Limit the spread of COVID-19 with the right humidity levels

Back in 1986, researchers concluded that if you use good ventilation, viruses don't stand a chance, regardless of the humidity. The concept was easy: with adequate ventilation, you blow pathogens out of the building [1]. However, lots of things have changed since then. With a stronger focus on energy saving during the last decades, people have stopped ventilating spaces as often as they used to. This gives viruses free rein to make people sick in our buildings, even if we don't even get together in space. You're probably wondering: how is that possible? 

How does indoor air climate limit the spread of viruses?

Viruses spread through tiny droplets of moisture that are released into the air (Figure 1). For example, when we sneeze, cough, talk, or flush the toilet. Therefore, as a side note, our first tip for everyone: when you go back to the office, flush the toilet with the lid closed. This keeps viruses in the toilet bowl and reduces the risk of contamination. 

Anyways, coming back to the core idea. The indoor climate in a building has a lot of influence on the effectiveness of virus drops. Humidity, for example, is one of the ways to make these drops less effective. The greater the difference between the humidity in our lungs (100%) and the outside air, the more droplets are formed due to breathing, coughing, or sneezing [3]. Starting at 80% relative humidity, droplets absorb so much water that they fall from the air due to their weight.

What are the safestlevels of relative humidity?

Based on what we just discussed, you would think that the trick is to increase the relative humidity as much as possible, right? Unfortunately, it is not that easy, since other forms of pollution benefit from higher levels of relative humidity. For example, at 80% relative humidity, fungi are given free rein to quickly grow and expand. It's all about finding the right balance.

Some viruses, such as the Coronavirus, have a protective fat membrane [5]. However, this protective layer is less effective at high humidity, and decreases the virus' chance of survival at relative humidity levels above 40% [6]. We can use this mechanism to our advantage to prevent the spread of viruses [7]. Nonetheless, we need to be very careful, since this only works up to a relative humidity of 60%. Why? Well, viruses that do not use a fat membrane thrive at relative humidity levels above 60% [3].  

Another factor that comes into play is our own immune system. Relative humidity lower than 20% affects our immune system. As a result, the defense mechanism in our lungs no longer works properly. Our lungs do their job best with 50% relative humidity [9], [10].

So far, we know that:

• 80% relative humidity might encourage Fungi to grow,
• Relative humidity above 60% encourages viruses without a protective fat membrane to spread, 
• Our lungs do their job best with 50% relative humidity,
• Relative humidity levels above 40% help viruses with a protective fat membrane to spread,
• Relative humidity below 20% affects our immune system

We now see an ideal scenario emerging where the relative humidity is between 40 and 60%. Viruses get the least chance to spread effectively, fungi don't get the chance to grow and our own immune system remains unaffected. 

How is the air quality at your workplace or in your building? Measure it with a quick scan from Clairify and gain insight into the humidity in your building. 

Sources

[1] Arundel AV, Sterling EM, Biggin JH, Sterling TD. Indirect health effects of relative humidity in indoor environments. Environ Health Perspect [Internet]. maart 1986;65:351–61. Beschikbaar op: http://dx.doi.org/10.1289/ehp.8665351

[2] Bjarne OW. Workshop: How much ventilation and how to ventilate in the future? [Internet]. Geotabs. 2011 [geciteerd 28 april 2020]. Beschikbaar op: https://www.geotabs.eu/Publication/workshop_b_o/at_download/Leuven_Ventilation_Olesen.pdf

[3] Wan Yang LCM. Mechanisms by Which Ambient Humidity May Affect Viruses in Aerosols. Appl Environ Microbiol [Internet]. oktober 2012 [geciteerd 14 april 2020];78(19):6781. Beschikbaar op: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3457514/

[4] Franchimon F. COVID-19 in relatie tot gebouwen [Internet]. TVVL. [geciteerd 20 april 2020]. Beschikbaar op: https://tvvlconnect.nl/thema/klimaatinstallaties/documenten/1851-covid-19-in-relatie-tot-gebouwen

[5] Cascella M, Rajnik M, Cuomo A, et al. Features, Evaluation and Treatment Coronavirus (COVID-19) [Updated 2020 Apr 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Beschikbaar op: https://www.ncbi.nlm.nih.gov/books/NBK554776/

[6] Jabr F. Why Soap Works [Internet]. 2020 [Quoted 14 April 2020]. Available at: https://www.nytimes.com/2020/03/13/health/soap-coronavirus-handwashing-germs.html

[7] Memarzadeh F. Literature review of the effect of temperature and humidity on viruses. ASHRAE Trans [Internet]. 1 January 2012 [Quoted 14 April 2020];118(1):1049–61. Available at: https://go.gale.com/ps/i.do?p=AONE&sw=w&issn=00012505&v=2.1&it=r&id=GALE%7CA295268299&sid=googleScholar&linkaccess=abs

[8] PubMed Dev, Lowen AC E al. Influenza virus transmission is dependent on relative humidity and temperature. — PubMed — NCBI [Internet]. [Quoted 23 April 2020]. Available at: https://www.ncbi.nlm.nih.gov/pubmed/17953482

[9] Kudo E, Song E, Yockey LJ, Rakib T, Wong PW, Homer RJ, e.a. Low ambient humidity impairs barrier function and innate resistance against influenza infection. Proc Natl Acad Sci U S A [Internet]. 28 May 2019 [Quoted 14 April 2020];116(22):10905–10. Available at: https://www.pnas.org/content/116/22/10905.abstract

[10] Moriyama M, Hugentobler WJ, Iwasaki A. Seasonality of Respiratory Viral Infections. Annu Rev Virol [Internet]. 20 March 2020; Available at: http://dx.doi.org/10.1146/annurev-virology-012420-022445