If i have never had a reaction to home hair dye should i worry about one ? can i suddenly become allergic?

Hair Dye: The culprit is a common chemical ingredient in permanent hair dyes, called para-phenylenediamine, or ppd. Ppd is found in more than two-thirds of commercial dyes, the researchers say, including many of the top-selling brands. Patients with severe PPD reactions commonly develop painful rashes around the hair line or on the face, which often require treatment and can lead to hospitalization.

Yes: You can indeed become suddenly allergic but the incidence is not that high. So if it does happen, you will need to avoid it altogether. Black dye is the most common offender.