  This paper presents a mechanism that may modify the extinction law for SNIa
observed at higher redshift. Starting from the observations that (1) SNIa occur
predominantly in spiral galaxies, (2) star-formation ejects ISM out of the
plane of spirals, (3) star-formation alters the extinction properties of the
dust in the ISM, and (4) there is substantially more star-formation at higher
redshift, I propose that spiral galaxies have a dustier halo in the past than
they do now. The ejected material's lower value of $R_V$ will lead to a lower
average value ($\bar{R}_V$) for SNIa observed at higher redshift.
  Two relations in SNIa observations indicate evolution of the average $R_V$:
the relation of observed $R_V$ with inclination of the host galaxy at low
redshift and the matching of the distribution of extinction values ($A_V$) for
SNIa in different redshift intervals. The inclination effect does point to a
halo with lower $R_V$ values. In contrast, the distributions of $A_V$ values
match best for a $\bar{R}_V(z)$ evolution that mimics the relation of SNIa
dimming with redshift attributed to the cosmological constant. However, even in
the worse case scenario, the evolution $\bar{R}_V$ can not fully explain the
dimming of SNIa: host galaxy extinction law evolution is not a viable
alternative to account for the dimming of SNIa.
  Future observations of SNIa --multi-color lightcurves and spectra-- will
solve separately for values of $A_V$ and $R_V$ for each SNIa . Solving for
evolution of $\bar{R}_V$ (and $A_V$) with redshift will be important for the
coming generation of cosmological SNIa measurements and has the bonus science
of insight into the distribution of dust-rich ISM in the host galaxies in the
distant past.
