All About ASTM
When the pandemic initially started we were all trying to cover our faces as quickly and efficiently as possible, with little knowledge as laymen, as to what worked best. Once we got past the first stage it became apparent that certain types of masks worked better than others and that getting ones with an ASTM rating were the best. Dr. Fauci eventually recommended that everyone have an ASTM level 3 mask for daily wear and that became the standard by which surgical face masks were and are judged. Below I will explain simply how that applies to protecting yourself and those around you.
Surgical face masks were originally given ASTM designations to ensure they met minimum standards for surgery that would protect both the surgeon and the patient. The main purpose was to keep the blood of the patient from penetrating the mask of the surgeon and to keep the surgeon's respiratory molecules from getting into an open wound on the body. The ASTM standard is F2100 and is broken down into levels 1, 2 and 3 to determine the level of efficacy that its 5 criteria meet. I will go over the 5 criteria and their relevance to daily use in the following paragraphs.
The first and least important of the criteria is flammability. Due to the majority of surgical face masks being made of the same materials they should all easily fall into class 1. The purpose is to ensure that no open flame will cause the mask to easily ignite and potentially melt onto the user. Thankfully most surgical face masks are made with polypropylene and while they aren’t fire proof they are more than safe enough to be worn around an open flame.
The next criteria is resistance to penetration by synthetic blood which is measured in mmHg, which translates to millimeters of mercury. MmHg is defined as the extra pressure a column of mercury 1mm high will create. The important thing to note about mmHg is that this is the metric by which the protection a mask will give a surgeon should arterial blood spray and hit the mask. Maximum blood pressure that would likely be experienced is 160 mmHg which is coincidentally what an ASTM level 3 mask protects against. To make a real world comparison, if someone were to sneeze directly into your face mask it would have a pressure of 135 mmHg, so a level 3 mask would completely protect you. Odds are this won’t happen, however, for maximal protection you would want to have ASTM level 3 to prevent that. Otherwise, the respiratory droplets that would hang in the air would likely not challenge even an ASTM level 1 mask, thus why this is the 2nd least important criteria.
Subsequently we come to the 3rd of the criteria which has reasonable importance, especially for prolonged wear, and that is differential pressure. This is measured with a Delta-P score to determine the breathability of a surgical face mask. The measurement system determines essentially the comfort level of the wearer as they breathe into the mask. With a score of 1-2 the mask is cool and comfortable and as it progresses into 4-6 the mask becomes more stifling and hot, although for short wear times this effect is largely negligible. Thankfully the ASTM standards are looking for a lower Delta-P score while also maintaining the other criteria but they allow a higher score for better filtration and fluid resistance as the ASTM levels increase. Regardless, the breathability should not affect the wearer in most environments and I have worn masks with a Delta-P of close to 5 all day and have not found myself to be uncomfortable.
The most important criteria are equal in their usefulness and that is PFE (particle filtration efficiency) and BFE (bacterial filtration efficiency) which are where you get your coronavirus (and other airborne pathogens) protection from. This is provided by the meltblown filtration layer which is electrostatically charged and woven into a web that creates a tortuous path to prevent particles and bacteria from penetrating. PFE and BFE are both equally important and the main difference between them is the density of molecules being forced through the mask at one time, with BFE usually being higher. Either way they are both very important and the filtration level requirements for both to meet or exceed ASTM level 3 are the same. The main difference is that most meltblown manufacturers use BFE as a measuring stick for how effective their product is and is how face mask manufacturers determine which meltblown vendor to use. We have tested several different meltblown fabrics and have seen a strong correlation between the results of PFE and BFE and thus feel confident when using BFE as a way to measure the efficacy of meltblown.
In conclusion I would say that striving to use ASTM level 3 is the best course of action and there is no reason to do otherwise. The cost to manufacturers to use materials that meet the standard is minimal (aside from the testing of the materials) and is a mark of a good quality product. Most vendors of face mask materials will have already vetted their products through independent testing so selecting good raw materials will almost always result in passing the necessary standards.