Found this on YouTube.

Repairing a minifig helmet with solvent (solvent welding).

Yeah, definitely don't leave an open container of Dichloromethane or Methyl Ethyl Ketone in an enclosed space unless you like extreme migraines and nausea lasting several hours, and permanent optical nerve damage. Certainly don't use it without gloves, because it's readily absorbed by the skin, can dissolve subcutaneous fat, and in large enough amounts will metabolise into Carbon Monoxide.

It's safer (and cheaper) to just buy a new helmet.

^This. I work in a lab where we use a lot of solvents, and most people who haven't had to handle the stuff in large quantities don't realize how even a little of the stuff can be harmful depending on the nature of your exposure.