As a professor of environmental health, an exposure scientist who studies air pollution for a living, and someone who is fully vaccinated against the coronavirus, I still wear a mask.

I am fortunate to live and work in Massachusetts, where vaccination rates are high and rates of infections remain low. Most, if not all, of my neighbors have made the same choice to be vaccinated. For this I am grateful. But I worry there is a false perception of invincibility that replaces their masks. The virus that causes covid-19 is still here, which means there’s no reason to let our guard down.

I wear a mask when I go to an indoor environment, like the store, and I understand it can be awkward to be the only person wearing one. I suspect that people wonder if I am against vaccines or immunocompromised. They may even worry that I actually have covid. I’m none of those things, and, to the best of my knowledge, I’m not sick. Instead, I wear a mask for the unvaccinated people in my life.

Americans face intense and persuasive social and political pressures on masks. Many have rushed to cast off masks as a sort of badge to show that they’ve done their part. Others might feel a premature sense of vindication or relief for making it over the covid finish line. And others view masks with disdain for their intrusion on their lives. They shouldn’t. It is not a sign of weakness to wear a mask but a sign of compassion for others. It is not a political statement; it is one based in science.

It doesn’t help that we continue to see mixed public health messaging around the ongoing value of masking. For example, the American Academy of Pediatrics recommends universal masking in schools, even for the vaccinated. Until Tuesday, the Centers for Disease Control and Prevention had recommended indoor mask-wearing in schools, but for those who are unvaccinated. That has changed: The CDC now recommends masking for some vaccinated people in some cases and that everyone in K-12 environments mask up, whether they’re vaccinated or not. What was a parent or teacher to believe before the guidance started to shift? Which guidelines should they follow now that things are changing, especially in states where local guidance is different? And the beginning of the school year for most children is just around the corner.

As is now abundantly clear, even the 160 million Americans who are fully vaccinated can still become infected with the virus. These “breakthrough infections” are real, as we have seen in high-profile baseball teams and tourist destination clusters. This does not suggest that the vaccines don’t work — they do: Many of the breakthrough cases are without symptoms, and the vaccines clearly lead to more moderate illnesses in most of those who are symptomatic. But it tells us that covid-19 remains part of our world, and we have to remain vigilant.

For the most part breakthrough infections are probably most often diagnosed in limited populations who are required to be regularly tested for some reason — professional athletes, clinical staff or those who are hospitalized for unrelated medical reasons, for example. Daily coronavirus tests in the United States have rapidly declined, which suggests that those who might be harboring a breakthrough infection simply aren’t being tested. It’s a good bet that if most of your neighbors are vaccinated, they haven’t been tested for months.

We have little definitive information about whether a breakthrough infection can be passed to someone who is unvaccinated. But the CDC still recommends isolation for anyone showing symptoms, even if they are fully vaccinated. It would be reasonable to ask why they make this recommendation, and the reason is simple: You might give it to others, even if you are vaccinated.

There are more than 150 million Americans who remain unvaccinated: many by choice, others for age, medical or religious reasons. Meanwhile, there are the 160 million vaccinated Americans who could be unsuspecting carriers of a breakthrough infection. While I’m not much of a gambler, I don’t like those odds. I believe a precautionary approach is still warranted.

There is still discussion in the academic world about how this virus is transmitted from person to person, and this is a surprisingly complicated topic. But what is not in dispute is that wearing a mask — whether a simple surgical-type or cloth mask or a more sophisticated N95 respirator — captures and holds fast to virus fragments as they enter or exit our breathing passages. Simply put, they reduce our risk of being infected by others and in passing our own infection on to others.

The burden of mask-wearing is sometimes compared to the burden of using a seat belt in a car, something almost all of us do without much thought. This is not the right comparison, because the injury that might happen in a car accident when not wearing a seat belt usually results in injury to the person abstaining and not directly to anyone else; refusing to wear a seat belt is ultimately a personal choice with primarily personal consequences. By contrast, I align mask-wearing more closely with the burden of using a turn signal when driving. When you use your turn signal to indicate your intentions, you are protecting yourself. But you are also protecting others around you who may be at risk. It is a literal signal to others that a potential risk might be in front of them.

Most of us have worn masks routinely during the pandemic. Masks remain easy to wear and are still inexpensive. Simply put, there’s no good reason to not wear a mask when you’re in contexts where going without might put you or others at risk. They are perfectly safe, do not make my life difficult and I’m happy to keep mine on if it makes others safer. Especially for those who remain unvaccinated.

And about those 48 million children in the United States who are not yet eligible for any vaccine. They probably won’t be eligible for a vaccine for months. In mid-June, child cases of covid represented about 10.1 percent of total cases. In mid-July, children represented 22.3 percent of covid cases. This is going exactly in the wrong direction.

So we are faced with a choice: We can continue to assert our “freedom” to not wear a mask, but do so knowing that it sustains virus transmission. This means we should expect another surge in the fall, just like last year. Or, we can be vigilant and do everything that is in our power to reduce the spread of this virus — including wearing masks in any close-contact environments and keeping our social distance. The choice is ours to make.

I choose to keep wearing my mask.

Read more: