What is Relative Humidity

The air can hold a given level of water vapour at any given temperature, with warmer air being able to hold more than cooler air. Relative humidity is the amount of moisture in the air, compared to what the air can hold at that temperature. A reading of 100% relative humidity would mean that the air is completely saturated with water vapour and cannot hold any more – resulting in dew, condensation or rain.

We are extremely sensitive to humidity; our skin relies on the air to get rid of moisture. If the air is too humid, sweat cannot evaporate and we feel hotter. If the humidity is low, sweat will evaporate faster and we feel colder. We tend to feel most comfortable with a relative humidity of around 40 – 45%, and this is the level that health professionals recommended maintaining – more on this later.

Think of Air like a Bottle of Water

Warm air holds more water vapour than cold air, so when the air begins to cool, relative humidity begins to rise (the air cannot hold as much water vapour so is closer to becoming saturated).

Try imagining a litre of water inside a two litre bottle. As the temperature drops, the bottle gets smaller but the amount of water stays the same – a litre worth. As the bottle gets smaller, the proportion of water that’s being held increases – the water takes up more room, but the amount present doesn’t actually change, there’s still a litre of water. When the bottle has reduced in size enough (ie the temperature has decreased), the bottle is full and if it decreases in size any more (the temperature drops even further), then the water will start overspilling. In reality, when the air has cooled enough, it can no longer hold as much water vapour as it did at the warmer temperature. This results in liquid water dropping out of the air as condensation – much like the water overspilling from the smaller bottle.

The temperature at which condensation begins – when there is 100% relative humidity because the air is fully saturated – is called the dew point.

But why is there Condensation on Surfaces, such as my Windows?

When moisture laden air comes into contact with a colder surface, it causes condensation. This is because as the warmer air travels towards the colder surface, it begins to cool, thus the relative humidity increases as the air can no longer hold as much water vapour (think of it like the bottle above). When the air hits the colder surface, it is brought below the dew point (the temperature at which the air becomes fully saturated) and so water begins to drop from the air as condensation.

Humidity and Health

Besides comfort levels, there are health risks to having too much or too little humidity in the air. As mentioned earlier, health professionals recommend a relative humidity level of around 40 – 45% all year round.

Health Effects of Too Low Humidity Levels

The dry air which is present with low humidity levels can pull moisture from the skin, leading to dry, cracked skin – often referred to as “Winter Itch”. It can also lead to dry, itchy eyes as the dry air evaporates tears more quickly.

Sore throats and irritated sinuses are also potential risks. Long term exposure to low humidity levels risks the drying out and inflammation of the mucous membrane lining your respiratory tract. If this stops working, your chances of catching a cold, the flu or other infections rises. Studies have also shown that the flu virus survives longer and spreads easier in low humidity.

Health Effects of Too High Humidity Levels

Perhaps the most obvious effect of high humidity is feeling increasingly uncomfortable. In high humidity, sweat doesn’t evaporate as quickly, therefore our bodies are not as efficient at cooling down.

High humidity is also a breeding ground for all sorts of microscopic organisms, whereas no bacteria or virus can live on dry surfaces with a humidity of less than 10 percent. Dust mites also thrive in high humidity, one of the biggest problems to asthma and allergy sufferers.

However, the biggest threat to our health caused by high humidity levels is the mould that’s often associated to these conditions, and how that affects our health. We cover this more in a later blog.

But what about the Health of my Building?

The health of a building is often overlooked when considering current humidity levels but it can have costly consequences if you ignore it.

Materials such as wood, leather, paper, cloth and carpet all contain water and will swell and shrink in line with gaining or losing water. The quantities of water they contain changes with the surrounding humidity, and therefore affects their size and weight.

As an example by BOMI International, a cubic foot of wood at 60 percent relative humidity weighs 30 pounds and holds three pints of water. However, at 10 percent relative humidity, the wood is lighter and only holds one pint of water.

Now imagine that kind of change in the woodwork and materials within your home or workplace – how a constantly changing moisture content can affect joists, studs and plaster; by swelling then shrinking, which in turn can cause cracking, some of which can be significant.

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Changing humidity levels can also have adverse effects on wooden furniture, art, books and even your wooden flooring. Think of how wood is bent without cracking it. It has to be exposed to steam for a period of time. And a high humidity level has the same effect on the wood in your home. High humidity can result in creaking floorboards, damaged furniture, peeling paint and even wet patches on your ceiling or walls.

Your wooden flooring, whether solid or laminate and no matter how it was installed or what type of wood, is also affected by the humidity levels in your home or workplace. Like skin, wood is most comfortable in humidity levels of around 40 – 45% – too high and it can swell and buckle, damaging the edges of the panels meaning that it no longer sits flush when it shrinks back to size, and too low can see the floor panels shrink, resulting in large gaps between the boards.

How Can I Adjust the Relative Humidity Levels?

Before you think about ways of adjusting relative humidity levels, you first need to ascertain what they currently are. There are several ways to do this. You could buy a hygrometer from most DIY stores and attempt to gain a reading yourself, you can look for visual clues, or if you are worried about excess moisture, you can hire the professionals to carry out an in-depth moisture survey.

Visual clues that the humidity levels are too high include:

• Condensation
• Unexplained wet patches on ceilings or walls
• Mould and mildew
• An increase in allergies, asthma attacks or generally feeling unwell

Visual clues that the humidity levels are too low include:

• Dry, cracked lips and skin
• Dry, itchy throat
• A large amount of static electricity
• Increased problems with electrical equipment

If after carrying out your own tests or having a moisture survey it is decided that you need to adjust the humidity levels, try the following tips.

To increase humidity:

• Use a humidifier
• Let wet clothes and dishes air dry
• Put bowls of water on the windowsill and let the sun evaporate the water back into the room
• Let the hot water from sinks and baths cool before emptying them (not recommended if there’s children around)
• Add more houseplants – as long as they are regularly watered, they will continue to release moisture into the air

To decrease humidity:

• Use a dehumidifier
• Ensure good ventilation which includes the use of extractor fans and opening windows
• Increase indoor temperature – warmer air can hold more water vapour
• Decrease the temperature of your water when running sinks, showers and baths
• Avoid drying clothes indoors
• Invest in some houseplants such as Peace Lily, Reed Palm and English Ivy – all of which absorb moisture from the air.

Remember, high relative humidity levels have a direct impact on condensation which is the biggest cause of mould growth. To get rid of mould permanently, you have to first get rid of the excess moisture. If ignored, it can have devastating consequences.

In our next blog, we explore the risks of mould to both your health and the health of your building.