In researching hair extensions and the damage they can do to your scalp, HowStuffWorks.com contacted several trichologists -- physicians who specialize in treating maladies of the hair and scalp. It's worth noting that the general consensus among the trichologists we corresponded with is that, when done properly, hair extensions pose little or no threat to your scalp. It's when they're put in haphazardly that problems can arise.
One such issue, the aforementioned traction alopecia, may be the most common problem with hair extensions. When hair extensions are put in, they're often woven into the real hair. If the weave is too tight, hair extensions will tug at the real hair they're attached to as they're removed, resulting in loss of the real hair. This tugging of the scalp can also lead to inflammation at the site, since the hair has literally been pulled out by the extension [source: Kingsley].
The attached nature of most types of hair extensions can also lead to inflammation and irritation of the scalp when the hair is brushed or combed, since the extension can be caught in the tines. This also leads to tugging of the scalp. Because there aren't many ways around this, extensions should only be used for the short term.
Some types of systems use heat to apply the hair extensions, a method known as heat sealing. In this method, a solid solution used to bond the extension to the hair at the root is liquefied using a heated clamp. When the solution cools, the hair and extension are fused together. Heat sealing requires the use of a shield that protects the scalp from burning; this method is safe in the hands of a trained professional, but there's always the potential of burning the scalp whenever a hot instrument is used near it.
The solution used in heat sealing and the glue used in other ways to bond extensions to real hair can cause an allergic reaction on the scalps of some people. This reaction is called contact dermatitis; the symptoms include an uncomfortable, red, itchy rash, akin to one caused by poison ivy [source: Mayo Clinic]. The chemicals used to remove the bonding agents in order to remove the extensions pose the same risk. It's difficult, if not impossible, to tell how your scalp will react until these chemicals are applied.
For these reasons, it's a good idea to use only reputable salons staffed by trained professionals when opting for hair extensions. When it comes to sparing your hair and scalp from pain and damage, it's worth the money.