How do ASTM F3502-21 masks compare? It depends on the test.

It might feel like ancient history now, but during the summer of 2020 the U.S. Centers for Disease Control's NIOSH division was scrambling to help the public make sense of masks. Did fabric masks work? Were they to protect yourself, or only to protect others? What good was a filter?  Together with industry, NIOSH set out to establish minimum performance standards for face-mask filtration and breathability, and provide some much needed guidelines for mask manufacturers to follow.

The CDC-approved standard became official on February 16, 2021, when industry standards group ASTM published F3502-21, Standard Specification for Barrier Face Coverings (BFC).

More than two years later, you might STILL be wondering what makes an ASTM F3502-2 mask better than your average face covering. Why are they guaranteed to be more breathable, and offer better filtration, than that fabric mask you bought from  Old Navy back in 2020? Does it make sense to swap out your surgical mask or even that KN95 respirator for a tested cloth mask? We'll compare the standards and discuss how performance and breathability are measured differently for all three types of masks. But first, let's answer the question:


What Are We Actually Comparing?

The ASTM F35021-21 specification assigns each mask two numerical scores: one for filtration efficiency, and another for breathability.  Depending on the score, the barrier face covering is rated as either Level 1 (Lower Performance) or Level 2 (Higher Performance) in each category.  A compliant ASTM F3502-21 label always features these scores, so you know exactly what you're getting.

 So, how are these new categories measured, and what do the specifications tell you about the breathability and protection the mask actually provides in comparison to surgical masks and medical respirators?

Figuring Out Filtration

Per ASTM F3502-21 testing requirements, masks need to reach a minimum sub-micron filtration score of 20% in order to qualify for the standard. This means stopping at least 1 in 5 nano-meter-sized particles from penetrating the mask material and reaching your lungs. Fine aerosols are the tiniest type of airborne particulates, within the same ballpark size as virions like the coronavirus.

It makes sense that the CDC would use fine aerosols filtration when measuring protection efficiency. Medical respirators like the N95 are designed to filter out 95% of the same size particles. In other words, under the new CDC standards, a Level 1 Mask needs to be about one-fifth as efficient as a N95 respirator when it comes to fine aerosols in the sub-micron range. 

You might be already thinking to yourself, are ASTM and the CDC really saying 20% filtration efficiency is enough? More importantly, will 20% keep me safe? The good news is that according to aerosol experts, a mask that's able to filter out even a small percentage of fine aerosols is going to be exponentially more effective at filtering out larger aerosols.

For example, a Level 1 mask should be able to block at least 80% of particles larger than 4-5 microns. With the science now suggesting the importance of viral load, 80% might be a good place to start. Also worth remembering is the FDA doesn't require surgical masks to be tested at all against aerosols this small. F3502 and filtering respirators like the N95 are unique in this regard.



Measuring Breathability

Filtration efficiency is only one of two important scores on your Barrier Face Covering label. A certified ASTM F3502 mask must also be breathable enough to wear comfortably all day. Too much inhalation or exhalation resistance will disqualify a mask from being able to say "Meets ASTM F3502."

Air flow resistance, or breathability, is measured in millimeters... and lower is better!  The highest (least breathable) score permitted for F3502 is 15mm, although you might see the scores for medical respirators added to the label for context (N95 respirators can go all the way up to 35mm... that's a lot of work for your heart!).

The F3502 standard uses 15mm as the cutoff because this masks with level of breathing resistance were not expected to require a pre-use medical evaluation for most users, making them safer for general use.

NIOSH-certified respirators have two different breathability limits: one for inhalation and one for exhalation. For inhalation, the air flow resistance limit is 35mm, more than double the maximum allowed under the F3502 standard.  Surgical masks, on the other hand, don't always have any limit when it comes to breathing resistance. That's because surgical masks are not traditionally designed to offer protection from aerosols, so the material didn't necessarily need to be breathable. The exception is surgical masks made to ASTM F2100 standards, which require a maximum pressure drop of 5mm for Level 2 and 3, and 4mm to for Level 1.


What kind of masks don't qualify under the new F3502 standards?

In most cases, the masks that won't get the ASTM F3502-21 label will masks that don't prevent at least 20% of fine aerosols from reaching your nose and lungs.  This will disqualify the majority of the single-layer masks and most, if not all, of the stretchy-style masks and neck gaiters.

On the other end of the spectrum are masks that trade too much breathability to achieve that high level level of filtration. This may prohibit a lot of the thicker cloth masks, some KN95 and similar medical-style respirators, and maybe some non-ASTM surgical masks from getting the ASTM F3502 label.

How do I know if a mask is ASTM F3502 compliant?

According to the scope of the new consumer mask standard, a compliant mask will be labelled as such, and will always have the filtration and breathability scores discussed above.

ASTM typically requires a statement that the product "meets ASTM specification" to be displayed prominently on the package. Look for a disclaimer that the "barrier face covering" is not a medical respirator certified by NIOSH. This helps to differentiate between masks with a protection efficiency score suitable for everyday use from masks designed for medical professionals, which are still in short supply.

What are Workplace Performance Masks?

Workplace Performance and Workplace Performance Plus are NIOSH-endorsed mask standards that build on the F3502-21 requirements. All F3502 masks require fit testing, also known as a leakage assessment. But whereas standard F3502 coverings can be fit-tested using either a "quantitative" (measured) or "qualitative" (non-measured) method, the fit testing for Workplace Performance masks must be quantitative, and the resulting "Leakage Ratio" goes right on the label.

To be considered a "Workplace Performance Mask", a fit-tested mask needs to qualify as Level 2 under the F3502 standard, with a Leakage Ratio of at least 5% (higher is better). To qualify as a "Workplace Performance Plus Mask", the same fit-tested mask would need to reach a filtration efficiency of at least 80% under the F3502 method, with a Leakage Ratio of 10% or higher.