Nov 15, 2020
How to Reduce the Spread of Coronavirus by Monitoring Indoor Carbon Dioxide
In March 2020, a new coronavirus (official name SARS-CoV-2) brought the world to a standstill. Politicians all over the world are guiding their citizens through the first pandemic of their professional careers implementing more or less successful measures to combat the spread of the virus, alongside snappy reminders such as “Hands, Face, Space” referring to hand hygiene, face covering and social distancing. But despite all our efforts, SARS-CoV-2 is in the middle of raiding through Europe a second time.
Unlike bacteria, viruses multiply inside cells “utilising” the machinery of its host’s cells to make and assemble new virus particles. To do this, SARS-CoV-2 particularly (but not exclusively) targets cells along the airways and inside the lungs. Once the virus has multiplied, the host cell is destroyed and the freshly produced new virus particles are released into the environment going on to infect other cells. Transmission from person to person happens either through inhalation of respiratory droplets of an infected person or getting in contact with virus-contaminated surfaces followed by touching the nose, mouth or eyes.
So how does this link with carbon dioxide?
Carbon dioxide is a gas in our atmosphere which we mostly know about from discussions about global warming, greenhouse effect and car exhaust fumes etc. The global average atmospheric carbon dioxide in 2019 was 409.8 parts per million or in short ppm (https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide). In percentage terms, this is around 0.04% of all gases in our atmosphere and — despite this surprisingly small number — is going to cause us a lot of trouble in the years to come. But this is not what this article is about.
What is less known is that carbon dioxide is also a by-product of our daily activities.
To supply ourselves with the energy we need to go about our everyday lives, we eat food. Among other things, the food we eat contains carbohydrates. Once inside our digestive systems, these carbohydrates are broken down and inside our body cells aided by hundreds of helper molecules called enzymes the energy stored in the chemical bonds is set free, allowing us to move about, work, exercise, read, think.
In very simple terms, this is the reaction of carbohydrates with oxygen (from the air we inhale) which supplies the energy needed for our cells to function, also known as respiration. The “waste products” of this process, carbon dioxide and water vapour, are released into the environment by exhaling. Along with these two gases, all sorts of other things from our respiratory tract can leave the body through exhalation. We therefore call these emissions respiratory droplets.
We humans tend to spend quite a lot of time in enclosed indoor spaces.
The carbon dioxide concentration indoors can range from 600 to over 3000 ppm. For example, imagine 32 students and a teacher in a poorly ventilated classroom for 60 minutes working and thinking heavily.
As a Science teacher at a secondary school which recently underwent a costly refurbishment, I have been privileged to be mostly teaching in laboratories with carbon dioxide monitors installed. In my personally conducted micro studies, I found that on a normal day in spring, and in absence of any ventilation the carbon dioxide concentration easily rises above 1000 ppm within 15 minutes and above 3000 ppm by the end of the lesson.
No wonder one feels a bit drowsy and head-achey at that point as these are the typical symptoms of mild carbon dioxide poisoning which start to appear from about 1500 ppm upwards. But again, this is not what this article is about.
“At least 30 of 41 choir members contract coronavirus after indoor rehearsal.”
This was a headline in the Independent on the 25th September 2020. One can’t be sure about whether social distancing was observed in this rehearsal, however, this is just one example of many. So how can we explain this?
Let’s stay with our classroom scenario: 32 students, 1 teacher, 60 minutes in a poorly ventilated room, but this time, we add an asymptomatic student infected with SARS-CoV-2. In fact, a UK study found that over 80% of people are “silent spreaders”. This are people who are infected with the new coronavirus and don’t experience any symptoms, but still “shed” (release into the environment) substantial amount of virus particles and therefore are able to transmit the virus.
With each exhalation, our contagious student releases carbon dioxide as well as respiratory droplets now containing virus particles into the surrounding air. Whereas real droplets (anything larger than 100 micrometers) fall to the ground within seconds, the smaller aerosol particles (anything smaller than 100 micrometers) will stay in the air, just like the smoke of a cigarette, accumulating inside the room as time goes by.
