The project focuses on the study of the effect of the internal structure of temperature and density in ionized nebulae on the determination of their chemical composition. The main objective is to carry out different independent verifications and analyze in depth the repercussions of our recent work published in Nature (Méndez-Delgado et al. 2023), where we found a correlation between the internal variations of the electron temperature and the abundance discrepancy in the HII regions, a problem that had been unsolved for more than 80 years. The project focuses on three subprojects. The first is based on the global study of the deep spectrum database compiled by our group over the last 20 years (DESIRED) and an extension with good-quality data from the literature (DESIRED Extended) and the obtention of new observations of local low-metallicity HII regions. The other two subprojects constitute two independent tests of the hypothesis of Méndez- Delgado et al. (2023) and the problem of the discrepancy of abundances both in HII regions and planetary nebulae. On the one hand, the exploitation of the nebular part of the WEAVE/SCIP survey, with the determination of chemical abundances of HII regions in Galactic star formation zones and their comparison with the abundances obtained from the associated massive stars obtained by other WEAVE research groups. On the other hand, the third subproject will be based on obtaining mid-infrared (MIR) spectra with MIRI MRS in the JWST of HII regions from our DESIRED optical spectroscopy database to calculate the abundance discrepancy by comparing abundances determined from collisionally excited lines of a given ion that present lines in both the optical and the MIR. Our objective is to clarify what type of emission lines provide the correct abundances and thus determine, with better precision, the data on metallicities that we have available to understand the chemical composition of the Universe and its temporal evolution.