The smell is caused by bacteria, which grow in the damp conditions and attack sweat residues and some of the materials. The worst place for this is between the plastic foam footbed and the cardboard liner. All shoes need to be aired after use, which, with synthetic materials, means removing the footbed and allowing everything to dry. Once the smell has developed it's difficult to eradicate but a wash wth hot caustic water, as in a washing machine with detergent, will go a long way to removing smells. Be sure to dry the shoes thoroughly after washing and after use. Sunlight is good because the UV will kill bacteria.