A foil balloon is made of a special type of film. This film limits how quickly the helium atoms or air molecules can escape from the balloon. How permeable the film is determines on how quickly air or helium escapes from the balloon. If you used a powerful microscope, you could notice small holes in the film where the helium escapes. Helium atoms are smaller than the air molecules. Air cannot fit through the holes in a balloon while helium can.