Humidity is a key factor in helping influenza viruses thrive and spread, especially at temperatures above and below a narrow window between 21-24 degree Celsius. New research from group led by the Scripps Institution of Oceanography at the University of California San Diego has used a technique called empirical dynamic modeling to analyse 20 years of global influenza data from the World Health Organisation to uncover this association between flu outbreaks, humidity, and temperature across latitudes.

Finding such links if relevant because flu outbreaks typically occur in temperate countries with the onset of winter but occur in warm, tropical countries with no seeming direct link to seasonal change.

The study finds that the spread of flu is influenced by absolute humidity, which is a measure of the amount of water vapour in the air, and not by relative humidity, which is the percentage of the total amount of water vapour at any temperature. The research team says that flu outbreaks and the factors of humidity and temperature do not have linear correlation and so cannot be understood by studying them independently. The only way to find a pattern between outbreaks in these environmental factors is to look at the interdependence of humidity and temperature.

In cold climates, the lipid and protein envelope that protects a virus is prone to disruption with water moving into the tightly packed structure. In such environments therefore, drier air with low humidity help the the spread of flu. In warm environments, viruses are prone to desiccation of their envelopes with water moving out and so more humidity propagates the flu. The empirical dynamic modeling analysis published in the journal Proceedings of the National Academy of Sciences suggests that the switch between the two effects of humidity happens at the temperature of about 24°C or 75°F.

The relevance of this finding lies in possible population-level interventions to prevent flu outbreaks such as installing humidifiers in schools and hospitals in regions that experience cool and dry winters and using dehumidifiers or air conditioners set above 24°C in public buildings in tropical countries.