhalation resistance. Neither Level 1 nor Level 3 masks caused a significant increase in inspired CO₂.

NIOSH cautions that the study’s scope was limited, assessing only nine EHMR configurations and using small sample sizes. For more information, download the report from the agency’s website.