Please Please if you ever dye your hair, using L'oreal or any dye please do a skin test everytime you dye your hair but leave it 3-4 days before dying your hair.
I started getting allergic reaction to hair dyes 3 years ago, I've always done skin tests and have found the cheaper dyes (aldi/poundland) are the only ones I can use as they seem to use less of the PPD's. Even they now are starting to give me reactions. 
3 weeks ago I used a Wilkinsons own brand( cheap and have used before) and did a skin test. 48 hours afterwards I had no reaction, so dyed my hair, 12-24 hours after I used the dyeon my hair, my skin test flared up.
My scalp/ears/neck flared up 4 days after I applied the dye to my hair, this was despite washing my hair daily and using antihistamines. Luckily with the cheap dyes I 'only' get blisters and irritation on my scalp.
3 weeks later my scalp is only just clear of itchy weepy blisters and the skin on the backs of my ears have stopped peeling even though they are still itchy.
I honestly think if I used a more expensive brand I too would end up hospitalised.
The most important thing is leaving the skin test longer -all the women have done the skin tests and had no reaction so dyed their hair, and have gone on to either lose their lives or be extremely ill.
Please leave the skin tests longer before applying the dye