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Prior to filming, director Alexandre Aja and Gregory Levasseur had already conceived an idea for the mutants' appearance. Aja stated "We based all our descriptions and directions on real documents, pictures, and footage that we found on the effects of nuclear fallout in Chernobyl and Hiroshima"
The gas station set was built on location in Morocco. It was so authentic that occasionally someone not involved in the production would stop to buy gas.
The mutants were created by CGI, physical effects or both. Most of the mutants with full deformations were created by prosthetic makeup. But the deformed children who were playing were digitally distorted using tracking dots on their faces. Ruby was a mixture of the two, using the tracking dots for distortion and prosthetic teeth.
(at around 3 mins) The photos of mutations that play during the opening credits are not of atomic mutation, but birth defects caused by the use of Agent Orange in Vietnam.