These points are candidate PNe, however, there are three other types of objects that would also exhibit such an emission line that must be filtered out: HII regions, supernova remnants, and Lyα galaxies.
The first is well known and can be taken from sources such as reddening maps computed from H I measurements and galaxy counts or from IRAS and DIRBE satellite experiments.
Observational data and models support that this holds true for other galaxies, that the bright edge of the PNLF is primarily due to PNe in front of the dust layer.
Consequently, a reduction in oxygen density only lowers the emergent [O III] λ5007 emission line intensity by approximately the square root of the difference in abundance.
Only in extremely metal-poor PNe does the brightness of the PNLF cutoff dim by more than a small percentage.
[3] The relative independence of the PNLF cutoff with respect to population age is harder to understand.