Cause
The main cause is foot sweat. Sweat itself is odorless, but it creates a beneficial environment for certain bacteria to grow and produce bad-smelling substances. These bacteria are naturally present on our skin as part of the human flora. Therefore, more smell is created with factors causing more sweating, such as wearing shoes and/or socks with inadequate air ventilation for many hours. Hair on the feet, especially on the toes, may contribute to the odor's intensity by adding increased surface area in which the bacteria can thrive.
Given that socks directly contact the feet, their composition can have an impact on foot odor. Polyester and nylon are common materials used to make socks, but provide less ventilation than cotton or wool do when used for the same purpose. Wearing polyester or nylon socks may increase perspiration and therefore may intensify foot odor. Because socks absorb varying amounts of perspiration from feet, wearing shoes without socks may increase the amount of perspiration contacting feet and thereby increase bacterial activities that cause odor.
Read more about this topic: Foot Odor