  As electromagnetic showers may alter the abundance of Helium, Lithium, and
Deuterium, we can place severe constraints on the lifetime and amount of
electromagnetic energy injected by long-lived particles. Considering up-to-date
measurements of the light element abundances that point to $Y_p=0.245\pm
0.003$,$({\rm D/H})= (2.527\pm 0.03)\times 10^{-5}$,$(^7{\rm Li/H})=1.58
^{+0.35}_{-0.28} \times 10^{-10}$, $(^6{\rm Li/^7 Li})=0.05$, and the
baryon-to-photon ratio obtained from the Cosmic Microwave Background data,
$\eta=6.104 \times 10^{-10}$, we derive upper limits on the fraction of
electromagnetic energy produced by long-lived particles. Our findings apply to
decaying dark matter models, long-lived gravitinos, and other non-thermal
processes that occurred in the early universe between $10^2-10^{10}$ seconds.
