Seven problems related to the determination of the primordial helium   abundance
  Recent advances on the quest to determine the primordial or pregalactic helium abundance, Yp, are reviewed. There are seven problems affecting the He/H abundance determinations of H II regions that are briefly discussed: the underlying absorption lines in the observed spectra, the ionization, temperature, and density structures of each object, the collisional contribution to the intensity of the He I and H I lines, the optical thickness of the He I lines, and the extrapolation to derive Yp based on the Y and O/H values. 
Manuel Peimbert, Antonio Peimbert, Valentina Luridiana, & Maria Teresa   Ruiz