Airing at night in the wintertime in case of CF

Question

Hello,
I come across again and again differing opinions form physicians, concerning the nightly tilting of the window in the wintertime. One physician says, it is better to keep the windows closed, as outdoors the cold air would be too dry. The other arguments in favour of opening the windows, as the house is heated and even if there is no heating on in the bedroom, the air would be better in any case for CF patients with an open window. Could you give me a clearer answer here? I do already the following: wet towels and water bowls on the radiators, regluar airing and switching off the heating during sleep.
Many thanks in advance

Answer

Dear questioner,
in order to be able to answer your question satisfyingly, one has to go a little bit into physics. Air contains humidity, that means water(steam); how much water steam the air can take up, until it is saturated (over the the saturation limit the water would fall out as fog or drops), depends on the temperature of the air. Warm air can take up more water per cubic meter, before the saturation limit is reached, than cold air. An example: 10°C cold air can take a maximum of 9.4 g water per cubic meter, 30°C warm air however already 30.4 g water; then the air is in both cases saturated with water to 100%.
The relative air humidity quotes the actual content of water(steam) in the air as percentage of the maximal possible content of water(steam); in this example, thus the relative air humidity was 100% in both cases (thus the maximum). The absolute air humidity however, was for the 10°C cold air and 100% only 9.4 g, for the 30°C warm air however 30.4 g. A relative air humidity of 50% is comfortable for humans.
Indeed it is like this, that the relative air humidity decreases after airing in the winter: assumed, in the winter there are 0°C outside and there would even be optimal humid circumstances outside: fog, that means 100% relative air humidity. One can then find in tables, that air of 0°C contains with 100% saturation about 5 g of water per cubic meter. In the living room for example, there had been 20°C and 50% air humidity before, with this, the air contained 9 g water per cubic meter air (all these values can be derived from tables). Now you air until there had been a complete interchange of air, that means in the living room are now also 0°C and 5 g water per cubic meter air. Then you close the windows and heat the air again to 20°C, this air, however, has now only 5 g instead of before 9 g water per cubi cmeter and therefore now the relative air humidity is only about 30%.
Therefore the statement, that the cold air outside is dryer, is correct (depending on the temperature itself, the lower the temperature, the less water the air contains, even in case of 100% humidity), respectively in our example the air outside at 0°C was as dry as air in a 20°C heated room that had only 30% relative air humidity (these conditions can indeed be found sometimes as room climate during the heating period in the winter). In the bedroom, that is not heated, one has often values with 16°C, 60% relative air humidity, corresponding to about 8 g water per cubic meter. Optimal conditions for the heated living room would be 20°C with 50% relative air humidity and therefore about 9 g water per cubic meter. Even in case of outside temperatures of 7°C and 60% relative air humidity and therefore about 4.5 g water per cubic meter air, the outdoor air still contains markedly less water than the indoor air in bed- and living room.
Now to the circumstances for sleeping. It is unambigous, that the optimal room temperature for sleeping is between 15-18°C, in the ideal case with more than 50% relative air humidity. Besides temperature and air humidity the oxygen content of the air is of course important for a good sleeping climate. Therefore, it is generally advantageous, to have always a good oxgen content via the open window. However, how the open window influences the air humidity, strongly depends on the outside temperatures and outside air humidity; the colder the outdoor temperature and if the outdoor air is additionally dry (e.g. only 30% relative air humidity in case of sunny, but cold high pressure weather in the winter), it can become fast too cold in the room due to the open window, as well as indeed also too dry.
In here, this also depends on the extent of the opening of the window: wide open, tilted, with or without down scrolled window shades. In case only few air reaches the room, it does not come immediately to a complete interchange of the air inside and outside, but however only to a decrease of the room temperature of 1°C and to a small decrease of the air humidity.
A room that is too cold, does often lead to a disturbed sleep due to freezing, additionally it can come the the follwoing phenomenon: if it is markedly colder in the bedroom than in the rest of the appartment, warm air enters the cold room and mixes there. As warm air can have a higher humidity content and meets cold air and walls that cannot take up as much humidity, the humidity can condensate and lead to mould formation.
In case of such a dry and cold weather, it is better, to air before sleeping, in order to optimize the oxygen content and then to close the window. It would probably be advantageous to have a "weather station" in the bedroom and to make the decision according to the actual room temperature and air humidity. In case the room temperature is in the upper range at 18°C and the relative air humidity is quite high, than one can have the windows rather sometimes open with small air flow in the wintern in case of cold dry air, if one likes to sleep with an open window. An alternative to sleeping with open window could also be an open window in the neighbouring room with open doors, but also here, the room should not get too cold and therefore too dry.
I hope this answer helps even it cannot be said with a single sentence.
Yours sincerely,
Dr. Daniela d'Alquen

14.01.2014