The sample is collected from the indoor water vapour by decomposition into solid frost, which then melts and forms water droplets on the steel surface. This is one way how water vapour moves in nature as well. From the sea, the water vapour evaporates into the clouds, and rains down as either water or snow. There is always outdoor air first and it turns into indoor air when it is brought inside a building.
When outdoor air is brought inside a building, certain contaminants get stuck in its water vapor. Indoor air also contains particles and gaseous substances, but they are collected by different methods. These parts have been studied for more than 30 years, but the impurities in the water vapor have only been studied since 2014 as a result of Johanna Salo’s diploma thesis and the sampling method developed by Aattela.