  Dust extinction is generally the least tractable systematic uncertainty in
astronomy, and particularly in supernova science. Often in the past, studies
have used the equivalent width of Na I D absorption measured from
low-resolution spectra as proxies for extinction, based on tentative
correlations that were drawn from limited data sets. We show here, based on 443
low-resolution spectra of 172 Type Ia supernovae for which we have measured the
dust extinction as well as the equivalent width of Na I D, that the two barely
correlate. We briefly examine the causes for this large scatter that
effectively prevents one from inferring extinction using this method.
