Most, if not all, red LEDs also emit small slivers of blue and/or green light. You can't see them with the naked eye as they are overwhelmed by the safe red light. But if you go into a completely dark room and carefully observe the red light as reflected from the business side of a common CD, you might be surprised. The CD acts as an inexpensive prism, breaking the light up into its separate wavelengths.
Because the slivers are small, if your normal working procedures only expose the paper to the safe light for a short period of time, you might be all right. But if, like me, you often find yourself in a complex and lengthy dodging and burning session, you might begin to experience subtle fogging. I did using 635nm red LEDs.
The solution is to filter the red LEDs with a single layer of Rubylith.