What’s the Best Mask?

Last update 2023-02-16

This is a description of the peer-reviewed literature on the filtration efficiency (how well do they remove aerosols) of different kinds of masks. The results of this kind of testing apply to infectious aerosols and removing particles (such as PM2.5 and PM10) from wild-fire smoke. They do not apply to removing gases (NO2, CO2 and ozone) or volatile organic compounds (VOC) that are gases at room temperature.

The best masks for respiratory protection are respirators: N95s, Can99s, and FFP3s. In Canada, and likely elsewhere, there is adequate supply for frontline workers. Information on finding N95s is here.

We also recognize that they are not within everyone’s budget. Information on extended use of N95s is here. Lower-cost alternatives to N95s and their expected filtration efficiency are shown in the infographic above.

A summary of the evidence, based on fit testing on humans:

N95s without fit testing: 82 - 99% particle filtration (Reponen, O’Kelly, Du, Duncan)

KN95s, KF94s: 55% (1 mask, 7 participants, 1 replicate, O’Kelly), 84% (1 mask, ≥8 participants, Duncan) and 97% (1 mask, 3 men, 4 replicates, Popov).

Overmasking, cloth over medical mask. Improved over either alone (Brooks):

• 38 - 44% particle filtration for medical alone (Blachere)

• 23% particle filtration for cloth mask alone (Blachere)

• 52 - 75% for the combination (Blachere)

Medical mask with minor modifications (mask hacks) was improved over medical mask alone (Clapp, Blachere): up to 85%

Medical mask on ties: 72 - 80% (Clapp, Davies)

Medical mask on earloops: 39% (Clapp)

Medical mask, certification not specified, design not specified: 50 - 75% (Duncan)

Cloth mask 23 - 86% (Davies, O’Kelly, Clapp, Blachere, Duncan)

Well-fitting 2-layer cloth mask on overhead ties; quilter’s cotton, T-shirt or fashion fabric 55 - 60% (Drouillard)

Non-certified disposable mask: no published data. Likely improved by overmasking with cloth.

What does it mean?

Any mask is better than no mask. Respirators, even without fit testing, provide the best efficiency. Non-certified disposable and single-layer, badly-fitting cloth masks are likely the least efficient. In between - all in the ‘good’ range - are well-fitting cloth masks with 2 or more layers, medical masks, and the combination. Many KN95-type masks likely fall in this ‘good’ range, based on informal testing.

The international standard for masks for community use (sometimes called barrier face coverings) is ASTM International F3502. These are tested using different methods, but it is very likely that masks that meet this certification would fall in the 50 - 70% range, and some would be better. Masks meeting these standards are listed here. NIOSH guidance on masks for the workplace is found here. F3502 is in no way meant to replace respirators in the workplace where they are needed.

To compare like with like, the percentage filtration efficiencies quoted here were extracted from studies of protection of a human wearer, using particles 0.02 to 3 mcm (Clapp), 0.02 to 1 mcm (Drouillard) and 0.02 to 0.1 mcm (Davies, O’Kelly, Blachere, Reponen, Du). Other studies examining source control (Blachere, Brooks), using manikins (Brooks, Ueki), and using larger particles (Brooks) come to similar conclusions.