In the framework of spherical pure hydrogen models of stellar envelopes photoionized by photospheric (or chromospheric) radiation we study the dependence of their ionization structure on an outward density decrease. To avoid the problem of radiative transfer we make use of the two classical approximations of optically thin (A) and thick (B) HII regions. We show that all density decreases can be classified, by means of some simple criterion, as slow and fast ones. For slow density decreases the radius of HII region can be estimated in the way which gives a straightforward generalization of Strömgren radius to inhomogeneous envelopes. But for fast density decreases this generalization can fail, and the radius of the HII region crucially depends on the ionization parameter p near some critical value P_l. Namely, it grows sharply when p increases up to p_l and it becomes very large (essentially infinite), in comparison with the inner radius r₀, for p > p_l. This means that in the case of fast density decreases an HII region is essentially a threshold phenomenon: there is no significant HII region when p < p_l - Δp while the envelope is almost completely ionized when p > p_l, the value of Δp being much smaller than p_l. It is shown also that for sufficiently fast density decrease and p in the vicinity of p_l there is the possibility of a nonmonotonous distribution of ionization degree throughout the envelope. We present analytical and numerical results for the power law density distribution n(r) = n₀(r₀/r)^k in which case the slow and fast decreases correspond to k < 3/2 and k > 3/2. The case of a stationary flow with the Lamers velocity law is also briefly discussed and numerical estimates are given in the context of the Catala and Kunasz (1987) models of outflowing envelope around Herbig Ae/Be star AB Aur.