The aim is to achieve a state known as equilibrium to produce a so-called equilibrium modified atmosphere, or EMA. Here, oxygen and carbon dioxide can pass between the inside and outside of the package in such a way that as oxygen is consumed within the pack it is replaced by oxygen from outside, and similarly a constant level of carbon dioxide is maintained. Another advantage of a modified atmosphere is that it reduces the natural production by the fruit or vegetable of a gas called ethylene, which speeds up the ripening process.