Balmer decrements for α.btfr galaxies. The blue thick histogram represents the distribution of Balmer decrements for the galaxies in the α.btfr sample. Balmer decrements have been computed from the H_α and H_β line fluxes reported in the NSA catalog (refer to Sect. 2.2). The red histogram is the Balmer decrement distribution for a broad sample of galaxies that are high quality ALFALFA detections. This parent sample spans a broad range in galaxy inclination, stellar mass, and gas fraction. The green histogram represents the distribution of the subsample of galaxies in the parent ALFALFA sample that are oriented edge-on. The vertical dashed line denotes an H_α/ H_β ratio of 2.86, which is adopted here as the fiducial value for HII regions in the absence of extinction. The upper y-axis represents the implied dimming of the r-band luminosity due to internal extinction (Eq. (A.2)).