French term (edited): distance des effets l�taux/irr�versibles (DEL et DEI)

I think you should translate them and give the abbreviation in brackets, like you already did. Whenever it comes to terms that might not have a perfect equivalent in the target language, it's better to give the full translation for them. At least, that's my opinion. I do have one question, though: does "distance" refer to a future moment when such effects should occur? Because then, maybe, a better translation would be "apparition" or "manifestation des effets letaux/irreversibles".

French term (edited): distance des effets l�taux/irr�versibles (DEL et DEI)

I take it that your text has something to do with the Seveso directive. I've scoured the web, but I haven't managed to find an "official" English term. You'll come across the term in the link below, but it's not a "native English" text. Even so, endpoint or radius would work (IMHO) according to the meaning of the French, which indicates the distances from the source point (such as a chemical spill) at which lethal or irreversible effects would be caused.
You could therefore use the abbreviations LEE & IEE or LER & IER. Either way, it's better than the unfortunate DIE !!