I read this somewhere:

The average pair of feet gives off a half pint of moisture a day. Almost two quarts a week. And 25 gallons a year. Some feet give off much more. Where does that incredulous amount of foot moisture go? Most of it remains inside the shoe, absorbed by the linings. If the moisture isn’t properly absorbed and passed off, the foot experiences a sloshy, clammy feeling. Perspiration is 98 percent water and 2 percent salts and acids.