With over 2 million sweat glands on the feet, the average person sweats up to ¼ cup a day in their shoe. This sweat changes the pH level of the skin and creates an environment for bugs to grow. It is this bacteria that gives off an unpleasant odor. The build up of perspiration on the foot can lead to a build up of moisture on the feet or in the shoes and this can lead to a build up of bacteria which can cause odor issues.
Shoes are designed to keep moisture out and so they quite often also keep moisture in which exaggerates the issue. Materials in the shoe can hold the moisture thereby promoting bacteria growth. Products such as 'Cool Foot Spray' and 'Roll On for Feet' can help solve this problem