No, masks don't create a dangerous atmosphere!
00:00:00.000 | Hi, I'm Jeremy Howard. I'm a data scientist from the University of San Francisco, and
00:00:05.540 | today I want to talk about this video that I was sent. It's described as being something
00:00:12.300 | about the danger of face masks from a Canadian worker. It doesn't mention the gentleman's
00:00:16.800 | name, but I've been seeing a lot of these videos around on the internet, and they have
00:00:21.040 | a scientific error in them that I want to tell you about that makes them totally wrong.
00:00:26.440 | So what this gentleman claims is that, well, he does in fact do, is that he uses an atmospheric
00:00:34.040 | quality measuring device, which he describes as being regularly calibrated and able to
00:00:40.360 | identify agents in the air like hydrogen sulfide and carbon monoxide, and then he uses it to
00:00:47.320 | check his exhaled air. I will tell you this, if you are exhaling hydrogen sulfide you have
00:00:52.560 | bigger problems than masks, but putting that aside, the key thing he points out is that
00:00:58.720 | he actually starts out by very appropriately using a scientific approach where he tests
00:01:08.280 | the air on its own, so that's kind of the before group, and then he tests the air underneath
00:01:13.600 | his mask, inside his mask, after breathing for a little bit. So this is the part where
00:01:18.760 | he puts his mask on, and then he breathes for a while, and the thing starts beeping like
00:01:25.400 | crazy, and he tells us that's because he's now breathing a dangerous atmosphere.
00:01:30.080 | Now what he says is that the air quality before he puts on his mask, well not the air quality,
00:01:35.720 | he says the oxygen level before he puts on his mask is about 20.5%, 20.9% is actually
00:01:41.560 | pretty normal, and afterwards it's about 17.5%. And his claim is that means he's now breathing
00:01:48.200 | 17.5% oxygen, which is too low. And if he was, that would be too low, 19.5% is kind of the
00:01:54.440 | normal cutoff where people say, oh, not enough oxygen. But actually that's not what's happening.
00:01:59.760 | Let me explain why. So you see, he's actually not breathing in the air that is 17.5% oxygen,
00:02:07.680 | he's actually breathing air that's inside the area around the mask, and let me show
00:02:13.240 | you why that is. That's because when I breathe out, the area between my mouth and the mask
00:02:17.980 | it's too small an area to hold all my breath, and I'll prove it to you. So if I put on the
00:02:24.160 | normal kind of cloth mask that we're all being asked to wear, and I get this bag and I make
00:02:28.240 | sure it's all squished out so there's no air in it, and then I breathe into it. So the first
00:02:40.680 | thing to point out is that this would not be blown up at all if all of the air was trapped
00:02:46.120 | between my face and the mask. In fact, a lot of the air has ended up in here. So we can
00:02:51.600 | estimate how much. So what I'll do is I'll grab a measuring tape, so here's a measuring
00:02:59.040 | tape which we can grab, and I can take this measuring tape and we can say, well, this
00:03:05.920 | is about 9 inches by about 6 inches by about 4 or 5 inches. We're going to call it 4 because
00:03:19.480 | it's kind of rounded off. So we'll call it 9x6x4. So actually what's happened is, next
00:03:25.840 | time I breathe in, this amount of air is going to come in from the outside of my mask, and
00:03:31.800 | a little bit of air is going to come from inside my mask. So let's head over to the
00:03:35.160 | computer and figure out what the percentage of oxygen I'm actually going to breathe in
00:03:38.800 | is. Come with me and we'll have a look. So here's a little spreadsheet I created, and
00:03:45.440 | what we're going to do is we're going to put in the height, width and depth approximately
00:03:49.880 | of the bag we blew into, and then we'll try to figure out also, we'll just make a guess
00:03:54.440 | at how much air might be inside the mask. And then in this spreadsheet we've got the
00:03:59.360 | starting oxygen level in the air at what the Canadian gentleman said is 20.5 percent, and
00:04:07.600 | then he said in his mask afterwards it was 17.5 percent. So the amount we exhaled into
00:04:12.880 | the bag was 9x6x4 or so, so about 216 cubic inches. So that's the amount that ended up
00:04:26.720 | in the bag, and then what ended up inside our mask? Well, it's probably about, I don't
00:04:31.920 | know, half an inch deep between my kind of mouth and the mask that's being probably generous,
00:04:38.280 | and then the area that's kind of not touching my face would be something around maybe 3x4.
00:04:47.000 | So that means that we have about 6 cubic inches, probably less than that in practice of air
00:04:52.880 | inside my mask and about 216 in the bag. So when I inhale again, it's going to be about
00:04:59.040 | the same mix. So that means about 3 percent of what I inhale is coming from that 17.5
00:05:06.360 | percent atmosphere, and the remaining 97 percent is coming from the 20.5 percent. So that means
00:05:12.440 | in practice, I'm going to get 3 percent times 17.5 plus 1 minus 3 percent, 100 minus 3 if
00:05:24.120 | you like, times 20.5. And so my actual oxygen that I'm inhaling is about 20.4 percent. So
00:05:32.400 | in practice, it looks like these masks are causing our oxygen levels to go down from
00:05:37.880 | 20.5 to 20.4, so we're losing about 0.1 percent oxygen, not getting anywhere close to that
00:05:44.380 | dangerous level of 19.5 percent. So in practice, this is not a good reason not to use a mask.
00:05:51.600 | In fact, wearing a mask can save the life of people around you, and removing 0.1 percent
00:05:59.480 | of oxygen from what you're inhaling are pretty unlikely to be a problem. So I would say go
00:06:04.280 | ahead and wear that mask and help keep the community safe. It also means that the economy
00:06:08.720 | will stay open more often, and for longer it will save jobs because you're helping to
00:06:13.040 | reduce the transmission of COVID-19 in your community. So wear a mask. It's not unhealthy.
00:06:19.760 | You're not going to get carbon dioxide poisoning. In fact, you're getting basically the same
00:06:23.280 | oxygen level that you were before. Thanks for watching.
00:06:26.080 | [BLANK_AUDIO]