On the total O/H abundance ratio in Galactic and extragalactic H ii regions

To determine the primordial helium abundance and to study the chemical evolution of galaxies it is necessary to derive the total O/H ratio in H ii regions. To determine the total O/H ratio in H ii regions it is necessary to add to the gas-phase component the dust-phase component of O atoms. Based on the Fe/O ratio and other considerations we estimate the dust-phase fraction as a function of O/H.