Why do wet things smell more than dry ones?
• Making something wet does not automatically make it more smelly. For instance, a wet clean towel smells no worse than a dry clean towel. However, the presence of moisture does allow the growth of bacteria, assuming that there is organic matter present for them to eat. As they grow and multiply, bacteria produce a whole range of smelly compounds of the kind you can detect in bad breath, for example. So given moisture and enough time for bacterial growth, wet things can smell worse. But if you prevented bacterial growth by, say, sterilising the wet item to kill all bacteria, then it wouldn’t develop such a smell.
Simon Iveson, Department of Chemical Engineering, University of Newcastle, New South Wales, Australia
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• Most chemical components of dank smells are products of microbial activity, and microbial activity requires water. Once the chemicals are present they can reach the nose only by escaping into the air.
Most are fatty acids, amino compounds and the like, with charged chemical groups that readily bind to non-volatile molecules such as large proteins and carbohydrates. Once they have latched onto, say, dry cloth or leather, they cannot float freely into the air so there is not much to smell. However, these charged groups have an affinity for polar molecules, and the most polar of common molecules is water. So when the object gets wet, water molecules prise loose the odour molecules, cocooning them in tiny mobile parcels of water. For good or ill many escape into the air, reaching nearby noses in vast numbers.
Accordingly, a powerful deodorising strategy is to release other molecules that immobilise pong molecules by binding them with complementary charged groups. combats smells partly by presenting a metal atom that binds the active groups of many smell molecules. Similarly, by binding key molecules, partly oxidised paraffin wax vapour from the smoke of burning candles also helps clear a room of the stench of cigarettes.
Jon Richfield, Somerset West, South Africa