The strength of silica optical fibers depends on the water activity at the glass surface. Polymer coatings are currently applied to optical fibers to protect them from mechanical damage during handling. They also act as a barrier diffusion of the surrounding humidity reaching the glass surface. Water is known to be one major factor of the propagation of cracks at fiber glass surface because it makes much easier the breaking of the Si-O bonds which build the vitreous network. The strength of the fiber is related to the water concentration at the glass surface. It is well known that flaws in glass subject to stress in humid condition grow subcritically. The crack velocity is related to the applied stress and also to the humidity rate . The kinetic of the reaction between silica and water changed at very low water concentration .