How well do DIY FACE COVERING materials compare against the N95 RESPIRATOR standard?

Some materials make better filters than others! The best materials balance breathability with filtration efficiency. To better understand DIY materials performance, samples of commonly used homemade mask materials were sent to TSI, Inc who tested them using the same machine and test procedure used to give N95 respirators their NIOSH certification [1].

From these test results, it appears the best material for homemade face coverings is Filti nanofiber. Two layers of blue shop towels is perhaps the most economical option. Keep in mind some materials tested here may not be approved for face covering or mask usage, contact the manufacturer for safety information.

how are N95 RESPIRATORs tested?

The data below shows the results of different materials tested by TSI, Inc using a TSI 8130a test machine [2]. This instrument is the gold standard in filtration media testing. The TSI 8130a is used by NIOSH to certify N95 respirators (42 CFR Part 84) and by respirator manufacturers around the world to test their products. These tests use a particle size distribution specified by the NIOSH standard.

“Filter testing is a complex process that requires precise control in all parts of the system to ensure accuracy:  particle generation, particle conditioning (humidity and electrical charge), flow rates, and photometer/electronics performance.  Even small errors in testing can generate incorrect and misleading results.

Another very important factor is the test system must have a high degree of precision (mathematical representation of repeatability) so measurements from different tests of the same filter media can be compared. This is also true for comparison of test results across different  filter media types.

Test conditions and requirements are strictly defined by government agencies (NIOSH in the U.S.). 

The test results below, 100% follow the NIOSH Standard (42 CFR part 84).   Any other test configuration is an approximation to the NIOSH requirements, and results may vary.”

When making a DIY face covering , there has to be a balance between filtration, breathability, and seal.

A material with a higher filtration efficiency will filter a greater amount of small particle aerosols and large droplets (non-oil aerosols). For example, an N95 filters 95% (or better) of test particles. That means only 5% (or less) of test particles penetrate through the material. Most N95 respirators use a special electrostatic media to achieve a high filter efficiency while retaining breathability. It is unlikely any DIY face covering material can truly achieve the N95 filter rating while remaining breathable, but the only way to know is to test them! FILTI does come close.

An N95 also has a resistance (pressure drop) target around 8 mmH20 — if the pressure exceeds 15 mmH20 you start to gain a vacuum effect and essentially start to suck air from the sides of your mask losing the benefits of the mask. It will also be uncomfortable and hard to breathe. Lower resistance is better.

As for fit — the tighter your face covering, the tighter your fit, the better you will benefit from the filtration efficiency and inhaling/exhaling particles out of your face covering. A loose style surgical mask will help block large particles but will not stop small particles from entering and exiting your mask. I highly recommend visiting www.FixTheMask.com- they have developed a mask “brace” which will allow you to seal a surgical mask.

Testing notes

N95 respirators are tested at 85 L/min and a typical respirator is ~160 cm^2. A salt aerosol (NaCl) is used as the test agent. Although salt particles are .1µm, the test results are based on a distribution of particle sizes specified by the NIOSH standard.

These tests were performed using a TSI 8130a tester with salt aerosol as the test agent and a standard filter holder area of 100 cm^2. Because of the difference in area the flow rate was adjusted to ~60 L/min so the velocity through the samples was approximately the same as on a mask. For some of the smaller samples an adapter was used that lowered the test area and the flow rate was adjusted accordingly.

With these adjustments in flow the pressure drop (resistance) should be similar to how the fabric would work as a mask and the % Penetration (or % Efficiency) should be approximately the same.

For another project TSI tested 20 N95 masks and the average efficiency was 98.2% and typical pressure drop was about 9 mm H2O. The limit on pressure drop for respirators is 35 mm H2O.

FUNDING - MaskFAQ and Chloe Schempf have not received any funding from TSI Inc. or Filti or any other manufacturer of tested materials. TSI Inc. conducted testing at no cost and all materials were sent blind.

Mask Fit & Handling

Regardless of material used, keep in mind that proper mask fit and proper donning/doffing procedure must be followed to achieve maximum respiratory protection and to avoid contamination. A leaky or dirty mask cannot be relied upon.

[1] Surgical N95 respirators are class II devices regulated by the FDA, under 21 CFR 878.4040, and CDC NIOSH under 42 CFR Part 84. See also the U.S. Department of Health & Human Services FAQ regarding N95 masks.

[2] These test results are provided for informational purposes only and do not take into account mask fit or real-world usage, should not be interpreted as recommendation or advice, and are not approved by any regulatory agency. Note that many DIY materials are not approved or recommended for filter usage, please contact the manufacturer with any questions or safety concerns. You can usually request or lookup the MSDS (material safety data sheet) or SDS for a product.