What exactly is fresh air?
We need fresh air to be able to live healthily. Without it, we don't feel well and may even get sick. Fresh air is therefore indispensable for us. Many people automatically associate fresh air with special and individual things: one likes the smell of freshly mown grass, another of fresh sea air. In the countryside, fresh air is also associated with natural smells from the environment. Everyone has their own ideas about fresh air. If you breathe deeply and consciously, you can notice a difference between "usual room air" and "fresh air". But what exactly makes this difference? Is it only the smell or do other things play a role? First of all, let's start with the smell of air: humans are able to automatically recognise whether air smells good or bad. This ability to distinguish good from bad smells made it easier for humans to survive. It is similar with taste. Bitter, for example, is automatically associated with poisonous and thus prevents the consumption of deadly foods. Furthermore, we are able to assign smells. For example, we can distinguish the smell of a cellar from that of a garage or workshop.
Humidity, however, can affect our sense of smell, both positively and negatively. At extremely low humidity levels, the mucous membranes in the nose and throat can become dry, which impairs the sense of smell. If you are exposed to this dry air for a longer period of time, the mucous membrane can also change slightly and is covered with a small layer of mucus to protect it from drying out. This "protection" of the mucous membrane is comparable to a cold and additionally impairs the sense of smell.
Extremely high humidity can also interfere with the sense of smell. From approx. 65 % - 70 % relative humidity, one has the feeling that everything is perceived as somewhat "stale". So we can literally smell and distinguish between fresh and stale air.
Let's look further at the topic of comfort. Comfort is a general term that means when we feel good. In contrast, we speak of discomfort when one or more factors do not feel right together or we simply do not like them.
We will look at these factors in more detail below. As described, we can often directly "smell" too high or too low humidity. But other senses also play a role: too high humidity usually makes us sweat easily or makes our clothes and fabrics, such as curtains, bed covers, etc., feel clammy or damp. The discomfort can even go so far as to cause dry and cracked skin or the feeling that it is difficult to breathe. Too low humidity can also lead to exactly this effect as our bodies try to counteract the drying out of our skin. In addition, viruses and bacteria can lead to infections, e.g. of the mucous membranes, more quickly in very low humidity. Diseases therefore have an easier time. We devote an entire chapter to the topics of humidity, condensation and their effects.
Another important comfort factor is the temperature. If it is too low, we are too cold; if it is too high, we are too warm. However, whether we perceive the temperature as too warm or too cold also depends on our gender and our personal comfort temperature. For example, men have more muscle mass than women. Muscles, on the other hand, generate heat even when you are not moving, which means that on average men do not get cold as quickly as women. If you now add personal sensation, it can lead to completely different sensations of comfort.
Moving air is another factor that is often not or only insufficiently taken into account. We usually speak of (uncomfortable) draughts. Everyone knows the feeling of being cold and having the feeling that a window is open somewhere in the house. Often, however, even small air movements, e.g. in front of a closed window, are enough to create this feeling.
Based on these factors, a comfort diagram was developed with different test persons. For this purpose, different temperatures were combined with different humidity levels and different levels of draught. If more than 20 % of all participants felt uncomfortable, the diagram was adjusted accordingly. So 80 % of all people interviewed agreed. The other 20 % did not, which shows that not everyone can always feel comfortable under the same conditions. However, this also means that 20 % of all people still feel uncomfortable in a place on average, even though the comfort criteria (sometimes also referred to as just the comfort criterion) are met.
This diagram is to be understood in such a way that we always feel comfortable purely in the presence of air humidity and a certain temperature when the intersection of both values lies in the blue, i.e. in the comfort range. The orange range is already somewhat worse and it is not recommended to remain permanently under these conditions. If the intersection point is outside the marked areas, we can almost certainly assume that we will not feel comfortable.
If draughts are added (red line), we will feel uncomfortable even more quickly. The colder it is and the lower the humidity, the faster even a small movement of air will make us feel uncomfortable. The area above the red line (also the previous area of comfort) now becomes equally uncomfortable.
Finally, there is another important point to consider: We humans are creatures of habit and quickly find things good that we have done many times before or that we know from the past. For example, we associate fresh air in the house with certain learned characteristics.
Fresh air is cool for most people, for example, because we were not usually exposed to heat recovery devices or pre-heated air as children or growing up. Instead, a window or door was opened and air flowed into the home. Usually this air was cool, either because it was colder outside than inside, or because it just seemed that way to us, as the air from outside was less enriched with moisture. What is perhaps even more important is the fact that the fresh air always flowed from the direction of a window or door. So we automatically associate fresh air with a window or door, or at least with the direction from which the air comes.
Ventilation devices can give fresh air completely new properties. Fresh air is no longer necessarily cool, is sometimes even humidified and comes from a completely different direction than our body might expect. So it may be that we get fresh air from outside, but do not perceive it as fresh air and thus cannot recognise it. What this means exactly and why modern ventilation devices often do not bring the benefits in practice as calculated is described in another paper.
Let us summarise:
Fresh air is something that we (usually) associate with something positive. Starting with a "good" smell to low humidity to (pleasant) coolness. We usually notice when the air is used up and we need fresh air again. Possible negative characteristics such as pollen or the like are left aside for the moment. Fresh air is important for all of us, because it is partly responsible for our well-being and health. But it is also important for maintaining the value of our houses and flats. We will deal with this topic in another article.