On foam products with porous covers, moisture doesn't increase as much as on most foam since the open cell structure of the covers provides a pathway through which moisture can diffuse. Nicholson showed that water vapor transfer rates were reduced by more than half when foam mattresses were covered with non-stretch and two-way stretch covers (Nicholson, 1999). Patient movement can increase transfer rates. Mean temperature increases of 3.4 degrees C and a 10.4% increase in moisture at the skin surface have been recorded by Stewart on foam products after one hour of contact .