High densities also affect the absorption and emission spectra of photoionized plasmas. Figure 14 shows a comparison of a warm absorber spectrum at densities of 10 cm (panel a) and 10 cm (panel b) due to oxygen in the 0.5-1 keV energy range. The ionization parameter is log()=2 and the temperature is 10K. In the high density case there is a prominent absorption structure near 0.65 keV, associated with resonances in the photoionization cross sections from the 1s2s configurations in OVII. These resonances are apparent in the cross sections shown in figure 2, and they appear in opacity due to the build-up of excited level populations at high densitiies. Such features are potentially observable in the spectra of astrophysical X-ray sources such as the partially ionized absorbers associated with Seyfert galaxies (e.g. [George et al. 1998]) if these objects contain gas at densities comparable to those considered here.