Photodegradable products react differently depending on many factors. The intensity of the UV radiation affects the rate of photodegradation. That means shade, cloud cover, and geographic location play a role in how much UV reaches that diaper your friend just flung out the car window.
The reality, though, is that most plastics won't see much time sunning on the beach. When a plastic product's use is complete, it usually winds up in a landfill or scattered in our environment. Only about 8 percent of plastics are recycled [source: EPA]. So when you consider that the United States alone creates more than 30 million tons of plastics each year, location matters, especially in terms of photodegradation.
In some cases, these light-sensitive plastics will break down in a landfill, but only if they receive a good dose of UV before they're plowed under. Other photodegradable plastics remain totally intact for untold years once they hit your local landfill. What's more, even those that totally disintegrate in the wild are often consumed by other organisms and as a result, can wiggle their way into our own food chain. No one can say for sure that the effects of plastics consumption are totally benign.
Currently, photodegradable kinds of plastics are really useful in situations involving littering -- intentional or otherwise. So for grocery sacks that blow into Grand Teton national park, at least there's hope that the sun will eventually take its toll.
In the end, this kind of degradable plastic is just one type of solution to a big problem: How do we humans deal with all of the plastic garbage we create? Right now, reusing and recycling are our best options, but in when plastics do go on jailbreaks and escape into the world, it's good to know that degradable plastics might not last as many generations as older types of polymer materials.