Some aspects of the chemical evolution of 4He in the Galaxy: the He/H   abundance gradient and the Delta Y/Delta Z enrichment ratio
  Two aspects of the chemical evolution of 4He in the Galaxy are considered on the basis of a sample of disk planetary nebulae by the application of corrections due to the contamination of 4He from the progenitor stars. First, the He/H radial gradient is analyzed, and then, the helium to heavy element enrichment ratio is determined for metallicities up to the solar value. 
W. J. Maciel