Measuring the fitted filtration efficiency of cloth masks, medical masks and respirators - PLOS ONE

Small study, but I thought it was interesting how well N95 respirators worked without any fit testing. I remember at the beginning of the pandemic there was a lot of concern about how well these type of masks would work for the average person without any fit testing.

Measuring the fitted filtration efficiency of cloth masks, medical masks and respirators

Amanda A. Tomkins, Gurleen Dulai, Ranmeet Dulai, Sarah Rassenberg, Darren Lawless, Scott Laengert, Rebecca S. Rudman, Shiblul Hasan, Charles-Francois de Lannoy, Ken G. Drouillard, Catherine M. Clase

Published: April 21, 2025

Abstract

Importance
Masks reduce transmission of SARS-CoV2 and other respiratory pathogens. Comparative studies of the fitted filtration efficiency of different types of masks are scarce.

Objective
To describe the fitted filtration efficiency against small aerosols (0.02–1 µm) of medical and non-medical masks and respirators when worn, and how this is affected by user modifications (hacks) and by overmasking with a cloth mask.

Design
We tested a 2-layer woven-cotton cloth mask of a consensus design, ASTM-certified level 1 and level 3 masks, a non-certified mask, KF94s, KN95s, an N95 and a CaN99.

Setting
Closed rooms with ambient particles supplemented by salt particles.

Participants
12 total participants; 21–55 years, 68% female, 77% white, NIOSH 1–10.

Main Outcome and Measure
Using standard methods and a PortaCount 8038, we counted 0.02–1 µm particles inside and outside masks and respirators, expressing results as the percentage filtered by each mask. We also studied level 1 and level 3 masks with earguards, scrub caps, the knot-and-tuck method, and the effects of braces or overmasking with a cloth mask.

Results
Filtration efficiency for the cloth mask was 47–55%, for level 1 masks 52–60%, for level 3 masks 60–77%. A non-certified KN95 look-alike, two KF94s, and three KN95s filtered 57–77%, and the N95 and CaN99 97–98% without fit testing. External braces and overmasking with a well-fitting cloth mask increased filtration, but earguards, scrub caps, and the knot-and-tuck method did not.

Spoiler: Image comparing different mask types

 

This proves that what I am doing has been working very well for me the last 3-4 years. I wear a surgical mask with a cotton mask over top. I haven't had a cold at all which is unheard of for me.

I caught Covid last year for the first time but I am sure it was from my sick neighbour and I was unmasked around her at the time.

I catch trains and buses with people who sound full of colds or Covid and I have found this double masking excellent so far.

The masks are put straight into a laundry bag after each use and I use the surgical masks up to 20 times before throwing out and starting with 10-15 new ones.

 

I always forget to mention that I sanitize my hands a lot when out too. So both measures have given me good protection.

 

I have been using FFP3 masks since March 2020, I had a packet from before the pandemic for working with MDF, they were at the time about £1.50 for 3! I wont be using anything less. Its pretty clear they work well, I have fit tested the ones I use now so I know they work as specified.

This pandemic could have gone a completely different way if instead of assuming the public couldn't handle to learn a 5 minute video on how to fit FFP2/3 masks they had spent all that focus on giving people that skill and ramping production. Instead they still deny airborne transmission of disease against all the evidence.