The effect of water soaking on the strength of silica glass is often studied in literature. When silica glass is immersed in warm water or water vapour and held there for an extended period of time, the strength increases over that of freshly damaged glass. The increase in strength is a consequence of water diffusion into exposed surfaces of the test specimen, which results in swelling of the glass and shielding of cracks present in the surface of the glass. For strength tests carried out in humid environment at various loading rates, so-called dynamic strength tests, we can simply show theoretically that the swelling effect by the reaction of water with silica must result in apparently increased crack-growth exponents. This prediction is in good agreement with results from literature.