Chernobyl Diaries

Chris (Jesse McCartney), his girlfriend Natalie (Olivia Taylor Dudley), and their mutual friend Amanda (Devin Kelley), are traveling across Europe. They stop in Kiev, Ukraine, to visit Chris's brother, Paul (Jonathan Sadowski), before heading on to Moscow, Russia, where Chris intends to propose to Natalie.
Paul suggests they go for an "extreme tour" of Pripyat, an abandoned town which sits in the shadow of the Chernobyl Nuclear Power Plant. Chris is against going on the tour because of the potential danger and would rather stay on the original plan of going to Moscow. They meet tour guide Yuri (Dimitri Diatchenko), and are joined by a backpacking couple, Norwegian Zoe (Ingrid BolsÃ¸ Berdal) and Australian Michael (Nathan Phillips). Yuri drives them through the wilds of Ukraine, before they arrive at a Chernobyl Exclusion Zone checkpoint, where they are refused entry by the Ukrainian military. He then takes them to an alternate entry he discovered years ago, towards the abandoned city.
The group stops at a river where Yuri points out a large, mutated fish apparently able to live on land. After arriving in the town, the group is worried about radiation poisoning, but Yuri assures their safety with a Geiger counter. After spending a few hours exploring, Yuri takes them to the upper floor of an apartment building and shows them the Chernobyl nuclear plant on the near horizon. After hearing noises at the other end of the apartment, it is found to be a bear which runs through the hallway past them, but not harming them.
The group prepares to leave Pripyat, and Yuri finds the wires in his van have been chewed through and destroyed. He tries to radio for help, to no avail. As night falls, the group decides on whether to hike to a nearby checkpoint which is 20 km away or to stay put and wait for help. Suddenly, strange noises come from outside, where Yuri and Chris go out to investigate. Shots are heard and Paul runs out, only to return with Chris whose leg has been severely mauled, and claims that Yuri has been taken.
The next day, Paul, Michael, and Amanda go out to look for Yuri. They follow a trail of blood to an abandoned cafeteria and find Yuri's mutilated body. They take his gun and are chased through the building before returning to the van. Amanda checks her camera and one of the pictures shows a humanoid creature inside one of the apartment buildings. Natalie stays with the wounded Chris while the others begin the hike to the checkpoint.
During the hike, Paul, Amanda, Michael and Zoe find a parking lot, where they find parts for the van. On the way back they are chased by dogs and also attacked by mutant fish in a stream. Night falls as the group returns to the van, only to find it ripped to shreds. They find Natalie's video camera, showing that she and Chris were taken and captured by humanoid mutants. While searching for the two inside an old building, the group is chased by more mutants.
During their escape, a traumatized Natalie is found and rescued, but when the group gets distracted by a mysterious young girl, Natalie is captured again. The rest of the group is swarmed by a horde of mutants and are forced to retreat. While fleeing through an underground passage, Michael is captured. As they continue, they find Chris' engagement ring for Natalie, with no sign of Chris. While climbing a ladder, a gang of mutants drag Zoe back down, forcing Amanda and Paul to leave her behind, emerging from the passage right beside the exposed reactor core. Paul recognizes that extremely high radiation levels are causing their skin to blister. They come upon Natalie's body before emerging outside, where they are confronted by Ukrainian military personnel. Blinded by radiation damage, Paul stumbles toward the soldiers, who fatally shoot him.
Amanda falls unconscious, and later awakens on a gurney. Several doctors, in protective hazmat suits, inform her that she is in a hospital and they will help her. The doctors reveal that the "creatures" were escaped patients, and after realizing that Amanda "knows too much," she is then forced into a dark cell and is swarmed by the recaptured patients, as the doctor closes the door's viewing shutter.

Question: What causes Paul's blindness?
Answer: radiation