I had a translucent container of propylene glycol exposed to sunlight. When I went to use it, I noticed it has a strong "acetone" like odor. Someone said it was a photo-oxidation reaction of the glycol into ketones or aldehydes. What happened and will the odor get worse even though it is is now removed from sunlight? Do I need to worry about these ketones or aldehydes in the glycol?
The thermal degradation of propylene glycol and its uses in e-cigarettes has been a discussed in a previous question here on Chem.SE.
Upon complete oxidation and degradation, propylene glycol will fully decompose to carbon dioxide and oxygen. However, if there is partial oxidation, you will get some intermediate species - formaldehyde or acetaldehyde.(1st answer)
Propylene glycol does not decompose on heating, it simply boils. In theory, under certain conditions (pressure/encapsulation/catalysts) it could dehydrate to either acetone or Propionaldehyde, probably the former.(2nd answer)
Also, this link shows the decomposition products of propylene glycol which are all acid based:
The main degradation product of propylene glycol was lactic acid (2-hydroxypropionic acid), with formic, acetic, and oxalic acids as minor products.
So, mainly degradation of propylene glycol in sunlight would yield a bunch of acids and small amounts of acetone. Worrying about the odors is a personal medical question and is off-topic in Chem.SE.