In a future not too near but not too far, pollution has gotten to an all time high.
Oil spills and the over-use of chemicals in mainland areas have gotten too far out of control to be contained. The air, now filled with toxins, has become more dense and polluted... making it harder to breathe and take in any clean air.
Many become sick and die, however, a large number of people living in populated urban cities are not severely effected by the changes in air. This is due to having already lived in areas with heavy air pollution and having fairly healthy immune systems. This becomes a nation wide phenomenon and in order to protect those who are not in any of these severely effected areas, the major populated cities are quarantined and restricted from leaving the city limits.
As a result of being stuck in the quarantined cities/ zones, those who are still alive begin to adapt even more to the environments. Some mutations are:
1) Skin becomes stained/ dirty. Also thickens to prevent more toxins from entering the body through pores.
2) Blood turns a blackish color.
3) Lungs enlarge and become sturdier in order to breathe and take in less toxins from the environment.
4) Limbs are stronger and grow longer in order to climb, dig, run, jump to scavenge/ find food.