{"id":2082,"date":"2021-12-31T19:32:04","date_gmt":"2021-12-31T19:32:04","guid":{"rendered":"https:\/\/www.uncharted-worlds.org\/blog\/?p=2082"},"modified":"2022-11-15T22:32:58","modified_gmt":"2022-11-15T22:32:58","slug":"using-a-carbon-dioxide-monitor-to-measure-air-freshness","status":"publish","type":"post","link":"https:\/\/www.uncharted-worlds.org\/blog\/2021\/12\/using-a-carbon-dioxide-monitor-to-measure-air-freshness\/","title":{"rendered":"Using a carbon dioxide monitor to measure air freshness"},"content":{"rendered":"

Explanation of an ingenious thing: how you can use a CO2<\/sub> monitor to check the air-freshness in a room, to guide your covid risk reduction.<\/em><\/p>\n

I looked into this a while ago and I thought I’d do a write-up!<\/p>\n

\""Using<\/a><\/div>\n

<\/a>What we can measure<\/h2>\n

Wouldn’t it be useful if we could somehow measure<\/strong> “Has this room got any covid particles<\/strong> hanging in the air at the moment?”?<\/p>\n

Unfortunately, we can’t yet do that in real-time<\/strong>. When scientists wanted to measure the covid levels in hospital ward air<\/a>, they had to gather the samples first, then take them back to the lab and do some extra processes to find out what they’d actually gathered. They commented \u201cThe sampling and detection of airborne viruses poses several technological challenges<\/span>\u201d :-)<\/p>\n

On the other hand, measuring carbon dioxide<\/em> in the air is something people have been doing for ages. And that can give us a clue<\/strong> about what the covid risks<\/strong> might be in a particular space.<\/p>\n

<\/a>What is carbon dioxide and why is it relevant?<\/h2>\n

(Skip if you know this bit :-) )<\/a><\/p>\n

Carbon dioxide is one of the gases which we naturally breathe out in our breath. We breathe in oxygen, our bodies make use of it, and we breathe out carbon dioxide.<\/p>\n

You’ll often hear it referred to as CO2<\/sub>, “see oh two”. The C means “carbon atom”. The O2<\/sub> means “oxygen atoms, two of them”. They link up to make a molecule with the carbon atom in the middle. (The “di” in “dioxide” means two, as well.)<\/p>\n

\"Sketch<\/a><\/div>\n

If CO2<\/sub> builds up in a room, we sometimes sense that as the room becoming “stuffy”. If you ever felt sleepy in a stuffy room, then more refreshed after someone opened a window… part of that was probably the fresh oxygen coming in, and the breeze sweeping away some of the built-up CO2<\/sub>.<\/p>\n

<\/a>Virus in breath<\/h2>\n

Meanwhile…<\/p>\n

As you might know by now, someone currently infected with covid can breathe out covid virus on their breath<\/strong>. The teeny tiny particles of breath-liquid, with even tinier specks of virus in them, can hang in the air<\/strong> for some time. In still air, they might hang about for a few hours – though obviously they’d get blown away much quicker than that if you were outdoors.<\/p>\n

Then the next person breathes in that same bit of air, and that’s probably the number one way that covid gets from one person to the next.<\/p>\n

\"A<\/a><\/div>\n

Fresh air will tend to sweep away any germs that are floating around – either from someone in<\/em> the room, or from someone who already left the room a while ago.<\/p>\n

(Even outdoors or in a well-ventilated room, you could still pick up germs from the breath of people close to you, same as you’d smell the smoke if someone was smoking right next to you. That’s why we still need good, face-fitting masks sometimes as well as<\/em> fresh air. But the fresh air really helps.)<\/p>\n

<\/a>Making the connection<\/h2>\n

The ingenious idea was: OK, we can’t measure covid itself. But both covid virus and CO2<\/sub> molecules float in human breath! So we can measure how much carbon dioxide<\/em> is building up<\/strong> in a room, or bus, or train carriage, or hospital ward, as the people there are breathing. And that would give us a clue to how much covid virus would<\/em> typically build up<\/strong> in that space, supposing an infectious person were there.<\/p>\n

