EW measurements for simulated intervening Na I D lines located at a rest frame (ɀ₀ = 5893 Å) on top of real sodium P-Cygni profiles from observed spectra of SN 1999em (left) and on top of simulated spectra of Dessart & Hillier (2011) (right). Horizontal lines show the EW expected from the simulated narrow Na I D line: EW1 = 1.1 Å, EW2 = 2.2 Å, and EW3 = 4.4 Å. The measured continuum slope at around 5893 Å is presented in the bottom panel. The horizontal line shows the cut applied in this study: a maximum continuum slope value of 0.002 Å⁻¹.