According to one artist, the answer is no.
Sally Davies has kept a Happy Meal in her home for 6 months, with no noticeable change other than "it's hard as a rock." You can compare the photos yourself at the link.
McDonald's accuses Ms. Davies of fabricating the whole thing. Well, I don't know ... have you seen this video of the Bionic Burger? And the museum of ancient McD "food"?