  Small angle scattering by dust grains causes a significant contribution to
the total interstellar extinction for any X-ray instrument with sub-arcminute
resolution (Chandra, Swift, XMM-Newton). However, the dust scattering component
is not included in the current absorption models: phabs, tbabs, and tbnew. We
simulate a large number of Chandra spectra to explore the bias in the spectral
fit and NH measurements obtained without including extinction from dust
scattering. We find that without incorporating dust scattering, the measured NH
will be too large by a baseline level of 25%. This effect is modulated by the
imaging resolution of the telescope, because some amount of unresolved
scattered light will be captured within the aperture used to extract point
source information. In high resolution spectroscopy, dust scattering
significantly enhances the total extinction optical depth and the shape of the
photoelectric absorption edges. We focus in particular on the Fe-L edge at 0.7
keV, showing that the total extinction template fits well to the high
resolution spectrum of three X-ray binaries from the Chandra archive: GX 9+9,
XTE J1817-330, and Cyg X-1. In cases where dust is intrinsic to the source, a
covering factor based on the angular extent of the dusty material must be
applied to the extinction curve, regardless of angular imaging resolution. This
approach will be particularly relevant for dust in quasar absorption line
systems and might constrain clump sizes in active galactic nuclei.
