  In the era of XMM-Newton and Chandra missions, it is crucial to use codes
able to compute correctly the line spectrum of X-ray irradiated thick media
(Thomson thickness of the order of unity) to build models for the structure and
the emission of the central regions of AGN or X-ray binaries. In all
photoionized codes except in our code Titan, the line intensities are computed
with the "escape probability approximation". In its last version, Titan solves
the transfer of a thousand lines and of the continuum with the ``Accelerated
Lambda Iteration" method, which is one of the most efficient and most secure
for line transfer. We find that for conditions typical of the AGN or X-ray
binary emission medium, all escape approximations commonly used lead to an
overestimation of the soft X-ray lines which can reach one order of magnitude
for intense lines.