If a lot of CO2<\/sub> is building up, it’s a clue that a lot of covid virus could build up in that room too, if anyone there was infected. And the other way round: if the CO2<\/sub> level is low<\/em> while people are there, then plenty of fresh air must be coming in to carry away what the humans are breathing out.<\/p>\n

It’s what in science terms would be called a “proxy measure<\/strong>“: not exactly<\/em> the same thing as what you’re really interested in, but related (at least some of the time) in a useful way.<\/p>\n

<\/a>Example of a CO2<\/sub> monitor<\/h2>\n

The Aranet 4 Home<\/a> has been recommended<\/a> (by scientists I consider reputable) as a good-quality CO2<\/sub> meter. It uses a “nondispersive infrared” sensor, more accurate than some of the other kinds.<\/p>\n

\"Aranet4<\/a><\/div>\n

It’s a dinky little thing – about 7cm by 7cm on its front face, similar to the palm of a smallish hand. It would easily fit in a pocket, and has a hole in the back where you can hang it over a screw-head on the wall.<\/p>\n

Via Bluetooth, you can link one or more units to your phone, and view the results in an app which you can download for free. The app can memorise 7 days’ worth of how the levels went up or down, for you to look at afterwards.<\/p>\n

Cost of that Aranet one is \u20ac199.00 plus taxes & delivery: expensive for the average family, but not enormous compared to running a school, a hospital or a business.<\/p>\n

There are cheaper ones too. If you’re getting one, make sure it’s the “nondispersive infrared<\/strong>” type, also known as NDIR<\/strong>.<\/p>\n

This kind<\/em> of thing is what the UK government has promised to UK schools, although I don’t know if they’re buying the Aranet or a different model.<\/p>\n

<\/a>What number to aim for<\/h2>\n

The CO2<\/sub> level in air is measured in “parts per million”, often written “ppm”.<\/p>\n

(Little devices like this won’t measure it to an exact number – it’ll be more like “give or take 50ppm”, which is good enough for ventilation monitoring.)<\/p>\n

Outdoor<\/strong> air would typically read at about 400ppm<\/strong> to 450ppm<\/strong>, or higher if you’re near a busy road.<\/p>\n

What’s a “good” level?<\/p>\n

A study looking at a tuberculosis outbreak<\/a> found the outbreak stopped after the ventilation was improved enough that the CO2<\/sub> level went down to 600ppm<\/strong>. Getting it below 1,000ppm<\/strong> also made a significant difference to the chance of infection. (Summarised here<\/a>.)<\/p>\n

Tuberculosis and covid both transmit via floating in the air, so we could make a guess that similar levels might be good for covid too. So it would make sense to say 600ppm is good to aim for<\/strong>. But if you can’t get it that<\/em> low, 700ppm is better than 800ppm, 800ppm is better than 900ppm and so on.<\/p>\n

The excellent FAQs on Protecting Yourself from Aerosol Transmission<\/em>, by a group of top professors, summed up like this<\/a>:<\/p>\n

The exact level considered \u201csafer\u201d for CO2<\/sub> varies and we have seen various recommendations from 500 to 950 ppm.<\/p>\n

Choosing a general level like this is a compromise to make the method feasible and simple enough for many people. This is the same reason that a single distance (e.g. 1 m or 2 m) is quoted for social distancing (even though we know that 1.5 m is better than 1 m, and 2 is better than 1.5 m etc.).<\/p>\n

A key goal is to make clear that the many shared spaces with 2000 or 3000 ppm CO2<\/sub> are unsafe, so that people realize that they have to take action to improve the situation there.<\/em><\/p>\n

Surveying classrooms, offices etc. with a CO2<\/sub> monitor can be useful to determine which ones may have the worst ventilation, and prioritizing our actions there.<\/p><\/blockquote>\n

(paragraph breaks added by me for ease of reading)<\/p>\n

They also explain:<\/p>\n

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