According to Arc's Guide to Reentry v1.1, TVOC measurements in ppb can be converted to μg/m3 using a conversion factor of 3.767. The guide can be accessed here.

The LEED v4.1 O+M Beta Guide also provides the option to “Use correction factors and/or conversion factors applicable to the instrument being used when converting from ppm to μg/m3 and note any assumptions in the evaluation report.”