 Just how effective are face masks at reducing the spread of COVID? The easy answer is that if you spend time with someone who has COVID or you're infected with the virus, wearing a face mask is better than not wearing one. But as you might expect, the science of how effective masks are is a little more complex, especially when you take into account different types of masks and how they're worn. The good news is that there are some useful science-based rules of thumb here, but before we get to these, it's worthwhile going over a couple of basics. The first is that the risk of becoming infected with COVID increases with how much viruses in the air you inhale and how long you're exposed to it. Reduce either and you reduce the risk. And the second is that not all masks are created equal. Some, for instance, let more airborne particles through than others. A simple cloth mask, for example, will never be as effective as an N95 respirator. Just as importantly, leakage where the edge of the mask fits against your face matters, and some masks leak more than others. The more air that passes between the edge of a mask and your face as you inhale and exhale, the less effective it is. But just how big a difference does all of this make? Are we talking really subtle effects here or night and day differences? As with most things, the answer is it depends. Fortunately, though, there's pretty solid science to help decide on the pros and cons of wearing something you bought on Etsy, for instance, versus an N95 respirator. Imagine, for instance, that you're hanging out with a friend and one of you may have COVID. If neither of you are wearing a mask, the chances of infection increase substantially over time. The US Centers for Disease Control and Prevention defined a window before the risk becomes substantial as 15 minutes. This isn't strictly based on science, but it is useful for estimating increases and decreases in possible risk under different circumstances. From here, for instance, we can begin to explore how much longer you can hang out together with some degree of safety if one or both of you are wearing a mask. First off, imagine that just one of you is wearing that Etsy cloth mask. That'll give you an extra five minutes over CDC's 15-minute rule of thumb. This increases to an extra 11 minutes if you're both wearing cloth masks. Replace these with surgical masks now and you extend your extra time by 45 minutes. Things are definitely looking up at this point. Of course, this does depend on you both being good mask-wearing citizens. If one of you takes their mask off, we're down to 15 minutes of extra time, which is better than nothing, but not great. Things get a lot better with a Nyush approved respirator, something like an N95 respirator. With both of you wearing one of these, you get an impressive 6 hours and 10 minutes of extra time. Even if one of you takes their respirator off, you're still looking at over an hour before COVID transmission becomes a serious issue. But the gold standard here is a fully fit tested N95 respirator. This is one where you've been trained to use it properly and tests have shown that you can put it on without it leaking. If both of you are wearing fit tested N95s, you get a whopping 24 hours and 45 minutes extra time over not wearing any face mask at all. Of course, there are compromises to be made for this level of safety. Wearing a respirator for 24 hours isn't everyone's idea of a life well lived. Thankfully, fit tested N95s are designed to be used for work shifts in hazardous places rather than round the clock. Perhaps more usefully here, if you're wearing a fit tested N95 respirator and the other person's not wearing anything, that CDC recommended 15 minute window extends to two and a half hours. And if the other person is wearing an N95, even one that isn't fit tested, that goes up to a massive 12 and a half hours. At the end of the day, how and when you wear a face mask will depend on what works best for you and importantly those around you. But whatever you decide to wear, far better that you decide based on evidence rather than habit, assumptions or simply peer pressure. For completeness, here's the full set of estimated times before the infection risk becomes serious for different combinations of mask wearing. Be warned though that it comes with a really important proviso and that is that these are just rough estimates and the most important takeaway is the size of the differences, not the precise numbers. For more information, please do check out the links in the blur below and as always, stay safe.