Dutch pollster and data analyst Maurice de Hond exposed a relationship between the spread of the COVID-19 virus and the weather (especially specific humidity). He calls for further scientific research on this phenomenon. Scientists are currently analysing his recent findings.

By Maurice de Hond
A highly respected virologist sent me the link to an article he wrote together with two colleagues (called Seasonality of Respiratory Viral Infections published in the Annual Review of Virology). Therein - and in several other articles they refer to - they deliver proof of my observations. It largely aligns with my interpretation of the regional patterns and shows how to return to a normal society quickly.

It's relatively complex material, but their findings are clear. Underneath, you'll find the most important ones. (But of course, you can read the full article yourself, as well as the articles referred to).

* Experiments on guinea pigs show that the spread of the influenza virus is much faster in a relative humidity below 35% and a temperature of 20 degrees, which equals a specific humidity less than 5,5 g/kg;
* In very high relative humidity (above 80%, i.e. a specific humidity above 13 g/kg), the virus stopped transferring through the air;
* Plus, they concluded that at 30 degrees, like in the tropics, the virus does not spread regardless of the humidity;
* Even at 5 degrees and a relative humidity of over 70%, the virus showed minimal spreading.

I also found this graph in one of the articles they referred to.

These findings are consistent with my own conclusions regarding the spread of COVID-19 under different weather conditions across numerous regions, including the relationship between indoor and outdoor humidity. It may be useful to emphasise again that relative humidity is not the same as specific humidity. It is the latter that is important for the described effects on the spreading speed. Here's an example:

Last night I slept with the windows open. The heating was low. Currently (9 AM), it's 11 degrees outside, and the relative humidity is 70%. Inside my house, it's 21 degrees and the relative humidity is 34%. It seems as if the humidity outside is way higher than the humidity inside. But when you change the relative humidity (by using this converter) to specific humidity, it appears the humidity is the same. Both cases show 6 g/Kg. This is because air can hold more water at higher temperatures. (So the dew point notably rises).

The humidity in houses and buildings shows a strong correlation with the water vapour content of outdoor air. However, when we examine the relative humidity percentages, we don't observe this. In winter, indoor relative humidity drops considerably because the heating system is turned on. The specific humidity, however, doesn't change that much.

We can raise or lower the water vapour content of indoor air by opening or closing windows and doors, ventilating, using air conditioning, and using humidifiers, to some extent. This is why now may be the best time to consider installing an air conditioning unit by a company such as Morris Jenkins, to make it easier to control the indoor air quality. However, outdoor humidity is always the dominant factor in how much water the air holds indoors. And if the amount of moisture in the open air is lower than that important value of 6 g/kg, the chances of infection via aerosols are significantly larger.

I have already said that contact within 1,5 meters contributes less to the rapid spreading of the virus than large indoor group gatherings do. Combining low specific humidity with poor ventilation indicates that, even if only one individual is infected, nearly everyone comes into contact with the airborne virus (microdroplets/aerosols). This seems to be especially the case in indoor locations that host large groups of speakers or singers, such as churches, choir rehearsals, and pre- and post-game football fan gatherings.

The WHO mentions "super spreaders". You'd think they mean one person that meets many others and thus infects them, but it's more about the conditions that enable someone to become such a super spreader. Take a soprano, unaware of his infection, going to a choir rehearsal. Even when this choir adheres to the 1,5-meter rule, it will contaminate the air with infectious aerosols and consequently infect most of the singers. This also explains the fast spread in the Dutch Bible Belt.

That said, the virus also spreads in warmer and more humid areas, but, without exception, major outbreaks seem to result from large gatherings such as Mardi Gras in New Orleans on the 25th of February or the large 6-day Tabligh gathering in Kuala Lumpur with over 16,000 people in a mosque.

For the time being, these are my most important conclusions:
* The fact that Africa, townships in South Africa, favelas in Brazil and refugee camps have not had outbreaks is mostly due to the reasons described above. Because life occurs mostly outdoors and people live in "houses" without windows, the likelihood of outbreaks is low. If we find multiple infections in such areas, the best measure would be to allow citizens outdoors only when wearing masks. These masks can be highly primitive, as seen in Cuba.

* For indoor spaces where many people need to gather, the rule of thumb is to maintain a temperature of 20 degrees and a relative humidity of at least 45 to 50%. The risk of infection is much lower under these conditions. Supermarkets and schools, among others, must regulate their temperature and humidity. The good news is that the weather from May through the fall will be such that the preferred humidity level is achieved naturally. However, there will be "risky days," and on those days, we should monitor humidity.

* Shops, restaurants and offices should all take extra care in securing a good humidity level. (Places with many cooling devices, like supermarkets or butchers, already show higher levels of humidity than their surrounding places).

* Gatherings and activities involving many people should, given the right humidity, continue like they used to. However, if the required humidity is not achieved, attendees should wear masks. (Ventilation is important in all cases.

* The risk seems to be much lower outdoors. Until this has been proven by hard scientific evidence, wearing masks in low-humidity conditions should be mandatory. I suppose that when people wear such protective equipment, the 1,5-meter distance rule won't be relevant. In the busy streets of Japan and Korea, almost everyone wears masks.

The weather in Northern Europe over the coming months is favourable (more water in the air), so we can resume many of our activities. Still, creating policy that keeps infections (very) low and the economy running when winter comes, and we still may not have a vaccine or cure, requires a lot more research.

Maurice de Hond is a well-known Dutch pollster and entrepreneur. This article was first published on his own website.