Condair Australia business development manager, Ian Eitzen, explains the impact of low humidity on our health.

Under the Work Health and Safety Act 2011, employers have a duty of care to provide the highest level of protection against harm to health and welfare from risks arising in the workplace, so far as is reasonably practicable. Many organisations in the building services industry, including CIBSE and BSRIA, recommend that the ideal indoor humidity for health is in the range 40-60%RH.

So how great is the risk to health if an indoor humidity falls outside of these conditions and is it reasonably practicable to expect an employer to take steps to maintain a “healthy” indoor humidity?

Scientific research into the effects of humidity on the air, specifically with regards airborne infection, was being carried out as early as the 1930’s and 40’s. Early experiments with mice, by Loosli et al in 1943, showed airborne flu remained active and infectious for much longer at low humidity under 40%RH.

More recent studies around the same hypothesis by Noti et al in 2013, used mechanical nebulizers to “cough” flu virus into a room at varying humidity levels, and air samplers to assess its infectious nature. The results were similar and demonstrated a dramatic drop in the flu virus’ ability to survive in air at more than 40%RH.

Numerous other studies, taking place between these two, have also shown that atmospheres maintained at above 40%RH significantly curtail the airborne transmission of flu and other viruses.

A review of over 20 such investigations since 1960, carried out by Jane Metz and published in the Journal of Infection in 2015, concluded that “the prospect of reducing influenza-associated morbidity and mortality by increasing the absolute humidity in ... public spaces is an exciting and novel potential strategy for disarming ‘flu”.

Two reasons are put forward to explain the detrimental effect humidity has on airborne germs. Firstly, it’s been shown that droplets remain airborne for longer if they are less than 5 microns in size. If an atmosphere is dry, droplets breathed or coughed from an infected person rapidly shrink through evaporation, leaving more at 5 microns or less in size and capable of remaining airborne for long periods. 

Secondly, when viruses are contained in a tiny airborne hydrated droplet, the dissolved salts and proteins are harmful to the virus, rendering them inactive. In dry atmospheres, the droplets lose all their moisture from evaporation.

The virus is preserved in a dry airborne particle and remains infectious without the salt and protein solution to attack it. So dry air has the double-effect of causing more infectious droplets to remain airborne and prolonging the infectivity of the suspended floating airborne viruses.

Beside the risk of promoting airborne virus transmission, low humidity also directly impacts on the human body. Few of us need scientific evidence to believe that our skin is drier and cracks more in the winter than the summer.

We feel and experience it for ourselves. However, there are still plenty of scientific studies that have quantified the detrimental effects, such as Sunwoo et al 2006.

Groups of young and old males were exposed to air at 10, 30 and 50%RH and were monitored for the effect on their skin, eyes and the function of “mucociliary clearance”.

This is the respiratory system’s primary line of defence against airborne infection and involves mucous in the nose and throat capturing airborne pollutants and transporting them to the digestive system, where they are destroyed.

Sunwoo’s results showed that 20% less moisture was lost from the skin at 50%RH than at 10%RH, that people’s eyes were drier at lower humidity causing them to blink significantly more to compensate, and it took significantly longer for pollutants to be cleared from our nose in a dry room. Interestingly it also showed that people felt colder at lower humidity, indicating higher room humidity could result in a 2°C reduction in the required room temperature.

Given that dry air promotes airborne viruses, the fact that it also detrimentally affects our body’s defence against them, is the most worrying of these effects of low humidity on the human body. As it occurs inside our nose and throat, it is also the most unnoticeable.

Salah et al, 1998, specifically examined the effects of breathing dry air by depositing saccharin into subjects’ nostrils, while they breathed either dry or regular room air, and having them swallow every 30 seconds until they could taste it. When breathing dry air, the time it took the subjects to taste the saccharin increased by more than one third.

Salah concluded that “breathing dry air results in excessive water loss by the nasal mucosa, which may in turn reduce nasal mucociliary clearance”.

So given that there is a lot of scientific evidence showing health is impaired when the humidity falls below 40%RH, why aren’t humidity levels in workplaces monitored and maintained in the same way temperature is?

The main reason is that unhealthy humidity levels are not easily perceivable. The human body can sense a cold room but it cannot recognise a dangerously low humidity. It is not unusual in Australia for indoor humidity levels in hospitals and offices to drop below 40%RH in the winter due to heating, or in the summer due to air conditioning.

However, people tend not to associate the increased levels of flu and colds they experience in these seasons to a dry indoor atmosphere.

Maintaining an indoor humidity with humidification systems is relatively easy to do. There are many options for any type of building, no matter how big or small and the cost is comparable, if not less, than that for temperature control systems.

Employers accept the need to provide heating in the winter and cooling in the summer for the health and productivity of employees but humidity control is nearly always ignored.

In 2018, there were 125 deaths in Australia confirmed as being due to flu, and around 60,000 cases. In 2017, there were 1,163 deaths and quarter of a million cases[1].

Airborne infections, such as flu, have a highly detrimental impact on Australian society, and cause a substantial drop in productivity due to absenteeism. There can be no doubt that airborne cross infection at the workplace presents a clear risk to health and welfare.

As the science all points to a healthy indoor humidity being 40-60%RH, the legal and moral obligation on employers to provide a healthy workplace should extend to managing indoor humidity levels throughout the year.


1.        [1]Neil, M. (2019, July 11). Flu deaths in Australia hit 300 as experts describe flu season as 'moderately bad'. Retrieved from:



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