The effect of confinement in quantum well on the lifetimes of excited shallow impurity states is theoretically investigated. It is shown that quantum well potential causes a slower asymptotic behavior of the wave functions at large wave vectors (q^-3) and may lead to an exponential increase of the transition rates due to interaction with short wavelength ( λ ≪a_B) acoustic phonons. The model explains experimentally observed reduction of lifetimes of impurity states in quantum wells and allows us to predict the parameters of quantum wells to maximize impurity states lifetimes.