Photoionization models of the giant H ii region 30 Doradus are built and confronted to available UV , optical , IR ( ISO ) and radio spectra , under black-body or CoStar spectral energy distributions for the primary source and various density distributions for the nebular gas .
Chemically homogeneous models show very small rms electron temperature fluctuations and fail to reproduce the heavy element optical recombination line ( ORL ) spectrum of the nebula .
Dual abundance models incorporating small-scale chemical inhomogeneities in the form of hydrogen-deficient inclusions which are in pressure balance with the normal composition ambient gas , provide a better fit to the observed heavy element ORLs and other nebular lines , while most spectral features are satisfactorily accounted for .
The inclusions , whose mass is \sim 2 per cent of the total gaseous mass , are 2–3 times cooler and denser than the ambient nebula .
Their O/H abundance ratio is \sim 0.9 dex larger than in the normal composition gas and have typical mass fractions of X = 0.687 , Y = 0.273 and Z = 0.040 .
Helium is found to be about as deficient as hydrogen in the inclusions , while elements heavier than neon , such as sulfur and argon , are quite possibly enhanced in proportions similar to oxygen , as indicated by the most satisfactory dual abundance model obtained .
This suggests that the posited H-deficient inclusions may have arisen from partial mixing of matter which was nucleosynthetically processed in a supernova event with gas of normal LMC composition .
The average gaseous abundances of the chemically inhomogeneous models are \sim 0.08 dex higher than those of the homogeneous models , yet they are lower by a similar \sim 0.08 dex than those derived from standard empirical methods ( fully corrected for inaccuracies in ionization correction factors and differences in atomic data ) which postulate temperature fluctuations in a chemically homogeneous medium .
Attention is drawn to a bias in the determination of H ii region helium abundances in the presence of H-deficient inclusions .
It is argued that these results provide evidence for incomplete small-scale mixing of the interstellar medium ( ISM ) .
The case for the existence of abundance inhomogeneities in H ii regions is examined in the light of current theoretical considerations regarding the process of chemical homogenization in the ISM .

Key Words : ISM : abundances – H ii regions – ISM : evolution – ISM : individual : 30 Dor – galaxies : individual : LMC