Why Not Face Shields Too?

Last March, I wrote a post suggesting that enough informal evidence was emerging that masks might provide protection against COVID-19 that states might consider enacting mask mandates, requiring at least cloth-type masks. I also suggested that randomized controlled trials (RCTs) of masks could be helpful in helping ascertain the effectiveness of masks. The rest is history. Mask mandates followed. RCTs did not.

This post asks a simple question: Why has there been no movement to require face shields in addition to masks in public places? My instinctive cost-benefit analysis suggests that such a regulation might be worthwhile, but that's not the point of my post. After all, I recognize that there was no causal effect of my first post on the adoption of mask mandates, and I assess the plausibility of shield-plus-mask mandates at close to zero. Rather, I'm interested in the inquiry as a puzzle and as a window into how the public thinks about COVID risk-reduction measures.

Let me start with the affirmative case. Face shields are cheap, less than $1 each for reusable ones. They can be worn concurrently with other personal protective equipment. By themselves, they are not as effective as masks by themselves at protecting wearers from aerosol-sized droplets, and so the CDC does not recommend them "as a substitute for masks." Still face shields at least "block the initial forward motion of the jet" of air from a cough, potentially reducing inhalation of virus particles. A study found that "wearing a face shield reduced the inhalational exposure of the worker by 96% in the period immediately after a cough." A study suggests that a type of shield provides extra protection for ophthalmologists. Perhaps most impressively, after 19% of a group of community health workers in India working with Covid-19 patients themselves tested positive, face shields were added to their existing PPE, and there were no further positive tests.

None of these studies is a gold standard RCT. But the studies of face masks generally aren't RCTs either. One face mask study randomized some people to receiving a recommendation to wear a mask, and it found only statistically insignificant benefits of receiving the recommendation. The medical community has shrugged at the absence of evidence from controlled trials, pointing out that there are other forms of evidence and claiming that randomized controlled trials would be unethical.

The failure to run effective RCTs for both masks and face shields strikes me as a major failure in global efforts against this pandemic. Even if one believes the anecdotal evidence that masks are beneficial is clear, there are many seemingly intelligent people (not just Alex Berenson) who point to anecdotal evidence that mask-making doesn't make a difference. Consider, for example, the comment from "Oleg" that Covid is spreading rapidly in Toronto despite widespread universal mask use. I wouldn't assign much weight to such analysis in comparison to meta-analyses that conclude that masks have some benefit, but some people are genuinely skeptical that masks make much of a difference.

RCTs showing effectiveness wouldn't convince all of them, but it would convince many. Moreover, it's quite possible that many people are overestimating the benefits of masks, viewing them as get-out-of-social-distancing-free cards. Meanwhile, the ethical argument against RCTs does not seem strong. One need not discourage some people from wearing masks to run an effective study. A trial can select some people to receive an intervention encouraging mask wearing. The study giving a recommendation is an example of this, but the intervention could be stronger than a mere recommendation. For example, the treatment group could receive financial incentives, such as a promise that if they answer a video call at some random time quickly while wearing a mask, they will receive some money. Granted, such trials would be expensive, but given the global scale of the pandemic, any improvement in information on an issue so vital is likely to produce benefits greatly in excess of costs.

In principle, RCTs also could provide better information on which type of masks are most successful. There are, of course, many studies assessing masks in the lab, but, so far as I have been able to find, none in the field. Clearly, an RCT could give the control group free cloth masks and the treatment groups some higher quality of mask, such as this Honeywell mask with polypropylene filters, which the Wall Street Journal reported "aimed to combine the comfort of a cloth mask with protection near what an N95 affords."

Even absent RCTs, one could imagine toughening mask mandates to require some of these seemingly higher-quality masks. Yet, I am not aware of any jurisdiction that requires higher levels of protection. There are several easy explanations for this, though. It may be difficult or impossible to determine whether people are wearing compliant masks, and we don't want shoppers fighting with employees about whether their masks meet the relevant standard. Some of the highest quality masks may still be in short supply, and some may still want to reserve them for health care workers. Individuals have spent a lot of money on masks, and they would be frustrated if their purchases were made obsolete.

Whatever the merits of these arguments against toughening mask requirements, the complete lack of interest in requiring face shields with masks suggests that something else is going on. It's easy to tell if someone is wearing a face shield and face mask. Face shields are not in short supply. And a hypothetical mandate would not mean that anyone could throw away their masks. So why is there absolutely no discussion of whether we should require face shields in addition to masks?

One unsatisfactory answer is that the cost-benefit calculus for adding the requirement of face shields is much worse than the cost-benefit calculus for the initial mask requirement. On the cost side, maybe face shields are much less comfortable. I'm skeptical of this; the added increment of discomfort is fairly low. Similarly, I will readily concede that they look ridiculous (and I felt a little embarrassed wearing one recently to a doctor's appointment). But masks looked equally unflattering when barely anyone was wearing them. The benefit side may be more plausible. Perhaps the marginal benefits of a face shield, for the wearer or for source control, may be considerably lower than the initial benefits of a face mask. But are they so much lower that costs exceed the expected benefits, taking into account society as a whole? That seems doubtful. And so one might expect those who point out that masks aren't so uncomfortable–just wear a mask!–to also highlight that it isn't a big deal to wear a face shield on top of a mask.

So what is the explanation? Perhaps the best answer is that we're all conventional and highly influenced by what people around us are doing. Very few of us are willing to take, let alone urge, steps that no one else is taking. It's easy to paint people who don't wear masks as bad people. But the vast majority of people who don't wear masks are part of groups where non-mask-wearing is the norm. It is personally costly to break a social norm. I wish that everyone would wear masks, at least when associating with people outside their household. But I don't think that their basic decisionmaking calculus is all that different from the approach of people who wear masks but don't wear face shields. People who are willing to be weird for the social good are rare and underappreciated.

One might argue that last year's shift in many social groups to widespread mask-wearing is inconsistent with my claim that concerns about conventionality have inhibited widespread adoption of face shields. Certainly, those shifts demonstrate that in response to an exogenous shock, norms can change quickly. Over a short period of time, for some, wearing a mask went from weird to cool. But once norms are established, they are hard to change. Mask wearing may increase slowly over time. There are some people who have mixed social groups, and so when Covid gets worse, one should expect mask-wearing to increase in a virtuous cycle. But it requires a lot for the norm to change dramatically, especially in the absence of clear new evidence, and in the absence of RCTs, that is unlikely to materialize. And so we should expect face shield use to be common only in certain occupational settings, like emergency rooms and care homes. Even with highly contagious variant Covid-19 strains, it seems unlikely that face shield use will pick up appreciably, let alone that we will see regulatory requirements.

Is there any possibility of some change that would encourage greater face shield use? I can think only of one. President Biden plans to call for 100 days of mask wearing when he becomes President. Can't hurt, but it's hard to see such a request as doing much good. Those who would do it for him are largely already wearing masks, and Trump supporters who don't wear masks are unlikely to change because he says "pretty please." Similarly, general calls to "be careful" won't meaningfully change much behavior at this point, especially given recognition that those making such calls sometimes turn out to be hypocrites. But it is plausible that Biden could induce some of his supporters to take some steps that they are not taking now, especially if he models good behavior for them. If President Biden started wearing a face shield in addition to a mask, maybe ten or twenty percent of the public would do the same. But he'd look kind of silly. I'm not counting on it.