Relative humidity indoors drops significantly during winter, causing flu like symptoms.

Indoor Air Changes With the Seasons

As the weather gets cold outside, indoor air seems to always feel drier.

It might sound surprising that during winter, the relative humidity might reach a 100 % – then why does your skin, throat and eyes feel so dry?

Air humidity can be measured in two ways. Absolute humidity tells how much water there is in the air, for example, as grams per a thousand liters. Relative humidity tells, how much water there is in the air relative to how much it can actually contain.

This in turn depends on the temperature: cold air is able to bind less water than warm. That causes the relative humidity to often be higher the colder it is, but as the amount it actually contains water is small, the relative humidity drops as the air gets warmer indoors. This is why the air feels dry indoors during cold weather.

Seasons affect our indoor environment

Seasonal changes bring inevitable temperature and humidity changes indoors. Even though the air is mechanically processed, eventually it originates from outdoors. The recommended relative humidity indoors is between 30–50 %, but during cold weather, it can drop as low as 10 %.

For many people this leads to flu like symptoms, such as throat and mucous membrane irritation, itching of eyes, and skin problems. Especially for people suffering from allergies and asthma, these symptoms can show up easily.

A research project (HITEA, several studies) surveying dust and mold in school buildings in several countries found, that although in Spain and the Netherlands the amount of molds was up to 2500 % higher compared to Finland, only the latter had a statistically significant connection between mold and sick leave as well as respiratory symptoms.

One possible explanation the researchers came up with was the relative humidity. For example, in the Netherlands relative humidity is between 70–80 %, significantly higher compared to Finland. Dry air makes it possible for small particles to float in the air, while in more humid conditions, they bind to the water droplets and fall to surfaces. Dry air is recognized as one of the risk indicators of sick building syndrome.

Overly high relative humidity is not healthy either: even though it isn’t harmful in itself, it might cause bacteria, mold, and dust mites to reproduce. They are all known to cause irritation to the airways, allergies, and hypersensitivity.

High humidity also increases chemical evaporation to the air. For example, a 35 % increase in humidity raises formaldehyde evaporation from the building surface materials 1.8–2.6 times.

Both extremes bring several hindrances, but it should also be considered that optimal humidity brings other advantages than just avoiding them. Humidity and temperature are known to affect comfort, productivity, and performance. Pleasant humidity is often a neglected factor when it comes to health, wellbeing and productivity, but even small changes can lead to a big impact.

Relative humidity indoors drops significantly during winter, causing flu like symptoms.

How to improve indoor humidity during cold weather

  • Observe outdoor and indoor temperature and relative humidity

By investing in a meter that shows both temperature and humidity, it is easy to notice if there is something to improve.

  • Regulate temperature

The relative humidity indoors increases as temperature falls, and often the reason behind dry indoor air is in overly high room temperature. Keep it between 21–22 degrees celsius. The heating mechanism also matters: for example, wood heating can increase relative humidity, as the water contained by wood evaporates to the air.

  • Regulate air ventilation

Mechanical air ventilation is one possible factor causing dry air, so it is advised to inspect its adjustments.

  • Air humidifiers

Humidifiers may alleviate excessively dry air, but their maintenance and function should be taken care of: if not, they might act as possible sources of microbes. Green plants are also an effective way of increasing humidity in the air.

  • Avoid peaks in the humidity

Do not try to increase humidity e.g. by keeping the bathroom door open while showering. This way humidity might rise temporarily, often too high. Especially during cold weather it might cause damage to the building: the excess moisture that the air cannot bind in itself condensates to cold surfaces and might cause water damage. For this reason humidity should not rise over 45 % when a building is being heated.

If the procedures mentioned above seem a bit cumbersome, then perhaps the right solution would be a green wall. Naava green wall has a built in AI that follows humidity and temperature automatically, and adjusts its fans and watering accordingly. Naava keeps up a stable, optimized air humidity indoors.

Read more about what factors contribute to indoor air quality, and tips for improving them!

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