Temperature is easily understood and controllable, writes Dave Mortimer from Vapac Humidity Control. If it gets too hot, shed a few layers, too cold and you put a coat on, but when humidity levels start to fluctuate it is an entirely different proposition and people may not know how to tackle the problem.
Most people understand materials expand and contract with a rise and fall in temperature, but many do not know that with hygroscopic materials there is a greater rate of expansion and contraction with a rise and fall in humidity levels.

In a dry atmosphere, textiles such as carpet, wood, plastics and leathers can shrink, harden, crack and lose weight. Losing weight can be a problem when produce is sold this way.

The human body can also suffer in much the same way. In a dry atmosphere moisture migrates from the body through the eyes, nose, mouth and skin. Moisture migrating from the skin causes adiabatic cooling which will reduce the comfort of a room's occupants.

At 21°C, with a low humidity level, moisture migrating from the skin will make a room feel cool to its occupants. Raising the humidity level will reduce the ability of moisture to migrate from the skin and the occupants will feel warmer.

The normal reaction would be to increase the temperature of the space by 1ºC to 3°C. The amount of power required to produce the temperature rise can be greater than the power required to bring the humidity up to a reasonable level.

There have been instances where the temperature in a working environment has been reduced to nominally 19°C and the humidity increased to 60%. The comfort level was much improved and the power saving substantial.

Health

But comfort isn't the only benefit. It has been accepted for sometime that low humidity is more uncomfortable than unhealthy but airborne bacteria can travel further and faster through a dry atmosphere than through a humid one and may increase the possibility of contamination.

Researchers have known since the 1940s that low humidity levels increase the survival time of the flu virus. This, together with airborne transmission which is also affected by low humidity, has been suggested as a factor for the seasonality of flu (Hemmes JH, Winkler KC, Kool SM, 1960)

But the latest research by Dr Jeffrey Shaman at Oregon State University goes much further. Shaman's research confirms humidity has a huge impact on our health but it could also have a significant part to play in the transmission and survival of viruses such as the influenza A/H1N1 in any areas where large numbers of people congregate such as hospitals, schools and universities. Lack of humidity is the key.

For many, increasing the environmental control of a building in winter would simply involve increasing the air temperature to a comfortable level of around 22°C.

However, this heating effect without the inclusion of humidity control reduces the RH levels to those below 25 per cent.

Defining factor

'Although increasing RH levels is beneficial, it is the absolute humidity (water in the air) that is the defining factor and can greatly reduce the viability of the flu virus and the transmission of influenza,' says Shaman.

Although absolute humidity is the defining factor over relative humidity, it is important not to let definitions cloud the issue. What is important is the inclusion of humidity and the fact that Shaman's findings nonetheless support a requirement for humidification.

With so many new buildings focusing on air tightness maybe the building industry and end-users alike should be asking many more questions?

Why has research conducted in the 1940s been largely ignored? The extent to which moisture has a part to play in our environments is well documented but its impact on our health poses many questions. The answers could have a major impact on building regulations and the way we manage the environment in our buildings.