 Hi, I'm Jeremy Howard. I'm a data scientist in the University of San Francisco and today I want to talk about this video that I was sent. It's described as being something about the danger of face masks from a Canadian worker. It doesn't mention the gentleman's name but I've been seeing a lot of these videos around on the internet and they have a scientific error in them that I want to tell you about that makes them totally wrong. So what this gentleman claims is that what he does in fact do is that he uses a atmospheric quality measuring device which he describes as being regularly calibrated and able to identify agents in the air like hydrogen sulfide and carbon monoxide and then he uses it to check his exhaled air. I will tell you this if you are exhaling hydrogen sulfide you have bigger problems than masks but putting that aside the key thing he points out is that he actually starts out by very appropriately using a scientific approach where he tests the air on its own so that's kind of the before group and then he tests the air underneath his mask inside his mask after breathing for a little bit. So this is the part where he puts his mask on and then he he breathes for a while and the thing starts beeping like crazy and he tells us that's because he's now breathing a dangerous atmosphere. Now what he says is that the air quality before he puts on his mask, well not the air quality, he says the oxygen level before he puts on his mask is about 20.5 percent 20.9 percent is actually pretty normal and afterwards it's about 17.5 percent and his claim is that means he's now breathing 17.5 percent oxygen which is too low and if he was that would be too low 19 and a half is kind of the normal cutoff where people say oh not enough oxygen, but actually that's not what's happening. Let me explain why. So you see he's actually not breathing in the air that is 17.5 percent oxygen. He's actually breathing air that's inside the air that's in the area around the mask and let me show you why that is. That's because when I breathe out the area between my mouth and the mask it's too small an area to hold all my breath and I'll prove it to you. So if I put on the normal kind of puff mask that we're all being asked to wear and I get this bag and I make sure it's all squished out that there's no air in it, okay, and then I breathe into it. So the first thing to point out is that this would not be blown up at all if all of the air was trapped between my face and the mask. In fact a lot of the air has ended up in here. So we can estimate how much. So what I'll do is I'll grab a measuring tape. So here's a measuring tape which we can grab and I can take this measuring tape and we can say well, this is a I don't know about nine inches by about six inches by about four or five inches. We're going to call it four because it's kind of rounded off. So we'll call it nine by six by four. So actually what's happened is next time I breathe in this amount of air is going to come in from the outside of my mask and a little bit of air is going to come from inside my mask. So let's head over to the computer and figure out what the percentage of oxygen I'm actually going to breathe in. It's come with me and we'll have a look. So here's a little spreadsheet I created and what we're going to do is we're going to put in the height, width and depth approximately of the bag we blew into and then we'll try to figure out also or just make a guess at how much air might be inside the mask and then in this spreadsheet we've got the starting oxygen level in the air at what the Canadian gentleman said is 20 and a half percent and then he said in his mask afterwards it was 17 and a half percent. So the amount we exhaled into the bag was nine by six by four or so. So about 216 cubic inches so that's the amount that ended up in the bag and then what ended up inside our mask. Well, it's probably about I don't know half an inch deep between my kind of mouth and the mask that's being probably generous and then the area that's kind of not touching my face would be something around maybe three by four. So that means that we have about six cubic inches probably less than that in practice of air inside my mask and about 216 in the bag. So when I inhale again, it's going to be about the same mix. So that means about three percent of what I inhale is coming from that 17 and a half percent atmosphere and the remaining 97 percent is coming from the 20 and a half percent. So that means in practice I'm going to get three percent times 17 and a half plus one minus three percent 100 minus three if you like times 20.5 and so my actual oxygen that I'm inhaling is about 20.4 percent. So in practice it looks like these masks are causing our oxygen levels to go down from 20.5 to 20.4. So we're losing about 0.1 percent oxygen not getting anywhere close to that dangerous level of 19 and a half percent. So in practice, this is not a good reason not to use a mask. In fact wearing a mask can save the life of people around you and removing a 0.1 percent of oxygen from what you're inhaling pretty unlikely to be a problem. So I would say go ahead and wear that mask and help keep the community safe. It also means that the economy will stay open more often and for longer it will save jobs because you're helping to reduce the transmission of COVID-19 in your community. So wear a mask. It's not unhealthy. You're not going to get carbon dioxide poisoning. In fact, you're getting basically the same oxygen level that you were before. Thanks for watching.