Do I Have the All-Clear?

How to tell whether it’s safe to remove your mask where you work, learn, and play.

Sam Johnston and Liz Ruark
Woman smiling, taking of her mask

I’m typically anxious about taking my KN95 mask off indoors when other people are around. But there are circumstances when I have no choice, like when I eat inside at a restaurant or go to the dentist (can’t have a root canal with a mask on!). At work, I may want to remove my mask while giving a presentation in the office. So, when I enter a place where I’ll need to remove my mask, I try to figure out how well this particular location is clearing COVID-19 from the air.

COVID-19 is airborne.

The number-one way that COVID-19 spreads is through the air. When people who are contagious breathe out, tiny bits of water that are carrying virus particles fly into the air. The largest of these fall to the ground right away. (That’s where the six-foot rule came from.) But the really tiny ones — called aerosols — can float in the air for a long time. You get COVID-19 when you breathe in enough virus to make you sick. The more virus is in the air, the more likely that becomes.

How can viruses be spread through aerosol transmission image

Obviously, when you enter a room, you can’t tell for sure how much virus is floating around. But you can look for clues that will tell you whether there could be a lot of virus in the air —or only a little.

How many people are there, and what are they doing?

COVID-19 doesn’t come out of nowhere — it comes from infected people. If the space isn’t crowded, it’s less likely to have virus floating in the air. High ceilings are another plus, because people’s breath can float up and away from where you’re breathing.

And the quieter the crowd is, the better. People send out the most virus when they’re shouting, singing, or exercising.

Is there plenty of fresh air?

One of the best ways to decrease the amount of virus in a room is to bring in fresh air that doesn’t have virus in it. That’s why being outdoors is safer than being indoors — the amount of “clean” air is so huge. Plus, the ultraviolet rays in sunlight help kill viruses.

So look for open windows and doors. Remember, unless there’s a breeze, a fan blowing in the right direction, or a temperature difference between outside and inside, the air might not flow into the room. If there are openings to the outdoors on either side of the space, that’s even better. It’s called cross-ventilation, and it helps air flow into, through, and out of the room.

If you’re in a place like an office or a school, you may have an HVAC system that moves air throughout the building. (Are there vents in the walls for air conditioning and heating? If so, there’s probably an HVAC system.) The person who takes care of the HVAC in the building should be able to tell you what the system’s “air change per hour” (ACH) rate is. ACH tells you how often the entire room’s worth of air is removed and replaced with fresh air. According to the Harvard T.H. Chan School of Public Health, you’re looking for an ACH of at least three — an ACH of six is ideal.

Are there air-cleaning devices?

If you can’t bring in fresh air, you can clean the air you have. There are two basic ways to do that: You can filter out stuff that carries viruses or you can kill the viruses directly.

You’ve probably heard of HEPA filters. They’re one of the main tools people use to clean the air in spaces where it’s tough to bring fresh air in. One thing to know: HEPA filters don’t actually pull viruses themselves out of the air. The viruses are too small. Instead, these filters remove the stuff the viruses are riding on, like bits of water and dust. If you’re entering a small space, look for standalone air-cleaning units. And remember, a bigger room will need a bigger filtering unit.

HVAC systems have filters inside them that clean the air as it flows through the ducts. If you’re in a building with an HVAC system, you can ask about the kind of filters it uses. HEPA filters can’t be used within most HVAC systems — it’s too hard for the fans to pull the air through such fine-meshed filters. The best filter most of those systems can handle is something called a MERV 13 filter (the higher the number, the better the filter). For some systems, even that’s too hard - the best they can deal with is a MERV 8 or 9.

HVAC systems and rooms with high ceilings may also have UVGI units, which kill viruses using ultraviolet (UV) light. Typically, UVGI devices are installed high up in a room. As warm air from people’s breath flows up, the viruses get zapped by the UV rays. UVGI is used this way so the UV light doesn’t hit people directly.

If you really want to be 100 percent sure, check the CO2 level.

This one’s for the folks who just have to KNOW. You know who you are.

The virus that causes COVID-19 gets into the air when infected people breathe out. So what you really want to know when you come into a room with other people in it is this: How much of the air in this room is made up of other people’s breath? You can find out by using a carbon dioxide (CO2) monitor.

Carbon dioxide is one of the main gasses that people breathe out. So if there’s a lot of CO2 in a room, it’s got a lot of other people’s breath in it. How much is too much? You want the CO2 level to be less than 800 ppm (parts per million). Just remember, if there’s a HEPA filter or UVGI unit in the room, the air might be clean even if it has a lot of CO2 in it.

How I surround myself with clean air.

If I don’t have any control over the building where I’m going to take my mask off, such as an indoor restaurant, I’ll ask about their filtration system or their ventilation. No filtration, no outside air, no business from me. I ended up switching dentists from one with no ventilation to one that had a portable HEPA filter in every room.

For my own office, where the windows don’t open, I brought in a portable HEPA filter that sits next to my desk. That way, I can take my mask off, and feel comfortable that the air surrounding me is clean.