  We remark on the utility of an observational relation between the absorption
column density in excess of the Galactic absorption column density, $\Delta
N_{\rm H} = N_{\rm H, fit} - N_{\rm H, gal}$, and redshift, z, determined from
all 55 Swift-observed long bursts with spectroscopic redshifts as of 2006
December. The absorption column densities, $N_{\rm H, fit}$, are determined
from powerlaw fits to the X-ray spectra with the absorption column density left
as a free parameter. We find that higher excess absorption column densities
with $\Delta N_{\rm H} > 2\times 10^{21}$ cm$^{-2}$ are only present in bursts
with redshifts z$<$2. Low absorption column densities with $\Delta N_{\rm H} <
1\times 10^{21}$ cm$^{-2}$ appear preferentially in high-redshift bursts. Our
interpretation is that this relation between redshift and excess column density
is an observational effect resulting from the shift of the source rest-frame
energy range below 1 keV out of the XRT observable energy range for high
redshift bursts. We found a clear anti-correlation between $\Delta N_{\rm H}$
and z that can be used to estimate the range of the maximum redshift of an
afterglow. A critical application of our finding is that rapid X-ray
observations can be used to optimize the instrumentation used for ground-based
optical/NIR follow-up observations. Ground-based spectroscopic redshift
measurements of as many bursts as possible are crucial for GRB science.
