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 have recently shown, 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. I will discuss the causes for this large 
scatter that effectively prevents one from inferring extinction using this method.