The Centers for Disease Control and Prevention (CDC), whose previous advice about face coverings as a safeguard against COVID-19 paid little attention to large differences in the effectiveness of different mask types, recently revised its guidance to acknowledge that N95 respirators work better than reusable cloth masks or disposable surgical masks. As with earlier changes in CDC guidance, it is not exactly clear why the agency waited so long to highlight useful information about COVID-19, although the spread of the highly transmissible omicron variant may have something to do with it.
The CDC's prior advice about "Types of Masks and Respirators" mentioned N95 masks as one option without giving any clear indication of how effective they are compared to the alternatives:
Masks are designed to contain your respiratory droplets and particles. They also provide you some protection from particles expelled by others.
Respirators are designed to protect you from particles, including the virus that causes COVID-19, and in doing so they also contain your respiratory droplets and particles so you do not expose others.
CDC continues to learn more about the effectiveness of different types of masks and respirators for preventing COVID-19.
The CDC noted that N95s "filter up to 95% of particles in the air when approved by NIOSH and proper fit can be achieved." But it did not provide any corresponding estimate of how effective cloth masks are, and its advice about when an N95 might be appropriate was opaque. "Some situations may have a higher risk of exposure to COVID-19 than others," it said. "So, you may want to consider the type of mask or respirator to wear depending on the situation."
The CDC's examples of "some situations" included using public transportation, especially in crowded conditions for extended periods of time; "taking care of someone who is sick with COVID-19"; and "working at a job where you interact with large numbers of the public." It also mentioned that people who are especially susceptible to COVID-19 because they are "older," have preexisting medical conditions, or have not been vaccinated might "want to consider the type of mask or respirator" they wear. Regarding N95s specifically, the CDC's most direct advice was this: "When supplies are available, individuals may choose to use a basic disposable N95 respirator for personal use, instead of a mask, in some situations."
By contrast, the CDC's latest guidance about mask types, published on Friday, acknowledges that "some masks and respirators offer higher levels of protection than others" and that "properly fitted respirators provide the highest level of protection." It warns that "loosely woven cloth products provide the least protection," while "layered finely woven products offer more protection, well-fitting disposable surgical masks and KN95s offer even more protection, and well-fitting NIOSH-approved respirators (including N95s) offer the highest level of protection."
The CDC adds that "a respirator may be considered in certain situations and by certain people when greater protection is needed or desired," although it also suggests that the more effective masks "may be harder to tolerate or wear consistently." The reference to limited supplies of N95s has been excised. So has the warning that N95s are incompatible with "certain types of facial hair," which linked to this amusing illustration indicating that soul patches, Zorro mustaches, and side whiskers are OK, while goatees are questionable and Garibaldi beards, wet noodles, and Dali mustaches are right out. The CDC does still note that "gaps can be caused" when "a respirator is worn with facial hair."
While the "disparity" between N95s and cloth masks "is widely known to the general public," The New York Times says, "the update marks the first time the C.D.C. has explicitly addressed the differences." Two years into the pandemic, the Times is saying, the preeminent U.S. disease control agency—the authority on which Americans are supposed to rely for timely, scientifically informed advice about how to protect themselves and their neighbors from COVID-19—is finally acknowledging an important fact that "the general public" already knew. The implication is that Americans are better off disregarding what the CDC says and seeking alternative sources of information.
That information has long been available. A laboratory study published in September 2020, for example, found that a valveless N95 mask was 99.9 percent effective at retaining droplets larger than half a micron generated by speech. While three-layer surgical masks and several kinds of cloth masks reduced the number of droplets detected by 80 percent or more, some designs—including a a "knitted mask" and a "two-layer cotton, pleated style mask" as well as a bandana—were substantially less effective.
A laboratory study published in October 2020 compared the effectiveness of different masks in several conditions involving two mannequin heads separated by 50 centimeters, one of which emitted "a mist of virus suspension through its mouth." When the "receiver" was fitted with a mask, "a cotton mask led to an approximately 20% to 40% reduction in virus uptake compared to no mask." The N95 mask "had the highest protective efficacy (approximately 80% to 90% reduction) of the various masks examined." When the "spreader" had a mask, "cotton and surgical masks blocked more than 50% of the virus transmission," while the rate for the N95 mask was 95 percent. When the edges of the N95 mask were "sealed with adhesive tape" to simulate a good fit, no virus was detected.
Similarly, a laboratory study published in December 2020 found that an N95 respirator blocked 99 percent of a simulated "cough aerosol." By comparison, a "medical grade procedure mask" blocked 59 percent and a "3-ply cotton cloth face mask" blocked 51 percent.
In the real world, of course, masks may not be clean, well-fitted, or properly worn, which is one reason to be cautious about estimating the actual impact of general mask wearing based on studies like these. The CDC has long emphasized the importance of a good fit, and it is clearly concerned that more effective masks will prove to be less comfortable, meaning they are less likely to be worn correctly and consistently. It recommends that "you wear the most protective mask you can, that fits well and that you will wear consistently."
Still, it is hard to understand why the CDC, once it decided that masks were a good idea after all, did not clearly and forthrightly lay out the pros and cons of different designs. It seems to be moving in that direction now because of the omicron wave, which prompted CNN medical analyst Leana Wen to declare that "cloth masks are little more than facial decorations," adding, "There's no place for them in light of omicron." But omicron was identified two months ago, and the latest data indicate that the current surge already has peaked in the United States, with newly reported cases beginning a sharp decline similar to what other countries have seen.
One of the CDC's main functions is to provide the public with accurate, up-to-date information about communicable diseases, based on its own examination of the scientific literature. Theoretically, anyone can delve into journals like Science Advances, Aerosol Science and Technology, and mSphere for the answers that the CDC fails to provide. But since most people are not inclined to do that, it would be helpful to have an authoritative source that reliably summarizes the relevant evidence in a timely fashion. In this case and others, the CDC is effectively telling people to look elsewhere.