Flat Irons: Wet or Dry? It Doesn't Matter!
Without a question, styling your hair causes damage. In studies evaluating the different methods of styling hair, the only one that doesn't hurt hair is just letting it air dry (Sources: Journal of Cosmetic Dermatology, July 2003, page 180; and Global Cosmetic Industry, January 2002, pages 30–32).
Unfortunately, the only way to get your hair to do what you want it to do (instead of what it wants to do naturally) is with styling tools that produce high heat. Flat irons, in particular, deliver intense heat directly to the hair shaft. One way to prevent damage when using a flat iron on your hair is to always be sure the hair is 100% dry before you start ironing it, as I've often advised. Otherwise, any excess moisture left in the hair could literally boil. Water boils at 212°F, and most flat irons get far hotter than that—over 350°F is typical. What happens when that kind of heat comes in direct contact with the hair, especially if you leave it in one place for too long, is that the water inside the hair shaft boils and turns to steam, which causes the hair to break or rupture at every boiling point. But all that has changed with a new generation of flat irons that can be used on hair whether it is wet or dry. A special venting system built into the plates of the flat iron allows the steam to escape instead of remaining in the hair shaft, thus eliminating the damage and creating straight smooth hair as the hair dries.
We tested two different flat irons with a TV station here in my hometown of Seattle (KOMO-TV, Northwest Afternoon). We compared the Remington Wet 2 Straight ($29.99) with the T3 Tourmaline Wet-to-Dry Flat Iron ($150). Both of them worked great for drying and smoothing the hair perfectly straight. The Remington came out ahead for making the hair feel smoother and working slightly faster! Either way, this new way to straighten hair is definitely a styling advantage, and you don't have to spend a lot of money to get superlative results.
You will find that these wet/dry flat irons work best on damp hair, as opposed to hair that is soaking wet. Dripping wet hair can take a long time to dry and smooth. Removing some of the moisture first with a blow dryer will make things go a lot faster. Wet/dry flat irons are also great for touch-ups on mornings when you aren't going to wash your hair. A stubborn area can be sprayed with water and easily and quickly restyled. As a reminder, all high-heat styling tools work better if you use light tension—that is, don't pull too hard. For best results, use minimal tension while keeping the implement moving over the hair instead of resting it in one spot for too long
Source: Paula Begoun