University of Utah Hospital officials say 23-year-old Derrick E. Martinez of Garden City, Kan., died Wednesday.
Martinez was found unconscious in a mobile testing trailer at the plant on Saturday.
Martinez was taken to a Pocatello hospital and then flow to the Utah hospital.
Exposure to high concentrations of ammonia in the air causes burning of the nose, throat and respiratory tract.
Martinez worked for a Simplot subcontractor, Blick's Phosphate Conversion. Blick's spokesman Kenny Kalb says the cause of the man's exposure is still being investigated.
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