**Table 1:** Methods for determining $N_{\rm H}$ /A_V ratios.
Method	A_V max	Medium	Length scale	Ref.
B stars (UV)	$\sim$ 2 mag	diffuse local	$\sim$ few 100 pc	Bohlin et al. (1978)
B stars (UV)	$\sim$ 2 mag	diffuse local	$\sim$ pc	Whittet (1981)
SNR (X-rays)	$\sim$ 30 mag	diffuse galactic	$\sim$ kpc	Gorenstein (1975)
SNR (X-rays)	$\sim$ 30 mag	diffuse galactic	$\sim$ kpc	Ryter et al. (1975)
SNR + compact objects (X-rays)^*	$\sim$ 20 mag	diffuse galactic	$\sim$ kpc	Predehl & Schmitt (1995)
Diskless T Tauri stars (X-rays)^*	$\sim$ 50 mag	dense clouds	$\sim$ sub-pc	This work

^* X-ray spectroscopy with imaging X-ray satellites.

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