Air exchange between indoors and outdoors
The air pollution measured in a room is controlled by the rate of generation of pollutants within the room, the rate at which pollutants move in from outside and the rate of absorption on the walls of the room. Separating these three processes is at present impossible, so all the pollution data we have from rooms, and from the environment, doesn't make us much wiser about the processes going on. There are several groups of scientists trying to improve this situation. The EU 5th framework project IMPACT has measured the rate of reaction of pollutants on various common indoor surfaces. There are data going back to the 1960's for individual pollutants and materials, but the IMPACT team are generating a systematic database.
Next, we have to measure the rate of air exchange. This is at present an expensive operation. A rare and unreactive gas, usually a fluorocarbon, is released at a steady rate into the room. This is done by enclosing the volatile fluorocarbon liquid in a glass tube with a silicone membrane, whose size and permeability allows a slow leak of the gas into the room. Elsewhere in the room is another tube with an absorber of the rare gas. This absorber is at the closed end of a narrow tube, or protected by a fine mesh, to ensure that the rate of absorption depends on the concentration in the air rather than the air velocity over the absorber. From the known emission rate and the measured absorption rate one can work out how fast gas is lost to the outer world. This is expensive, and the rate of air exchange varies minute by minute in a naturally ventilated room.
The typical museum archive has a large databank of indoor relative humidity and temperature. Pollution data indoors is much rarer. Even rarer is a database of climate indoors and outdoors and pollution indoors and outdoors.
Another 5th Framework project, MIMIC, has, as a bi-product of its main purpose, accumulated indoor and outdoor climate and pollution data. The data illustrate the great variety of indoor climates in a single building and also show the very powerful effect of buffering of both temperature and relative humidity by the materials of the building and by its furnishing. It also hints at the importance of air exchange measurement as a necessary means of distinguishing between rooms which are clean because pollutants do not penetrate into them and rooms which are clean because they absorb pollutants on the furnishing. To the human occupant, there is no difference to the health benefits, but to works of art there is a huge difference to their wellbeing, depending on which exclusion mechanism operates.
This matter is illustrated quantitatively in this article from some of the MIMIC team (in pdf, 1Mbyte)
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