If you're getting your hair color done at a new spot, and your colorist brings over a space-age looking heater, it could be totally okay—or it could be a recipe for disaster. Sitting under the dryer while coloring your hair is a step you may be used to, but it's actually not a necessity.
Stylists are incentivized to get clients in and out of the salon quickly because the shorter their appointment times, the more people they can see and the more money they make. While there are some healthy uses for heat during the lightening process, it's important to know whether you're a good candidate because there's a chance that the heat and desire for speed could be directly interfering with your desire for healthy hair.
Use a strengthening shampoo and conditioner to keep your colored hair healthy in between appointments.
Before we get into the "why's" and "how's" of heating color-treated hair, it's important to explain some background info on how hair is processed. When highlighting hair or going a lighter color, a colorist is going to use bleach. Normally, it's a powdered bleach mixed with 10-, 20-, 30- or 40-volume peroxide. The higher the number of the peroxide, the stronger it is.
A mixed bowl of bleach has a life of a couple of hours or more, although it may need a little refreshing. If bleach is applied well, it will stay moist up to one hour and will lift the entire time. Heat is not necessary for the bleach to do its job.
How Heat Affects the Bleaching Process
If heat isn't a necessity for lightening hair, why would do colorists use it? The short answer: time. All the heat does is speed up the lightening process, but it can cause some serious damage along the way by lifting moisture. You're at particular risk if your color isn't appearing light enough—it's easy for a colorist using a little heat to use too much. If that happens, all of a sudden you'll have a sea of "flyaways" at the top of the head after your hair is blown dry.
When Applying Heat Is Okay
In general, expert colorists say that the only time to use heat during processing is on healthy, virgin, black hair, as it's hard to lift. On the other hand, thin hair should never be put under heat, nor should hair that's consistently chemically treated. Anyone who can still see any effects of having hair bleached, permed, or chemically straightened should avoid going under heat.
If your colorist offers you the dryer while your hair is processing, you can request to skip it. If they insist, ask why they believe you need the heat, how long they intend to have you under, and how much breakage you should expect. Sometimes there's a good reason, like if you've had balayage and after 30–45 minutes the bleach is drying out in which case, five minutes or so of gentle heat is recommended. If you still don't think it's best for you, you can still decline. If it's between faster processing time and healthier processing, you can choose the healthier route.
Jeong MS, Lee CM, Jeong WJ, Kim SJ, Lee KY. Significant damage of the skin and hair following hair bleaching. J Dermatol. 2010;37(10):882-887. doi:10.1111/j.1346-8138.2010.00916.x