Fenthion is a commonly used insecticide because its harmful effects on humans and other mammals is minimal. Fenthion has a low bioavailability, meaning that it is not absorbed well when ingested, most cases of fenthion poisoning are caused by dermal exposure. It would take the ingestion of a very large amount of fenthion to cause a fatal case of poisoning, it's unlikely enough could be introduced into a carton of milk to cause death, not without the majority of the volume of liquid in the carton being fenthion and the victim drinking the entire carton which would be very unlikely as organophosphates have an extremely unpleasant odor and taste.