Finally, someone asking the real questions that Marvel's science consultants conveniently ignored! Oxygen molecules have a diameter of about 0.3 nanometers, so Ant-Man shrinking to subatomic size would indeed create a slight breathing problem. But hey, the same movie has him falling through "the quantum realm" while somehow maintaining consciousness, so scientific accuracy clearly took a vacation day. Next they'll tell us his mass stays constant while shrinking, which would turn him into a human black hole. Hollywood physics: where conservation laws are just gentle suggestions!