Immune to Purple

In a futuristic time, a zombie apocalypse occurred. Once the zombies were killed,the poison inside the zombies got released into the air, making the sky turn purple and leaving the non infected people unable to breathe in the outside air. The remaining survivors were gathered in a building that provides artificial air. fifty years later, scientist saw that they are running out of time and needs to find a way to make the outside air breathable. After a couple of years and tests they saw that some teenagers may be immune to the outside air. They then decide to make tests on all teenagers who are seventeen and see who can go outside and look for a cure. They need to pass three test in order to go out side: The first test is biological, the second is physical and the third is emotional.
This story is incomplete but feel free to comment ideas. Chapter updated and sometimes edited on Fridays or Saturdays evenings.