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This paper presents a mechanism that may modify the extinction law for Type Ia supernovae (SNeIa) observed at higher redshift. Starting from the observations that (i) SNeIa occur predominantly in spiral galaxies, (ii) star formation ejects interstellar medium (ISM) out of the plane of spirals, (iii) star formation alters the extinction properties of the dust in the ISM, and (iv) 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  for SNeIa 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 SNeIa 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 an  evolution that mimics the relation of SNIa dimming with redshift attributed to the cosmological constant. However, even in the worse‐case scenario, the evolution  cannot fully explain the dimming of SNeIa: host galaxy extinction law evolution is not a viable alternative to account for the dimming of SNeIa.  Future observations of SNeIa – multicolour light curves and spectra – will solve separately for values of   A   V    and   R   V    foreach SNIa. Solving for evolution of  (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.

Host galaxy extinction of Type Ia supernovae: co‐evolution of interstellar medium structure and the extinction law with star formation