if you're getting 120db because of the combine total signal going into your ears from the noise canceling process, you're a fucking idiot. noise canceling works by equalizing the wave to zero, or as close to it as possible. you're actually keeping your ears healthier by using this because you dont have to turn up your mp3 player to compensate for ambient sounds already taken out by noise canceling.
this reminds me of the plane on a treadmill question. idiot.
source:
HowStuffWorks "How Noise-canceling Headphones Work"
page 4 specifically.