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In our last post we got acquainted with temperature. Now time has come for the next indoor climate acronym –  RH – relative humidity. The ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Or more simply put, the amount of moisture present in the air.

Humidity can be described by a number of measures:

  • absolute humidity (g water / m3)
  • relative humidity (%)
  • dew point (° C)
  • wet temperature (° C)
  • and others…

Temperature and humidity

During winter, the relative humidity is at its highest, and here in Sweden it is usually between 85 and 95% throughout the country. The relative humidity changes rapidly as the temperature does. If the temperature rises one degree the relative humidity drops by 6% and an increase of the temperature by five degrees gives a decrease in the relative humidity by 26%. For this reason, heat (=dehumidifiers) is often used to keep the humidity down.

Similarly, as the temperature drops, the relative humidity increases significantly. If the saturation vapor content is reached, ordinary water will form from the air. Water may not sound that dangerous, but it can cause moisture damage.

Discomfort caused by dry air

We experience more discomfort connected to relative humidity in the winter time. Although it may be snowing and the RH-level outdoors is high, once that air comes into a building and heats up, its new relative humidity is very low, making the air very dry. It’s quite common that we get very RH-levels (10-35%)​, which can give irritated mucous membranes, causing lining nasal passages to dry, crack and become more susceptible to penetration of cold viruses. Low humidity is also a common cause of nosebleeds, dry eyes, dry skin, sinus discomfort, and sometimes also allergies and respiratory problems.​

Static electricity

You may have also experienced static electricity, which is also common when the air is dry. Shocks and sparks caused by static electricity are most often caused by substances being rubbed against each other, creating friction. For the friction to create a static charge, the RH-level must be below 40 percent. The phenomenon is also known as ESD (electro static discharge).

What can I do about dry air?

In recent years, dry air has become a buzzword and people have started asking for humidifiers. Often, the diagnosis of dry air is incorrect. Instead, it could be too high a temperature or too much pollution in the air. Lowering the temperature gives a higher relative humidity. Therefore, the first and easiest solution when the air feels too dry is to lower the temperature somewhat. If that doesn’t help, the solution could be to turn on your humidifier.

If you don’t have one, some easy ways of increasing humidity is to spray water on your curtains, boil water, or dry your laundry in your room. Doing this also saves your electricity bill compared to running a humidifier.​

How does too high humidity levels affect us?

Too high RH-levels (65-100%​) can cause moulds and other biological contaminants to thrive. They can also contribute to the growth and spread of unhealthy biological pollutants. Inhaling or touching moulds can cause hay fever-type symptoms like sneezing, runny nose, red eyes and skin rashes. Moulds can also trigger asthma attacks.​

Too much humidity can also cause damage to the building. Window sills and skylights are often exposed. In already humid rooms such as the bathrooms and laundry room, as well as inside very small rooms or closets, high humidity levels can cause damage by collecting and pooling water if ventilation is poor.​ Look out for fog or water droplets on the inside of windows, mirrors or glass surfaces, stale or “damp and chilly” cellar odor, moisture stains, bubbles in wallpaper or in layers of paint, or if you have plaster that falls apart.

What can I do about too high humidity levels?

When humidity reaches too high levels, you can either increase the ventilation, or turn on your dehumidifier to bring it back down. If you don’t have a ventilation system or a dehumidifier, using baking soda is a quick hack. Just put some in a bowl and cover it with some non-woven cloth.​

If you have really high humidity levels you may want to check your pipes and plumbing for water leaks. And if you see water rings on your ceiling or walls, that’s a clear indication that you could have leaks.

So, what is an optimal RH-level then?

If you do some research you will find somewhat various results, but our recommendation is that it should be between 40%-60%.​ That’s a healthy level both for people and buildings. Below you find an overview showing what ocurrs when you have different temperature, and RH-levels.

Overview temperature and relative humidity.

This covers the basics of relative humidity. Coming up in our next blog post is Carbon Dioxide. A parameter often used to control indoor climate. Until then, don’t hesitate to reach out if you have ideas about indoor air that you would like to “ventilate”.