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tokens: variations in the inferred and for highly reddened SNe if the dust distance is in the right range (Bulla et al. 2018 ). 2. For nonuniform ISM dust, the later phases will have values more consistent with each other while earlier phases will show larger and random (but correlated) variations. (For example, see Figure 8 for F13 : while the variations in F13 are over-stated, as discussed here, the trends are qualitatively correct). This effect is independent of the distance between the SN and foreground dust, and we find that typically it will be small even for highly reddened SNe. 4. Conclusions Even though there is clear evidence for ISM density fluctuations on scales of 10 au or even below, we show that due to the steep spectral index of the ISM column density power spectrum and because the SN photodisk is much smaller ( in diameter) than the typical ISM structure (), substantial variations on pc scales translate to much smaller fluctuations on the scale of SN photodisk. We provide a way to set the normalization for ISM density fluctuations on length scales relevant to SNe Ia based on MW dust emission observations (Miville-Deschênes et al. 2007 ). We have found that the expected time variations of EW(Na i ) and for SNe Ia due to nonuniform ISM would be typically much smaller than the predictions in Patat et al. ( 2010 ) and Förster et al. ( 2013 ), respectively. The observations of two SNe Ia highly reddened by at least mostly interstellar dust, SN 2012cu and SN 2014J, are consistent with the results of our analysis. The observed color variations for SN 2006X have been attributed to a light echo (Bulla et al. 2018 ) rather than nonuniform ISM, and so are also consistent with our analysis. We therefore conclude the following, if nonuniform ISM density in the SN host galaxy follows a scale-free power-law power spectrum. (1) The expected variation of extinction for SNe Ia with mag is negligible and is certainly smaller than what is currently scientifically relevant for SN cosmology. This, combined with mounting evidence that interstellar dust is likely the dominant source of reddening for SNe Ia, significantly reduces the concern about time variation of extinction for SNe Ia due to nonuniform dust density in cosmological studies. (2) For the study of extragalactic ISM density nonuniformity, it may be possible to detect small variations in extinction across phases for a highly reddened SN. The rms can be as large as ∼0.1 mag over a phase range of, say, from −10 to +50 days, with the shallowest spectral index (). A larger phase coverage would be better for this purpose (earlier observations would especially help, as they allow the observation to sample closer to the full extent of the photodisk at the last observed phase). Huang et al. ( 2017 ) demonstrated that in order for such a detection to be feasible, it is important to not only have an unreddened SN that is well-twinned with the dust-obscured SN, but the observed phases of these two SNe