[trebleshot]

Mostly it will just fade out the pigment in the dye used to color the plastic, making them look dull and pale. But it happens over years of exposure, so the less they are in contact with direct sunlight, the better. Soft-light bulbs are best for display lighting (and not shining them directly on figures helps too).

Smoke of any kind (cigarettes, fireplaces, etc) will basically stain the figure. There are some remedies to fix it, but it's rather laborious to do that and it's just easier to do preventative maintenance then attempt to restore them down the road.