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Why colds are more common in winter?

As we were warned today that the number of COVID-19 infections in the UK could soar to 50,000 a day by mid-October, let us expand our understanding of why we are more likely to catch colds as the mercury plummets. We have long since determined that there is no direct linkage, although our prior beliefs live on in the word “influenza” - a contraction of the Italian phrase “influenza di freddo” or “influence of the cold wind”.

That said, it is that cold wind that encourages us to spend more time indoors, huddled together, increasing the likelihood of spreading “cold” viruses. But there is much more going on under the surface: –

(1) Absolute humidity falls as the temperature falls. Virus-laden droplets will evaporate at lower humidity, liberating the virus particles and increasing the likelihood of others breathing them in. Partial evaporation leaves viruses vulnerable to increased concentrations of salts in the droplets. Heavier droplets are more likely to fall to the ground or surfaces. Infection is still possible through contact hand to mouth, explaining why infections are most likely in tropical regions when it is rainy and humid.

(2) Shorter days in winter means a higher likelihood of vitamin D deficiency. Vitamin D plays a key role in priming the immune system through receptors on B cells, T cells and antigen presenting cells. Vitamin D deficiency weakens the immune system and increases the likelihood of infection and disease.

(3) Viruses are trapped in the mucin that lines the inner surfaces of the nose. Bodily functions are exquisitely sensitive to temperature. Breathing in cold air through the nose may reduce the production of mucin, slow the conveyor belt of cilia cells that clear the mucin and reduce the activity of the immune cells that attack the viruses. Challenges that are exacerbated by low levels of humidity.

What might help as the nights close in? Keeping relative humidity as high as possible. Air conditioning and open windows have the exact opposite effect, sucking moisture from the air. As Professor Gordon Lauc from the University of Zagreb shared in a recent BBC Radio 4 podcast, humidifiers and even wet towels could help as we slowly dehydrate in our sleep.

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