HI metal enrichment from massive star--forming regions: sudden or delayed?

It now becomes possible to perform multiphase studies of the ISM from hot to molecular phases. HI and HII in star--forming gas-rich galaxies seem to exhibit differences in their metal composition in the sense that HII is less enriched in nitrogen, argon and possibly oxygen than the HII ionized gas. This observational evidence needs to be confirmed on larger sample and yet remains difficult to intepret. We review some possible interpretations. If, on the other hand, metals remain for a long time in hot gas they might ultimately cool down and mix with the original interstellar medium rather than being expelled into the intergalactic medium.