  Recent observations by Bania et al. (2002) measure He3 versus oxygen in
Galactic HII regions, finding that He3/H is within a factor of 2 of the solar
abundance for [O/H] > -0.6. These results are consistent with a flat behavior
in this metallicity range, tempting one to deduce from these observations a
primordial value for the He3 abundance, which could join D and Li7 as an
indicator of the cosmic baryon density. However, using the same data, we show
that it is not possible to obtain a strong constraint on the baryon density
range. This is due to (i) the intrinsically weak sensitivity of the primordial
He3 abundance to the baryon density; (ii) the limited range in metallicity of
the sample; (iii) the intrinsic scatter in the data; and (iv) our limited
understanding of the chemical and stellar evolution of this isotope.
Consequently, the He3 observations correspond to an extended range of
baryon-to-photon ratio, eta = (2.2 - 6.5) x 10^{-10}, which diminishes the role
of He3 as a precision baryometer. On the other hand, once the baryon-to-photon
ratio is determined by the CMB, D/H, or Li7/H, the primordial value of He3/H
can be inferred. Henceforth new observations of Galactic He3, can in principle
greatly improve our understanding of stellar and/or chemical evolution and
reconcile the observations of the HII regions and those of the planetary
nebulae.
