Published: Jun 2015 | Journal of Infection (2015) 71, S54eS58

Influenza and humidity – Why a bit more damp may be good for you!

Jane A. Metz, Adam Finn


Influenza is a major cause of winter-time morbidity and mortality in temperate climates. While the regular “coincidence” of flu epidemics and winter is obvious, the causal relationship is still not well understood. Studies on the role of relative humidity (RH) and temperature on viral survival, transmission and infection rates didn’t demonstrate conclusive trends. A series of exciting recent studies have instead focused on absolute humidity (AH) and demonstrated highly significant correlations with viral survival and transmission rates in both laboratory and epidemiological models.

The paper reviews the evidence for a causal relationship between absolute humidity and 'flu transmission and outlines, how this could lead to a new approach to curbing this and perhaps other viral epidemics in the winter months.


Quote: “There are data to suggest that outdoor AH levels affect humidity levels inside and that simple humidifiers may be adequate to raise AH to levels associated with a significant reduction in influenza virus survival”.

“The prospect of reducing influenza-associated morbidity and mortality by increasing the absolute humidity in nurseries, classrooms, hospitals, homes for the elderly and general public spaces is an exciting and novel potential strategy for disarming 'flu”.

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by Walter Hugentobler

Whether absolute humidity (volume of moisture) or relative humidity (% saturated), outdoors or indoors, is the primary cause that enables seasonal flu epidemics is rather academic. Since we have no influence on outdoor humidity, the practical consequence is always the same: we should maintain indoor humidity levels during the winter with hygienic humidification as part of our flu prevention strategy!

Low outdoor absolute humidity is the trigger for the onset of flu season but the resulting low indoor humidity is the causative factor, and one that can be avoided.

As air with low absolute humidity enters an unhumidified building, it creates the perfect indoor climate for flu virus transmission and survival. It also increases the susceptibility of occupants by weakening the mucociliary barrier function of the human respiratory system.

Other scientific studies relating to humidity and airborne infection