The PNLF method is unbiased by [[metallicity]]. This is because [[oxygen]] is a primary nebular coolant,; any drop in its concentration raises the plasma’s electron temperature and raises the amount of [[collisional excitation]]s per ion. This compensates for having a smaller number of emitting ions in the PNe. Consequently, a reduction in oxygen density only lowers the emergent <nowiki>[O III]</nowiki> λ5007 emission line flux by approximately the square root of the difference in abundance. At the same time, the PNe’s core responds to metallicity the opposite way. In the case where the metallicity of the progenitor star is smaller, then the PNe’s [[central star]] will be a bit more massive and its emergent ultraviolet flux will be a bit larger. This added energy almost precisely accounts for the decreased emissions of the PNe. Consequently, the total <nowiki>[O III]</nowiki> λ5007 flux that is produced by a PNe is practically
uncorrelated to metallicity. This beneficial negation is in agreement with more precise models of PNe evolution. Only in extremely metal-poor PNe does the brightness of the PNLF cutoff dim by more than a small percentage.<ref name="Ciardullo2004" />
