We explore the relationship between dust-emission derived reddening E ( B-V ) and atomic hydrogen column density N ( H I ) derived from 21 cm emission surveys .
We consider measurements at galactic latitude |b| \gtrsim 20 ^ { o } and E ( B-V ) \lesssim 0.1 mag where the interstellar gas is predominantly neutral and atomic and opacity corrections to 21cm H I profiles are small .
Over the Galaxy at large at lower resolution in H I , and on smaller scales at higher resolution , we find that the reddening is always much smaller than would be expected from the usually-quoted relation N ( H ) = 5.8 \times 10 ^ { 21 } \mbox { $ { ~ { } { cm } ^ { -2 } } $ } E ( B-V ) based on stellar reddening and uv absorption toward early-type stars .
On wide scales , we find N ( H I ) = 8.3 \times 10 ^ { 21 } \mbox { $ { ~ { } { cm } ^ { -2 } } $ } E ( B-V ) .
We cite various precedents for such a large N ( H I ) /E ( B-V ) ratio whenever wide-field 21cm emission surveys are considered , including when reddening based on galaxy counts or colors is substituted for the dust-emission derived reddening measure .