Blue compact galaxies ( BCG ) are ideal objects in which to derive the primordial ^ { 4 } He abundance because they are chemically young and have not had a significant stellar He contribution .
We discuss a self-consistent method which makes use of all the brightest He I emission lines in the optical range and solves consistently for the electron density of the He II zone .
We pay particular attention to electron collision and radiative transfer as well as underlying stellar absorption effects which may make the He I emission lines deviate from their recombination values .
Using a large homogeneous sample of 45 low-metallicity H II regions in BCGs , and extrapolating the Y-O/H and Y-N/H linear regressions to O/H = N/H = 0 , we obtain Y _ { p } = 0.2443 \pm 0.0015 , in excellent agreement with the weighted mean value Y _ { p } = 0.2452 \pm 0.0015 obtained from the detailed analysis of the two most metal-deficient BCGs known , I Zw 18 and SBS 0335–052 .
The derived slope dY/dZ = 2.4 \pm 1.0 is in agreement with the value derived for the Milky Way and with simple chemical evolution models with homogeneous outflows .
Adopting Y _ { p } = 0.2452 \pm 0.0015 leads to a baryon-to-photon ratio of ( 4.7 ^ { +1.0 } _ { -0.8 } ) \times 10 ^ { -10 } and to a baryon mass fraction in the Universe \Omega _ { b } h ^ { 2 } _ { 50 } = 0.068 ^ { +0.015 } _ { -0.012 } , consistent with the value derived from the primordial D abundance of Burles & Tytler ( 1998 ) .