What is the Optimum Humidity Level for my Home?


image of optimum humidity chart

Many people ask "What is the optimum humidity level for my home?" Its a hard question and opinions vary as to what is the optimum humidity. Rather than aiming for a specific relative humidty, you should aim to keep it within a range, we think that between 40% and 60% is the ideal range, lets look into the problems that can be associated with humidity levels that are too high or too low.

The Areas Affected

While the main effect of excessively high or low humidty is discomfort, there are many other problems which can be grouped into three areas:

  • Biological contaminants such viruses, fungi, bacteria and dust mites
  • Physical effects, timber based products can dry out and shrink
  • Chemical interactions such as ozone production

The chart illustrates the effect humidity has on the various items. As the red shaded area becomes bigger, the effect of that item is greater at that relative humidity. So for example, bacteria can thrive at very high and very low relative humidity, while dust mites will only thrive at higher humidity. The blue area shows the optimum humidity range.

Generally speaking, most homes especially those in mild and damp climates (the UK being a prime example) suffer from high humidity rather than low humidity. For those living in dry arid climates such as deserts the problem is reversed and humidity can be too low.

Biological Effects

Below 40% relative humidity, it starts to become uncomfortable. Your skin may become dry, your eyes may become dry and itchy, especially if you wear contact lenses. The membranes in your nose can also become dry, making you susceptible to nose bleeds. When your respiratory system is dry, it becomes easier for viruses to invade, increasing your chances of getting colds, flu and upper-respiratory infections. Experiments have shown that pneumococcus (bacteria that causes pneumonia) has a high mortality rate at relative humidity of around 50%, above or below this range the bacteria can survive for long periods.

Dust mites thrive in warm and humid environments, exactly like what is found in a modern centrally heated home. Dust mites do not drink water, instead they absorb moisture from the air. At humidity levels below 50% dust mites can not effectively absorb water and they die of dehydration. At 60% relative humidity and above dust mite activity increases and dust mite populations can expand quickly. Dust mites are a particular problem for those suffering from allergies or asthma.

Bacteria and viruses spread and survive more easily when the humidity level is either too high or too low. Bacteria need moisture to thrive, but in completely dry environments some species of bacteria can form spores.

Physical Effects

Humidty levels have a significant effect on wood. Most of us have seen how wood will expand when wet and shrink when dry. With wood being such a common material within the home the effect of excessively high or low humidity can be easily seen or even heard. Doors and drawers may stick or warp, stairs may start to creak, wood panelling can warp.

Musical instruments should be kept in an environment with stable humidity. Instruments made from wood such as guitars or violins can expand or shrink slightly which will put them out of tune. A piano's soundboard can shrink or expand which will affect your piano's tuning.

Excessive moisture in the air will lead to condensation on walls and windows. Mould will then grow on the damp walls and wooden window frames can rot.

Static electricty will be more likely to build up when the humidity is very low. At some point we have all experienced static, for example you walk across a wollen rug, touch the metal doorhandle and.... ZAP! a small spark jumps from your hand to the door. While static electricty merely causes momentary discomfort for us, some delicate electronics may be affected, or even damaged.

Chemical Interactions

Tools and other items made from steel or iron will start to rust if there is too much moisture in the air. At relative humidity levels below 40% steel and iron will not rust. Paper is hygroscopic, meaning it will absorb moisture from the air. If it absorbs too much it can speed up chemical reactions that are harmful to paper.

What is the Best Way to Achieve the Optimum Humidity?

The easiest way to protect your home and health from the effects of excessive humidity to buy a good quality dehumidifier. Look for a model that has a built in hygrometer so that you can set the desired level of humidity. You can then the unit set on automatic which means the dehumidifier will only turn on when required, saving you from having to turn the unit on and off.