When water diffuses into silica glass it reacts chemically with the glass forming nanometre sized pores that change the physical properties of the glass. In earlier papers and reports, we discussed the effect of water/silica reaction on the strength via volume swelling, and showed by use of damage mechanics that the water reaction reduces Young’s modulus E and intrinsic strength in thin surface layers. In this paper, the dependency between hydroxyl concentration and damage will be derived for the full damage range by using experimental results from literature. For small water concentrations, we used sound velocity measurements from literature. The suggested relations describe the dependence between the hydroxyl concentration and the Young’s modulus for the damaged glass and allow the strength decrease due to hydroxyl generation to be computed. From an example of application, it can be concluded that damage by hydroxyl generation has little effect on strength even in the case of completely damaged surface region so far the water-affected surface zone is much thinner than the bulk material.