Abstract
Influenza incidence and seasonality, along with virus survival and transmission, appear to depend at least partly on humidity, and recent studies have suggested that absolute humidity (AH) is more important than relative humidity (RH) in modulating observed patterns. In this perspective article, we re-evaluate studies of influenza virus survival in aerosols, transmission in animal models and influenza incidence to show that the combination of temperature and RH is equally valid as AH as a predictor. Collinearity must be considered, as higher levels of AH are only possible at higher temperatures, where it is well established that virus decay is more rapid. In studies of incidence that employ meteorological data, outdoor AH may be serving as a proxy for indoor RH in temperate regions during the wintertime heating season. Finally, we present a mechanistic explanation based on droplet evaporation and its impact on droplet physics and chemistry for why RH is more likely than AH to modulate virus survival and transmission.
Highlights
There has been a surge of interest in understanding transmission and seasonality of influenza and other infectious diseases
Among other factors, appears to play an important role in influenza transmission, and a confluence of recent studies has suggested that absolute humidity (AH) rather than relative humidity (RH) modulates influenza virus survival, transmission and seasonality [1,2,3,4]
Writing in Nature in 1960, Hemmes et al [83] claimed, ‘it became evident that the relative humidity and not the absolute humidity is the determining factor’, for virus survival in aerosols
Summary
There has been a surge of interest in understanding transmission and seasonality of influenza and other infectious diseases. While we have an intuitive understanding of humidity in general, the relationship between RH and AH is less obvious. AH is a mass concentration that describes the amount of water vapour per volume of air. Air contains water vapour at concentrations ranging from near-zero grams of water vapour per cubic metre of air in the driest deserts to approximately 40 g m23 in hot and humid environments. The amount of water vapour in air can be expressed as a partial pressure in units such as atmospheres or millibars. A related concept is specific humidity, which is the mass of water vapour per mass of air, such that absolute and specific humidity are related by the density of air
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