Chemical evolution of elements in NGC 6822 as predicted by model M4C. To reproduce the observed M_gas value and the different heavy element abundances obtained from collisionally excited lines for H ii regions, a well-mixed galactic wind lasting from 1.2 to 6.5 Gyr and an M_up = 40   M_⊙ are assumed in this model. The O/H value of H ii regions has been corrected for dust depletion, as explained in Sect. 4.1. The filled circles, filled squares, and filled triangles represent the average observational values for H ii regions, young, and old PN populations, respectively. The filled star in the Fe/H panel represents the value derived by Venn et al. (2001) for A-type supergiant stars. Observed C/H value is not shown because there is no observed value from CELs.