In EPA-speak, it contains substances which are known to cause or contribute to smog,
in presence of sunlight, Oxygen, and Nitrogen (oxides, I believe).
(At least, that's the context one always finds used when making this reference)

Literally, it appears to mean, "undergoes chemical change(s) when exposed to light".


Edit in response to seeing the above post... yeah, that makes more sense, come to think of it.
I know sunlight sure helps some things cure quicker