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Last week, a created consternation. The study took 45 kids (ages 6 to 17) and asked them to wear a mask. It measured rates of CO₂ inside the mask. The rates were high, and inversely related to age: the youngest kids appeared to have the highest CO₂ concentration.

Criticism came immediately. One pointed out that as children draw breath in, only a tiny fraction is from inside the mask (where it may be at a higher CO₂ level). The rest of the air is pulled through the mask, and the CO₂ will be diluted with room air in the lungs. There were many more objections raised (some legitimate), and the usual calls for retraction.

However, both the paper and the critics miss the point: Should kids be required to wear a mask and, if so, when?

It is a simple question, but it divides public health authorities. The World Health Organization (WHO) masking kids under the age of 5, and only masking 6- to 11-year-olds under some circumstances. The CDC masks be worn by any unvaccinated individual over 2 years old in indoor public spaces. This means the WHO and CDC are in diametric opposition on the decision to mask kids ages 2 to 4 in daycare or other public settings. Who is right?

The truth is there are potential benefits to masking kids, and potential risks. The biggest potential benefit is the possibility of reduced SARS-CoV-2 acquisition and transmission. The potential risks include normal language acquisition, speaking, and development. At very young ages (<2 years old) or while sleeping, there may be risks of suffocation, which both the CDC and WHO acknowledge.

Real life can complicate both the benefits and risks. Young kids may not wear the mask appropriately and it may slide off the nose. Saliva or mucous can soak cloth masks, which can diminish the putative benefit and potentially increase risks. During heavy exertion, masking kids may lead to fatigue or the subjective feeling of shortness of breath.

On balance then, does masking kids help?

Before I answer the question, let's consider one more complexity: It might not have a single answer. Here are several factors that might determine the answer:

1. Type of mask: cloth versus disposable surgical
2. Age of child/executive function: below some age, or some ability to self-control, masks could hypothetically not work at all
3. Indoors versus outdoors: at this point, nearly all authorities advise against outdoor masking
4. Rates of SARS-CoV-2 in the community: masks may provide benefit if community cases are above a certain threshold (e.g., 10-100 per 100,000), but in theory could have net deleterious effects at low rates when the virus is barely circulating
5. Duration of time indoors: masks may hypothetically work for kids in class for 15 minutes, or 2 hours, but all air may be exchanged by 8 hours in a room together (depending on ventilation), and they don't "work" over this duration
6. Cohorts: if kids are in cohorts together, masking may be superfluous, during normal length school days

Here is the real answer to the question of whether it's worth it to mask kids: No one has any clue. During the last year and half, the scientific community has failed to answer these questions. Failed entirely. We have no idea if masks work for 2-year-olds and above, 5 and above, 12 and above. No idea if they only work for some period of time. No idea if this is linked to community rates. No idea if the concerns over language loss offset the gains in reduced viral transmission, and if so, for what ages.

In an attempt to answer these questions, researchers conducted a series of studies trying different masking routines and examined what happened to kids in daycare and school. If anyone thinks the studies are unethical, they are not for two simple reasons. Number one: The WHO and the CDC disagree in their recommendations. When major international associations disagree, equipoise exists. Number two: Research is better than the alternative. It is to study interventions than deploy them on tens of thousands without knowing whether or how they help.

Despite research, the answer remains inconclusive because none of the studies are prospective experiments measuring clinical outcomes. I view the failure to answer this question as one of the greatest failures of the pandemic.

Some may argue that we do have the answer, and cite retrospective observational studies. For instance, that mask requirements for students in K-12 schools was associated with lower transmission (Figure S1 panel I in the study). In contrast, by the economist Emily Oster, PhD, and colleagues shows that students masking in schools had no effect on viral spread (teachers masking did). That finding is confirmed by (see Table 1).

However, the same critics who unleash their zeal at criticizing the CO₂ study have been silent. These studies can't prove anything.

Masking has become a political symbol tied to identity and tribe. The types of schools that don't have or don't enforce mask mandates have families and staff that are very different and have very different ideas about COVID-19 than the types of schools that religiously enforce mask mandates. The issue of unmeasured confounding looms large.

Worse, everyone -- including researchers -- has strong beliefs about whether masks help. Moreover, there are tens of thousands of datasets to probe. You could look at schools in Florida, or Georgia, or Georgia and Florida, or France, or any combination. When you combine tens of thousands of data sets with hundreds of researchers looking at the question, analytic flexibility and selective reporting results, meaning the resulting literature is little more than an opinion poll.

Others may argue that mechanistic science is sufficient to answer the question. By knowing the size of the virus and the properties of the mask, we can figure out if masks provide net benefit to kids. These folks are woefully misguided. Mechanistic science cannot answer questions of this size and scope. If mechanistic science were robust, all drugs in development would be successes. Most are not. If mechanistic science was sufficient, we would not run randomized trials of complex behavioral interventions (such as MERIT, STAR-ICU and PRISM).

Large, empirical studies alone can answer this question, and we did literally zero of them.

So, the latest JAMA Pediatrics study does not prove masks harm kids, and those who claim to have debunked the study have not proven masks benefit kids or anyone else. Major scientific bodies, funding agencies, public health authorities, and researchers have abdicated the hard work of running trials to reduce uncertainty and hopefully answer the question. Instead, we performed retrospective, confounded, selectively reported studies on a politically divided population with preconceived notations that will forever reach opposing conclusions.

A thousand years from now, on this question, our society will look as primitive and ignorant as the people who survived the plagues of Europe in the middle ages. The only difference is that we could have done better, and that's the real point of the recent mask study.

is a hematologist-oncologist and associate professor of medicine at the University of California San Francisco, and author of .

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