Greetings Dave:
Browning of foliage due to age and lack of sunlight causes interior needles to brown and fall to the ground--this of course, is natural. However, if needles toward the tip of the branches are browning and falling off, then you may have an issue which may warrant closer examination.
Feel free to email me (jhayek@illinois.edu) some high-resolution digital photos of the affected tree and foliage. In your instance, you can also take some photos and branch samples to the Morton Arboretum Plant Clinic in Lisle, IL.