Greetings fellow wavies and curlies!
There was a post recently on naturallycurly.com where a fellow board'er mentioned she had been having some burning and irritation on her scalp once she switched to a new product. Although the product was working it seemed as if she might have an allergy or sensitivity to something in the conditioner. Contact Dermatitis is common with cosmetic products including dyes, conditioners, shampoo's, etc.
It came to me then that I haven't seen much on the boards about doing a patch test when you get new product. This lets you know if your skin will be sensitive to it. It won't help if your hair just does not like an ingredient (that can be much harder to figure out but there are common culprits listed below).
The way I do it is I use about a pea to dime size of product and rub it into my inner arm. On the inside of my wrist, above my elbow, basically anywhere that my skin tends to be more sensitive anyway. If I'm worried I do it upper arm only--that way if there is a reaction I can cover it with long sleeves. I am on the PC all day at work so if a product caused a rash on my wrist I would be irritating it all day. Not good! You could also put it at the nape of your neck or behind your ear if you want to be closer to your scalp or even on a small area of scalp. The choice is yours.
I encourage you all to do this at night and then check in the morning. I know allergists sometimes recommend longer. Even though it might delay your instant gratification on the product its worth it in the end if your scalp is going to end up burning or itching or having sores.
Ingredients that may cause an allergic reaction such as contact dermatitis: Citrus (commonly essential oils of organge, lemongrass, etc), papaya (used as a nerve agent in medicines for its ability to encourage sensation in areas deadened), preservatives (too many to mention), cocamidopropyl betaine (shampoos).
Sometimes the patch doesn't show anything and it will still react on your hair but it is an easy way to at least get an idea. Remember that even something you've used before can turn into a sensitivity!
P.S. common ingredients that some peoples hair is sensitive too--can cause it to get hard, dry, limp, lifeless, greasy and more: mineral oil or other petro products, castor oil or other oils (olive, jojoba, etc), protein (hydrolized anything in a product), panthenol, BTMS.