Water vapor is always dissolved in air. And warm air can hold more water than cold air because it has a greater volume. In winter and spring the average temperature is lower than in summer, so there are fewer grams of water, as water vapor, in the air. When this cold air warms up (due to the heating), its volume grows and no moisture is added. So the relative proportion of water in the air decreases and the air becomes drier. 

How do I know if the air is too dry?
The air in the house is usually too dry in winter. The relative humidity is about 30%, while 50 to 60% is ideal. If you experience a dry cough, dry eyes, headache or other symptoms, it is likely that the humidity is too low. To be sure, you can use a hygrometer. This measures the humidity. Do not place a hygrometer on an outside wall or near a window, as the humidity is higher here. That would give a distorted picture.

Okay, the air may be a bit too dry, but what's the problem? The biggest problem is in the lungs. When air that is too dry is inhaled, the lungs can also absorb less oxygen. So there is less oxygen available to the body. In addition, water quickly evaporates in dry air, also in the throat and lungs. This dries out the throat, causing a dry cough or sore throat. In combination with dust, the problems can be exacerbated.

What are the symptoms?

Complaints of too dry air usually do not occur overnight. The symptoms only start to appear after days or weeks. They are often described as 'winter ailments' and nothing else is done about them. That while it is easy to prevent, as soon as the cause is clear. Symptoms related to dry air include;

• Dry throat
• irritable cough
• Dry skin (flaking)
• chapped lips
• Dry eyes, burning sensation in the eyes
• Irritated airways
• Headache
• Fatigue
• Difficulty concentrating
• Cold feeling, while it's not cold
• Decreased immune system
• Increased susceptibility to viruses 

At a relative humidity between 40 -60% you are least bothered by various environmental influences such as viruses, bacteria and fungi. The Sterling chart shows that all of these problems diminish when reaching 40 to 60% relative humidity. That is why it is important to provide your building or home with a humidifier.