If you can detect an odor or see mold growing on your Orbeez™, you should throw away the affected product and replace them with new Orbeez™.
You can keep your Orbeez™ fresh for a longer period of time by always washing your hands and feet before playing with, or placing them in Orbeez™.
Between playing, you should also store your Orbeez™ in a closed container (e.g. Zip-Lock bag, Tupperware, or the container you purchased them in) out of sunlight to keep them fresh for longer.