  Injection of electromagnetic energy - photons, electrons, or positrons - into
the plasma of the early universe can destroy light elements created by
primordial Big Bang Nucleosynthesis (BBN). The success of BBN at predicting
primordial abundances has thus been used to impose stringent constraints on
decay or annihilation processes with primary energies near or above the
electroweak scale. In this work we investigate the constraints from BBN on
electromagnetic decays that inject lower energies, between 1-100 MeV. We
compute the electromagnetic cascade from such injections and we show that it
can deviate significantly from the universal spectrum commonly used in BBN
calculations. For electron injection below 100 MeV, we find that the final
state radiation of photons can have a significant impact on the resulting
spectrum relevant for BBN. We also apply our results on electromagnetic
cascades to investigate the limits from BBN on light electromagnetic decays
prior to recombination, and we compare them to other bounds on such decays.
