I don’t love wearing a mask. I must greet people with my eyes. I find myself yelling to be heard on rounds in the hospital. Small blemishes appear on my chin. Masks are bothersome. But I wear one. And though the mask covers my mouth, it speaks volumes. It says that I recognize we are all interconnected and that I am making the choice to protect both myself and others. It also says that I understand the ample evidence that masks can limit transmission of airborne illnesses. In the midst of a pandemic that has left about 147,000 Americans dead, it is our first line of defense.

So, as a pulmonary and critical-care doctor, it worries me that those who object to masks are increasingly using scientific-sounding — but incorrect and misleading — discourse to promote their viewpoints. This isn’t just the usual idea that wearing masks somehow makes us less free, a premise that has created political turmoil throughout the pandemic, but the idea that the act of wearing a mask can itself result in physical harm. In a moment when we should be able to turn to science for collective safety, these claims instead make it all the more challenging for the public to separate fiction from reality.

Though much about covid-19, the disease caused by the coronavirus, remains unknown, we can say some things with certainty. The first is that wearing masks does limit the spread of the coronavirus, both by protecting others and by protecting those who are wearing them. Admittedly, this is not the message we were given by the Centers for Disease Control and Prevention early on in the pandemic, when we did not know the extent of asymptomatic spread and — officials now acknowledge — were trying to save masks for health-care workers. It is clear that the conflicting messages eroded trust. But we know now, from both laboratory studies and real-world epidemiologic data, that masks are effective. After a mask mandate for employees was put in place at the hospital system where I work, rates of coronavirus infections among health-care workers decreased. On a more granular level, in late May two hair stylists in Missouri had close contact with 140 clients while infected with covid-19. Everyone wore a mask. No one tested positive.

Despite such evidence, the Internet is full of testimonials and even diagrams claiming that masks lower oxygen levels and increase carbon dioxide. The idea here is that carbon dioxide is trapped within the mask and recycled when you breathe, leading to dangerously high levels in your blood, termed “hypercapnia” or “CO2 intoxication.” While such conditions are possible under certain circumstances, such as in a severe exacerbation of emphysema or after a drug overdose, this is not actually what happens when simply wearing a mask. The masks we wear in our daily lives, surgical masks or cloth face coverings, are not airtight. Carbon dioxide molecules are small enough to easily pass through. If this myth were reality, doctors and nurses would be collapsing on the job constantly. And yet my colleagues and I have worn surgical masks for hours, without any related health issues.

That same rationale applies to the related idea that masks cause oxygen deprivation. The mask is not an impenetrable barrier. Just as carbon dioxide passes through and around a mask, oxygen enters. In fact, I have been wearing a surgical mask for 12 hours at a stretch at work, with an N95 when seeing coronavirus patients, and have never felt the dizziness or weakness characteristic of a low blood oxygen level. With this in mind, some doctors have posted on Twitter about their normal oxygen levels after a long masked run. One layered six masks on his face while continuously measuring his oxygen level to prove that it remained unchanged.

But what about people with chronic conditions? Primary care doctors are increasingly fielding requests for “mask exemptions.” Here, too, there’s an attempt to hijack one of the reasons we’re all wearing masks — that is, to protect vulnerable people — to inveigh against wearing them at all. However, beyond the CDC recommendations exempting children under 2 and anyone who is unconscious or incapacitated, these requests are also based on misinformation. There is no evidence that people with breathing issues such as asthma will be harmed by surgical masks or cloth face coverings. To the contrary, those with lung disease are more likely to be severely affected by the virus, and thus would benefit most from any incremental protection.

Of course, with severe lung disease, masks might be uncomfortable and worsen shortness of breath, and the far tighter N95 mask might pose some issues for those with severe lung disease if worn for long periods of time. But no one is recommending that the public wear N95s in their daily lives. In any case, a good rule of thumb would be that if you’re too sick to be able to comfortably wear a surgical or cloth mask, you should either stay home or seek care in the hospital. And anyone who has spent time in a hospital has seen severely ill patients — including those with emphysema and cystic fibrosis, sick enough to be waiting for lung transplants — wearing masks to keep themselves safe, without any untoward effects.

There is also no evidence to support another Internet-friendly theory, the speculation that wearing masks could suppress the immune system. While some might object that what we don’t know can still hurt us, there’s no reason to invent new problems when we’re already dealing with real ones, and that’s precisely what those who worry about the effects of masks on our immune systems are doing.

This is a frightening time, partly because there’s still so much we don’t know and so much that we can’t do. We are all trapped in limbo, being asked to make immense daily sacrifice. Many Americans have lost their jobs. Without travel, it has been months since some of us have seen our parents. But that’s no reason to turn scientific language on itself, especially when we know both that masks are safe and that wearing them keeps us safe. When you pause to think about it, it is rather amazing that a simple, harmless piece of material has the power to save lives. We owe it to ourselves and each other to focus on what we already know about them — and what we absolutely can do with them.