To understand the positive role of humidifiers in preventing flu, we should look at some scientific evidence first.
The relationship between the relative humidity and influenza
RH and the viability of Influenza
Take influenza A viruses as an example. The research measured the viability of IAV in droplets consisting of various model media, chosen to isolate effects of salts and proteins found in respiratory fluid, and in human mucus, at relative humidities (RH) ranging from 17% to 100%. In all media and mucus, viability was highest when RH was either close to 100% or below ∼50%. When RH decreased from 84% to 50%, the relationship between viability and RH depended on droplet composition: viability decreased in saline solutions, did not change significantly in solutions supplemented with proteins, and increased dramatically in mucus.
So the findings appear to be three regimes of IAV viability in droplets, defined by humidity: physiological conditions (∼100% RH) with high viability, concentrated conditions (50% to near 100% RH) with lower viability depending on the composition of media, and dry conditions (<50% RH) with high viability.
RH and aerosol transmission
There is mounting evidence that the aerosol transmission route plays a significant role in the spread of influenza in temperate regions and that the efficiency of this route depends on humidity.
Research modeled the size distribution and dynamics of IAVs emitted from a cough in typical residential and public settings over a relative humidity (RH) range of 10–90%. The predicted concentration of infectious IAVs in air is 2.4 times higher at 10% RH than at 90% RH after 10 min in a residential setting, and this ratio grows over time. The inactivation rate increases linearly with RH; at the highest RH, inactivation can remove up to 28% of IAVs in 10 min.
From this experiment, we can see that humidity is an important variable in aerosol transmission of IAVs because it both induces droplet size transformation and affects IAV inactivation rates. Therefore, maintaining a high indoor RH and ventilation rate may help reduce the chances of IAV infection.
Humidifiers help you prevent and soothe flu symptoms
According to the researches mentioned above, purchasing a humidifier could be considered a preemptive measure since you can adjust the indoor relative humidity to lessen airborne virus transmission and inhibit the viability.
If you are already suffering from symptoms related to a cold or flu then a humidifier can also help in a few ways. This is because dry air can cause mucus to become thick and more difficult to pass and dry nasal passages can cause stuffy noses too. A humidifier can help loosen impacted mucus which will aid recovery. The moistening of the air can also lubricate your nasal airways resulting in easier breathing.
When the air is dry, clumps of fine particles have a tendency to break up and remain airborne for a much longer period, which means you’re much more likely to breathe them in. In damp air, they tend to remain intact and fall to the ground. So keeping your home and workplace at an improved level of humidity will make your surroundings less favorable for the survival of the virus, thereby reducing the likelihood that you’ll catch it.