  We present a spectral analysis of the narrow-line Seyfert 1 galaxy I Zwicky
1, focusing on the characteristics of the ionised absorbers as observed with
XMM-Newton in 2005. The soft X-ray spectrum shows absorption by two components
of ionised gas with a similar column density (N_H~10^{21} cm^{-2}) and
ionisation parameters log\xi~0 and 2.5. Comparing this observation with a 2002
XMM-Newton data set, we see a clear anti-correlation between the X-ray
ionisation parameter xi_X and the 0.1-10 keV luminosity. Viable explanations
for this effect include transient clouds or filaments crossing the line of
sight in a complex geometry or a gas observed in non-equilibrium. The outflow
velocity of the X-ray low-ionisation absorber is consistent with the outflow of
the UV absorber detected in a past Hubble Space Telescope observation. In
addition, the ionic column densities of CIV and NV derived from the X-ray model
are consistent with the UV values. This suggests that the low-ionisation
outflowing gas may survive for many years, despite large changes in flux, and
that there is a tight connection between the X-ray and UV absorbers that can
only be confirmed with a simultaneous UV and X-ray observation.
