Why do we catch more colds during winter?

Once autumn leaves and snowflakes start falling, the incidence of colds and flu peaks. We usually blame rainy days, negative temperatures or colleagues sneezing all around the office. However, the underlying cause is directly linked to the indoor climate – and thankfully this is something we can control.

According to the Center for Disease Control and Prevention (CDC) common colds, scientifically known as influenza, are the main reason why children skip school and adults miss work on several days each year. Every winter it infects 5-15% of the population.


There is a number of traditional explanations for these peak periods, including vitamin D deficiency, or even “You get colds because it is cold!”.  Vitamin D is well-known to have a strong direct effect on our bodies. However, recent clinical research shows that increasing the dose of Vitamin D does not significantly affect influenza resistance.


Another often heard explanation is that we spend significantly more time indoors during the cold seasons, increasing infection rates. Homes, offices, supermarkets, public transport etc. are as crowded with people as they are with viruses. During cold weather these viruses thrive indoors. But what is really happening here?

Dry indoor air provides optimal conditions for virus transmission

Coughing and sneezing creates an aerosol of virus-filled droplets. Dry air provides optimal conditions for the viruses to survive in these tiny water droplets; and to make matters worse, the dry air also makes these droplets shrink very small so that they stay floating in the air much longer. These two effects mean that the viruses are much more easily inhaled by other people in the vicinity of the sneezer when the indoor air is dry.


Perfect indoor climate is neither too dry, nor too humid 

In winter, the outside air coming into buildings is already quite dry. It is then dried further by the indoor heating system, making perfect conditions for viruses from coughing and sneezing to survive a long time in the air, leading to the highest virus transmission rates. Although colds and flu are naturally associated with the seasons, we are now able to minimize illness and employee absenteeism by monitoring and controlling humidity.


Humidity is usually measured as relative humidity (RH). It represents the percentage of water vapor in the air. In winter, RH can drop to below 30%, whereas in summer humidity is usually much higher, often above 70%. The good news is that by controlling relative humidity to between 40 and 60% the transmission of colds and flu can be reduced by up to 70%.


To learn more about humidity control, as well as the healthy indoor climate product portfolio of Siemens, refer to this link – here.




Picture credits: Hoxton/Gettyimage


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