  Although broad emission lines are the most reliable signature of the nuclear
activity of a galaxy and the location of the emitting material is well measured
by the reverberation method, the physical cause of the formation of the broad
line region remains unclear. We attempt to place some constraints on its
origin. We study the properties of the accretion disk underlying the broad line
region. We find that the effective temperature at the disk radius corresponding
to the location of the broad line region, as inferred from the H$\beta$ line,
is universal in all monitored sources and equal to 1000 K. This value is close
to the limiting value that permits for the existence of the dust. The likely
origin of the low ionization part of the broad line region is the strong local
dusty wind from the disk. This wind becomes exposed to the irradiation by the
central regions when moving higher above the disk surface and subsequently
behaves like a failed wind, thus leading to a local mixture of inflow and
outflow. This may provide the physical explanation of the turbulence needed
both to smooth the line profiles as well as provide additional mechanical
heating.
