Airborne viruses and relative humidity
In the range of 40-60% relative humidity, the time aerosolized virus droplets remain airborne is significantly shorter, as is the duration of a virus’ viability. It also minimizes risks to human health by other biological contaminants and chemical interactions. Just how this works, we will explain in the following.
Not so mysterious after all
Actually, much about the role humidity plays in the transmission of viruses is quite logical. Listen in to this great explatantory podcast to learn more.
In our INSIGHTS section there is an excellent short podcast that was recorded by Dave Marshall-George, Sales Director at Condair, with BusinessNet Explorer, on how indoor humidity mitigates the spread of airborne viruses and how we can better manage our indoor humidity for the health of the nation.
For instance, the higher the relative humidity, the less inclined are the airborne droplets to remain suspended in the air.
Instead, their increased weight due to their higher volume and weight of absorbed moisture causes them to plummet to the floor quickly, where they are less likely to invade our respiratory systems. The ideal indoor humidity level of between 40-60%RH has been scientifically proven to combat airborne flu infections.
In addition, the same ideal humidity level shortens the time airborne flu remains infectious, due to a more complex phenomenon which happens inside the virus.
Physiochemical reactions can disarm the viruses from within and these are more likely to occur in airborne droplets that contain higher levels of moisture.
Studies revealed that at room temperature, flu survival rate is lowest at around 50% relative humidity, due to salt concentrations of the host droplet being most damaging to the virus itself at this level.
Several medical studies revealing these absolutely fascinating natural mechanism for airborne infection control are to be found here.
Knowing this, it should come as no surprise to you that it is during the cold winter months when heating systems run constantly, causing relative humidity to fall below 40%, that you come down with the flu and other respiratory infections.
This infographic shows the three machanisms of preventing airborne infection with healthy humidity.
Furthermore clinical findings published 1985 by Sterling EM, Arundel A, Sterling TD - a study which was confirmed by several studies published along the following decades - reveal correlations relevant for comfort and health protection at different room humidity levels.
The risk posed by undesired microorganisms and the occurrence of specific symptoms of illness are minimal within the optimal range between 40 and 60% relative humidity.
The well-known Scofield/Sterling Diagram reflects their findings. The authors reviewed scientific literature that focused on humidity effects on biological contaminants (viruses, bacteria and fungi) causing respiratory disease, chemical interactions and the possible impacts on human health and comfort. 74 references are listed in the paper.
Their conclusion is that the optimal humidity range for minimizing risks to human health by biological contaminants and chemical interactions, is in the narrow range between 40-60%RH, at normal room temperatures.
Conclusions in a nutshell:
preferences of viruses and bacteria for low and high humidity are revealed, while fungi prefer humidity’s above 80%RH for optimal survival on surfaces
for airborne microbes midrange humidity was least favourable for survival
off-gassing of formaldehyde and chemical interactions increases above 40%RH and the concentration of irritating ozone decreases
 "Criteria for Human Exposure to Humidity in Occupied Buildings" by Sterling EM, Arundel A, Sterling TD