Of all the things you can be allergic to, add hair dye to the list (if it wasn't already there). Most hair dye products include a chemical substance known as paraphenylenediamine, which can result in numerous adverse effects such as skin inflammation to the eyelids and swelling of the face.
Imgur user thechosensinger recently documented an episode when her close friend had an allergic reaction to hair dye.
Here is a photo of her friend before the allergic reaction.
And here is a photo taken immediately after the allergic reaction.
As soon as her friend started having the reaction, she went to the emergency room.
The next day, she was still recovering.
The day after that, the swelling started to go down.
Five days later, her friend recovered and she is doing fine.
Her harrowing experience serves as a warning to watch out for hair dye. A way to find out if you have an allergy is to test a tiny amount of dye on your hands before applying it to your head and wait for a while to see if you experience a reaction. Or, if in doubt, consult a doctor.