Tuesday, June 16, 2026

Env. Microbiology: Environmental temperature and relative humidity shape post-emission aerosol fate and airborne influenza transmission



Photo Credit PHIL (Public Health Image Library)

#19,205

While it is no secret that winter heralds the arrival of flu season, one of the enduring mysteries about influenza is why it is predominantly a winter phenomenon – at least in temperate zones of the world.

Theories include:

  • During the colder weather people tend to gather indoors, with less fresh air ventilation.
  • Diminished sunlight exposure may reduced Vitamin D levels (see Study: Vitamin D And Flu-Like Illnesses)
  • With schools in session, millions of children co-mingle and more efficiently share viruses
  • But perhaps most importantly, at lower relative humidity (RH), evaporation of water from exhaled large droplets occurs rapidly, leading to the formation of  lighter, smaller, and more persistent droplet nuclei. 
This is a topic we've revisited often over the years (see links below), but due to using different methods (and different viruses), we've seen variable results.
In general, higher RH appears to decrease transmission, but one of the caveats from a 2019 study - mSphere: Environmental Persistence of Influenza Viruses Is Dependent upon Virus Type and Host Origin - was that they found considerable variation in RH tolerance among the 6 flu strains they studied, and between droplet and aerosolized particles.

But we've also seen evidence (see 2012's  Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum) that too much humidity may have the opposite effect.

Today's study tested 2 scenarios (20°C/50% RH (ambient indoor), and 7°C/73% RH (cold/high-humidity), using one flu virus (A/California/04/2009 (H1N1), in a swine model. Briefly, they found:
  • Donor pigs shed comparable nasal viral loads across both conditions
  • Naïve sentinel pigs housed 4 m away became infected 1 day earlier under ambient conditions
  • Breath and environmental air sampling showed cold/high-humidity conditions transiently increased viral RNA in exhaled aerosols at 1 day post-infection (dpi)
  • Ambient conditions supported greater and more persistent airborne viral burdens at 2–3 dpi, particularly at downrange locations.
This study argues that `. . . post-emission aerosol fate, shaped by environmental temperature-humidity conditions . . . ', heavily influences the viability and spread of the flu virus.   

This is obviously a complex study - with highly nuanced results - so you'll want to follow the link to read it in its entirety.

 I'll have a bit more after the break.
 
Airborne transmission is a major route of influenza virus spread, yet how environmental conditions shape the persistence and downrange transport of infectious exhaled virions is not fully understood. Using a physiologically relevant swine model infected with A/California/04/2009 (H1N1), we investigated how temperature and relative humidity (T/RH) influence airborne influenza emission, persistence, and transmission under two environmental conditions: 20°C/50% RH (ambient indoor), and 7°C/73% RH (cold/high-humidity).
Donor pigs shed comparable nasal viral loads across conditions, but naïve sentinel pigs housed 4 m away became infected 1 day earlier under ambient conditions. Breath and environmental air sampling showed that cold/high-humidity conditions transiently increased viral RNA in exhaled aerosols at 1 day post-infection (dpi), whereas ambient conditions supported greater and more persistent airborne viral burdens at 2–3 dpi, particularly at downrange locations.
Controlled aerosol generation experiments further showed that ambient conditions enabled substantially greater recovery of infectious virus with distance, even though RNA-containing particles were transported under both T/RH states. Together, these results demonstrate that, under the tested environmental conditions, infectious influenza aerosols persisted longer and transmitted farther under the ambient indoor environment than in the cold/high-humidity environment. These findings establish that environmental temperature-humidity conditions shape post-emission aerosol fate, and thereby constrain the airborne transmission range of the influenza virus.

IMPORTANCE

Influenza viruses spread efficiently through the air, yet the environmental conditions that determine whether exhaled virions remain infectious long enough to initiate new infections remain poorly defined. Using a swine model that closely replicates human expiratory aerosol output, we identify environmental temperature-humidity conditions as a critical determinant of airborne infectious range.
Cold/high-humidity conditions increased early viral RNA levels near the host but failed to sustain infectious particles at a distance. In contrast, ambient conditions supported prolonged airborne suspension and rapid transmission to distant recipients.
Controlled aerosolization experiments showed that infectious virus is transported far more effectively under ambient indoor conditions than in cold/high-humidity air despite similar RNA dispersal. These results reveal post-emission aerosol fate as the critical bottleneck in determining airborne influenza transmissibility. This mechanistic insight is essential for refining predictive models of influenza spread and developing environmental and public health strategies that more effectively limit airborne infection.

        (SNIP) 

In summary, environmental temperature-humidity conditions shape airborne influenza virus transmission under conditions where donor shedding magnitude is comparable. Ambient conditions supported persistence of infectious airborne virus and effective transmission, whereas cold/high-humidity conditions limited recovery of infectious virus at distance despite detectable viral RNA. These findings indicate that post-emission aerosol fate, shaped by environmental temperature-humidity conditions, influences whether exhaled virus remains airborne and infectious over distance.

 

Over the past decade there has been growing interest in the idea of raising the humidity inside homes, offices, schools, and health care facilities during times of heightened flu activity (see PLoS One Humidity as a non-pharmaceutical intervention for influenza A).

This is not exactly a new idea, as the Chinese have boiled vinegar for centuries in their homes to `disinfect the air' during epidemics (including SARS). While vinegar is unproven to add any beneficial effect, vinegar is 95% water, and boiling it undoubtedly raises the humidity inside their homes.

The irony here is that hospitals are normally kept cool and dry in order to inhibit the growth of mold and bacteria, but may be unintentionally providing an environment conducive to the spread of respiratory viruses like influenza, SARS & MERS.

Since today's study is based on a single influenza subtype, restricted to a swine model, and tested across only 2 scenarios (where the effects of temperature vs humidity were not fully isolated), it can't tell us the optimum environmental conditions to strive for to limit flu transmission. 

But it does provide additional reasons to believe that environmental controls could be useful NPIs (Non-Pharmaceutical Interventions), that could help slow the spread of a virus in an indoor setting.

As for determining where the `sweet spot' is, additional research across a wider range of viruses will be needed. 
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