The Department of Anesthesia is working assiduously with others throughout University of Iowa Hospitals & Clinics and other institutions to find creative solutions to shortages of personal protective equipment (PPE) in the midst of the coronavirus pandemic. As we are able, we will post the results of our experiments and investigations on this site.

Can we use plastic disposable face masks intended for mask ventilation with a filter affixed to the front as a substitute N95 mask?

Mark Gallion, CRNA
Patrick Hartley, MD
Patrick O’Shaughnessy, PhD
Cynthia A. Wong, MD
University of Iowa

Intrigued by the innovative approach to achieving the equivalent of an N95 mask with the tools of our trade (https://youtu.be/Es_iY5WJdmI), we tested the use of the disposable face mask used on anesthesia circuits with a filter (see below) attached to the front of the mask, as described in this video.

We used the two filters we are currently using on our anesthesia machines. The White filter is the filter routinely used on the inspiratory and expiratory limbs of the anesthesia circuit for all patients (EU303 filters - bacterial/viral/low resistance filter made by Vyaire Medical with a 99.8% viral filtration efficiency rating). The Blue filter is the one that has been recommended for COVID+ patients (Ultipor 25 made by Pall Corp with a 99.999% viral filtration efficiency rating).

We used the two filters we are currently using on our anesthesia machines. The White filter is the filter routinely used on the inspiratory and expiratory limbs of the anesthesia circuit for all patients (EU303 filters - bacterial/viral/low resistance filter made by Vyaire Medical with a 99.8% viral filtration efficiency rating). The Blue filter is the one that has been recommended for COVID+ patients (Ultipor 25 made by Pall Corp with a 99.999% viral filtration efficiency rating).

Background: Efficiency for a filter is not the same for all particle sizes. An N95 has its minimum efficiency for particles with an actual diameter around 40 – 60 nanometers but is nearly 100% efficient for smaller particles and larger particles.  An N95 should also have a pressure drop, or “breathing resistance” less than 30 mm of water pressure. A typical N95 is around 10 mm pressure when clean.

Results:  A qualitative fit test (QLFT) (to see if the mask leaks) was conducted on two male anesthesia providers with the 0.3-micron filter attached to the face mask. One passed and the other, who sported a short beard, did not.  

In the lab, we tested these two filters at a flow rate of 30 L/min (see Janssen LL et al. J Int Soc Resp Protec 2005;22:122-141 for justification) which is approximately the flow rate for minor exertion. Note that NIOSH tests N95 masks at 84 L/min, which is high exertion.

At 30 L/min flow rate the pressure drops were:
White: 58 mm H2O
Blue: > 70 mm H2O (exceeded pressure sensor upper level)

For efficiency (tested inside a chamber filled with salt aerosol, and measuring particle counts upstream and downstream of the filter) the minimum efficiency was:
White: 60% with nearly 100% above 1 micrometer
Blue: 98.5% with nearly 100% above 0.5 micrometer

We conclude that the Blue filter should not be used given its very high breathing resistance. The White filter will certainly protect well against the smallest droplets (see Yang SH, et al. J Aerosol Med 2007;20:4849-4, PMID 18158720) but may not protect against a free-floating virus (around 100 nm in diameter). Additionally, the White filter has fairly high breathing resistance even at moderate exertion so should not be worn for long periods. Finally, the mask-strap combination may not be leak proof for all individuals.