The companies we carry (with the exception of a few "high fashion" brands) specialize in making medical grade hair for women recovering from not only hair loss but illness as well. They take into careful consideration any sensitivities or immunity issues their clients may have along with providing high-end cap features for comfort. Medical wigs tend to have comfort features and high-end finishes. These include felt, silicone, or polyurethane in the caps to aid with grip and comfort when worn against a bare scalp. If they have lace, the lace is precut.
All of the non-heat-friendly synthetic hair in our store is virtually the same, with only minor variation. It is ALL kanekalon modacrylic (petroleum-based) fiber that has been treated with silicates and other chemical agents for static and flame-retardancy. Each company has its own proprietary blend of what they put into the hair and they keep it a trademarked, patented, tight-lipped secret. Our manufacturers do not provide us with detailed information on what is used to process their synthetic fibers.
The fashion wigs are not made of any product or processed with any chemical that would be considered dangerous to one's health. While we have not been provided with a full list of what each manufacturer puts into the wigs, we do know that these items are not harmful and we have never received any complaints from anyone saying they even had so much as an allergic reaction while wearing a fashion wig.
This is not to say that it can't happen because everyone's skin is different and reacts to wigs in a different way. This is rare since we have not had any complaints about it, but there are cases where people have had reactions when wearing wigs. All wigs, even medical wigs, are processed with some form of chemicals to help style them and maintain their shelf life. I'm not saying that your skin will react to these chemicals since they are non-toxic (they have to be worn by the public anyway), but if your skin is sensitive you may want to be sure you wear a wig cap with them.
I generally only wear fashion wigs and I keep my hair very short so the wigs are close to my scalp, and I have never had any reactions or issues wearing them. I have very sensitive skin also so I have to be careful with what I wear most of the time, even Bath and Body Works irritates me, but I have never had any issues with the fashion wigs.
If you still have a gut feeling telling you to stay away from the fashion wigs, I always say follow your gut, but I do not foresee you having any issues with the fashion wigs. Rest assured that the wig will bring no harm to your health and safety.
For a more in-depth response on a certain manufacturer's proprietary fiber treatment blend, I recommend reaching out to them directly.