Influence of Humidity on the Saisonality of Respiratory Infections

A correlation is established between relative humidity and transmission efficiency of respiratory infections. The theoretical considerations are confirmed on the basis of data on the seasonal course of the corona pandemic.

Respiratory infections are mainly transmitted via the air that an infected person breathes out. The viruses “travel” through the air in tiny water droplets, the aerosols. This airborne transmission route is inevitably subject to seasonal fluctuations. One obvious influencing factor is air humidity: the lower the relative humidity, the faster the water evaporates from the water droplets and the viruses lose their transport vehicle. Conversely, if the relative humidity is high, the longer the virus-laden aerosols can remain in the air. Consequently, a high relative humidity should favor the spread of the comparatively large influenza and corona viruses, because the longer the viruses can remain in the air in sufficient concentration, the more favorable for their transmission from host to host. In contrast, relative humidity should not have such a strong influence on the spread of the smaller adenoviruses and enteroviruses.

What is the connection between the time of year and humidity?

The relative humidity is generally higher in the winter months than in the summer months. There is also a pronounced diurnal variation, especially in the summer months - in the late evening and early morning hours, the relative humidity rises significantly and dew forms, while during the day the strong sunshine warms the air and lowers the relative humidity.

Exhibits: Relative humidity on a summer day compared to an autumn day (data from the German Weather Service for the Berlin-Buch measuring station, relative humidity at 2m above ground measured hourly on 26.6.2021 and 24.11.2021).

It is advantageous for the spread of a virus if an infected person encounters as many different potential hosts as possible and the efficiency of transmission from them to a new host is as high as possible. It stands to reason that the transmission efficiency is higher indoors with little air exchange than outdoors. On the other hand, indoors - at home or in the office, for example - we usually only come together with a small number of people, especially the same people, apart from “superspreading events” such as parties, club visits, choir performances and the like. On the street or on public transport, on the other hand, we meet a lot of people in passing every day. In these fleeting encounters, the probability of virus transmission is low, but not impossible, as the observation in a shopping mall in Australia has shown that transmission of SARS-CoV-2 can also occur in a fleeting encounter in passing. If the probability of transmission in fleeting encounters is improved by a higher relative humidity, this could have a significant influence on the spread of the relatively large influenza and corona viruses and explain their seasonal spread dynamics.

This is where our behavior comes into play: we have numerous fleeting encounters, especially in the morning hours (6-9 am) on the way to work and in the late afternoon hours (3-7 pm) on the way back from work and when running everyday errands. In summer, the sun shines at these times and pushes the relative humidity down, while the UV radiation also damages the pathogens in the air. In the winter months, on the other hand, we are mainly out and about at dawn and dusk, when the relative humidity is high and there is no UV light.

Empirical verification

The hypothesis developed here to explain the seasonal variation of some respiratory infections is initially speculative and needs to be verified. As the chain of reasoning is based on physical, biological and social correlations, this requires interdisciplinary cooperation between different scientific fields. In particular, it would be desirable to empirically confirm the dependence of viral particle suspension time on diameter and relative humidity, which has been derived only from theoretical considerations.

Even if the correlation between humidity and the spread of the virus presented here is not strictly proven, the course of the coronavirus pandemic provides some empirical evidence in this direction. The graph below shows the correlation between the number of daily new infections with SARS-CoV-2 in Berlin (weekly average for each calendar week) and the relative humidity measured by the German Weather Service at the Berlin-Buch station at an altitude of 2m (weekly average formed from the daily hourly values at 7am, 9am, 3pm, 5pm, 7pm).

Exhibit: Correlation between relative humidity and new infections during the 2020/21 coronavirus pandemic

At the onset of the coronavirus pandemic in spring 2020, the number of new daily infections in Germany soon began to decline, even though the lockdown restrictions had not yet come into effect and had not had a chance to take effect. Many commentators attributed this curious fact to the voluntary reduction in social contact that many people had already undertaken before the lockdown was imposed. However, the graph above also shows a correlation with the very low humidity that prevailed in Germany from mid-March to the end of April due to the influx of dry polar air. According to our hypothesis, this reduced the transmission efficiency of transient outdoor contacts, which, together with the restriction of social contacts, led to a very rapid collapse of the first wave of infection in spring 2020.

In contrast, one year later, in the spring of 2021, the number of daily coronavirus infections did not decrease for a long time, contrary to expectations, until the number of infections dropped sharply at the end of May 2021. In the media, the drastic decline was mainly attributed to the "federal emergency brake", which restricted contact between people. If this explanation were correct, the infection numbers should have risen very quickly after the contact restrictions were lifted in June 2021, but this did not happen. A more plausible causal link seems to be the relative humidity, which was significantly higher in March/April 2021 than in the same period of the previous year, and the infection numbers remained at a high level, while new infections dropped sharply when the cold and dry polar air moved in from the end of May to mid-June 2021.

The opposite effect was observed in the fall, when the relative humidity of the outdoor air was high throughout the day. In the fall of 2020, relative humidity was often between 80% and 90% in the mornings and afternoons from the end of September, while in 2021 this was not the case until November. As shown in the graph, coronavirus infections reached very high levels several weeks earlier in the fall of 2020 than in the fall of 2021.

The graph also shows the timing of measures taken to contain the pandemic and the emergence of new virus variants, which certainly had an impact on the pandemic. Despite these various influences, there is a clear correlation between the relative humidity of the outdoor air and the number of new infections. This remarkable finding suggests that relative humidity is indeed a relevant factor influencing the seasonal dynamics of certain respiratory infections.

Consequences

If the chain of reasoning presented in this article is correct, then the route of transmission through casual encounters deserves more attention than it has received. In fact, in a high percentage (approximately 80 percent) of corona infections, the source of infection was unknown and is likely to be found in casual encounters. As a result, it would be wise to wear a face mask outdoors during the winter months when traveling to and from work and running errands - at least in places where you may encounter many people in passing. Also, avoid indoor environments with high humidity - a visit to the indoor swimming pool is always a good way to come home with a respiratory infection.