How Humidity Affects Human Health

Optimum Humidity Zone – 40%-60%


What is Humidity ?

Humidity is the level of moisture that is contained in the air. It is actually “water vapour”, and cannot be seen, but humans can identify high and low humidity, because the air is noticeably damp. Our natural perspiration that evaporates quickly from our bodies in an atmosphere of low humidity, causes us to sweat and become “sticky” when humidity is high. In cooler air, high humidity causes us to feel damp and cold.

How does it get there ?

New Zealand is surrounded by the sea, wind and rain cause moisture to evaporate into the outdoor air, and so the outdoor air is generally well above 50% Relative Humidity, for most of the time. (Check the weather page of your local paper to see the humidity of each day)

What about indoor humidity?

If your home interior is the same temperature as outdoors, and if the doors and windows are open all the time, then the interior of your home will be approximately the same humidity as outdoors. Unfortunately this is not realistic. Our homes are often closed up, with all the occupants inside — as on a cold winter’s night. At first the humidity actually reduces, as the heating system of the home warms up the interior.


When any air is warmed its relative humidity is reduced — it still has the same level of moisture, but warm air feels like it is dry air. – A hair dryer is a good example — dry your wet hair with the heating ON and the hair dries very quickly. Try drying your hair with the heat OFF — it takes hours — yet the air is the same air — the same amount of moisture – why? – because the heating of the air by the hair dryer causes the humidity of the air to become “drier” and so the moisture from your hair quickly evaporates into the air.

OK so now the warm air from the hair dryer is in the interior of the home – right! – As soon as this air “cools down” to the room air temperature, the room air humidity increases, (because the moisture from your hair is now in the air). Then, if this air passes by a cold surface — a cold glass window, or cold aluminium window frame, or the cold water cistern or toilet pan, then the water vapour in the air condenses (as visible condensation) on to the cold surface.

What other sources are there for indoor humidity to increase?

Humans consume 1 to 2 litres of fluid each day — a lot of this fluid is evaporated from our bodies into the indoor air as perspiration — even though we feel dry our bodies are constantly evaporating moisture from every pore.

Every time we exhale — we breathe out moisture from our lungs — just because you only see this as visible “steam” when we breathe outdoors in winter doesn’t mean that we are not breathing out moisture all the time — this is a continuous process, indoors and outdoors, summer and winter.

Bathing / Showering – every time we use the shower, it adds moisture to the indoor air, and increases humidity. Drying towels give off moisture into the indoor air — how much ?? compare the weight of a wet and a dry towel.

Clothes dryers — unless vented to outdoors, clothes dryers literally saturate the indoor air with moisture. Drying clothes indoors does the same — a clothes horse in front of the fire is a major contributor to indoor moisture.

Cooking — how much water does actually evaporate from your favourite cooking?? Where does it go ?? Unless you have a very efficient range hood extraction system — this moisture ends up inside your home as humidity.

Washing dishes – whether by machine or by hand — the steam that is part of hygienic dishwashing ends up inside the home.

Hot Drinks and steaming hot food — the humble cup of tea or coffee gives off water vapour (visible as steam or invisible as water vapour).

All these and other sources of indoor moisture have been documented by Building Research Scientists, (see BRANZ BULLETIN 367 “Condensation”), who estimate that in an average four person home, this moisture totals over 20 litres per day.

Can you imagine how 10 litres of water on your lounge-room floor would look?

Yet 10 litres is only about half of the moisture that is released into the indoor air of a typical four person home every day.

Where does it go ?

Fortunately, most of this moisture escapes to outside, by opening doors when we enter and leave, other ventilation by a window that may be “open a crack”, or by forced ventilation, such as a bathroom exhaust fan or kitchen range hood.

Unfortunately — Indoor moisture is generated almost continuously, by one means or another, but the means to expel it outside is not continuous, or sufficient.

For example — a bathroom window that is “open a crack”, may control the moisture in the bathroom, but it is useless at ventilating to outside, the moisture that is generated by two persons breathing in a bedroom for 8 hours overnight, when the windows are shut tight. In this example, the air from the bedroom occupants breathing, is usually air that is very stale — after all it has just been expelled from their internal organs — it is odorous and damp. Some of this air will “give up” its moisture as “condensation” when the air passes over a cold surface (the cold window glass or frames). Some other of this moisture will be absorbed by porous surfaces that are cold — usually the walls and wall paper of the coldest wall of the bedroom, and usually low down — because that is the coldest part of the wall.

The same occurs in the lounge room — when the occupants are in the lounge and watching TV before the fire or heater, the heating system keeps the air humidity “low” even though the occupants are breathing moisture into the lounge room air. This increased humidity may not even be noticed, although the lounge room air may be described as “stuffy” — this is because it has been deprived of oxygen (the occupants have depleted the oxygen by normal breathing — and have replaced a lot of the oxygen with carbon dioxide).

After the occupants leave the lounge room to retire for the night, and let the heater run down or switch it OFF, the lounge room cools down, the humidity that was unnoticeable becomes higher (because the air is cooled down) and this humidity condenses on to cold surfaces, window glass and frames for example, and so, by morning, there are “puddles” of condensation.

In most NZ homes, in winter, warm air has the most moisture, even though its humidity may not be noticeable. The best way to dispose of this humidity is to exhaust moisture-laden (and stale) air to outside, while the moisture is still airborne and invisible as “vapour”, before it has time to condense on cold surfaces.


If the humid, warm air in the lounge was allowed to escape to outside, when the occupants retired to bed, it is most unlikely that there will be any condensation upon windows in the morning. It is for this reason that an HRV is the best way to remove excessive condensation.

In addition the HRV :

  • exhausts humid indoor air
  • recycles heat energy,
  • and reduces the relative humidity incoming air — by warming the outdoor air with available “recycled heat”

The cooling effect of humid air is often underestimated. Dry air feels warmer than damp air. Just as a warm dry towel or clothes, feel drier than dry clothes at room temperature. This has a hidden effect that is often not appreciated — if your home is damp with excessive moisture, then you have to use more heating and energy to feel warm. If your home is dry then you will feel warmer, even though the temperature indoors may be a few degrees C cooler.

The by-products of combustion from a gas stove are positively dangerous to human health.
Just read the instructions to see why.

Worst culprits for this effect are unflued gas heaters and gas cookers (ovens and hobs) — when these appliances burn gas, there are several very unpleasant side effects which are released into the indoor environment … ….. For every KG of gas burned —

  • 1.6 kg of water vapour — for every kg of gas (three pints!!)
  • 3.4 kg of carbon dioxide — this gas is deadly poison to humans, animals and birds.
  • Carbon monoxide — this is the same deadly gas that is from the exhaust of a motor car.
  • Nitrous oxide — this is laughing gas.


  • try to eliminate moisture release into the indoor environment of your home.
  • Dehumidifiers may remove moisture, but all of the other “nasties” in the indoor air are left untouched, and if the home is occupied these indoor air pollutants actually multiply.
  • An HRV is the most effective means to control excessive humidity in a home.


0800 DRY AIR  (0800 379 247) 


Products are New Zealand made by Avon Electric Ltd.


Avon Electric Ltd
25 Taurus Place, Bromley
P.O Box 19748, Christchurch 8030 
Ph: 03 381-5595  - Fax: 03 381-5596