An average human breathes about 10 to 12 times per minute.
A quick search online tells us that we exhale about 40 ml water vapour per exhalation. So using this number for a very rough estimate, that makes 400 to 480 ml water vapour per minute and 24 to 34 litres of water vapour per hour per person.
Even if only half of this volume stays in the air as aerosols, that is quite substantial. More so if we are physically active or talking or shouting as the infogram below shows (From: https://english.elpais.com/society/2020-10-28/a-room-a-bar-and-a-class-how-the-coronavirus-is-spread-through-the-air.html). To date it is unknown how many virus particles need to be inhaled to infect another person, but it is clear that the higher the exposure to virus particles, the more likely the person is to contract the disease.
How is measuring carbon dioxide levels indoors supposed to help us with this?
A recently published article on conversation.com on this exactly the same matter () brought my attention to a fascinating study conducted at Taipeh University. Taiwan had to tackle a couple of Tuberculosis (TB) outbreaks between 2010 and 2013. Although TB is caused by a bacterium rather than a virus, it is useful to look at this because — like SARS-CoV-2 — TB is a highly infectious airborne disease transmitted via respiratory droplets. This study reported that:
“…after adjusting for effects of contact investigation and latent TB infection treatment, improving ventilation rate to levels with carbon dioxide below 1000 ppm was independently associated with a 97% decrease in the incidence of TB among contacts.”
97% decrease, that almost sounds too good to be true, however, even if it was just a 50% decrease, that would be pretty impressive. By the way, this is not the only publication of that kind:
- https://www.cdc.gov/niosh/nioshtic-2/20038645.html
- https://www.researchgate.net/publication/272751195_Modelling_the_risk_of_airborne_infectious_disease_using_exhaled_air
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6072925/, https://www.bmj.com/content/370/bmj.m3223
- and many more...
Question: How difficult is it to achieve an indoor carbon dioxide level below 1000ppm?
To my own surprise, it’s not that hard. In my favorite Science lab, having a couple of windows half open and the classroom door fully open brings the carbon dioxide concentration nicely below 1000 ppm within 15 minutes and keeps it there. Using the installed extractor fan, it is five minutes. Obviously, this will vary hugely from room to room, number of windows etc.
So, ensuring sufficient ventilation combined with social distancing is a very effective (if not the most effective) way of reducing the risk of transmission of airborne diseases.
The WHO agrees with this and has published some guidance stating that ventilation is an “important factor in preventing the virus that causes COVID-19 from spreading indoors”.
On 18th November 2020 (finally), the UK Department for Health and Social Care published a press release stating :
“A new public information campaign has launched today by the government to highlight how letting fresh air into indoor spaces can reduce the risk of infection from coronavirus by over 70%.”
As part of this information campaign the following information video was released:
Once an indoor meeting exceeds 15 minutes, face masks unfortunately no longer provide much protection.
El Pais had a good look at the still unanswered question of how helpful face masks are. Their conclusion: Face masks are a great way of minimising exposure. As long as it is a brief encounter.According to El Pais, 4 out 5 people gathering in a room with one infected person (so 6 people in total) for longer than 15 minutes will contract the virus, even if all are wearing a face mask (). Obviously, this is quite a rough assumption as this also depends on the size and height of the room, how much virus the infected person is shedding etc, but still I found that rather disappointing.
Measuring the carbon dioxide concentration indoors allows us to determine how much air exchange is needed in order to minimise the risk of transmission.
The indoor carbon dioxide concentration in the air can be accurately measured using an infrared radiation sensor. Sounds like an expensive affair, but actually a good quality carbon dioxide monitor can be purchased online from about 80 British pounds. Not cheap, but not that expensive either considering the potential of bringing a bit of normality back into our lives and without having to sacrifice the lives of the vulnerable. In this spirit: “Hands, face, space, measure, ventilate”?
