These operate somewhere between 120 and 180 °C, roughly speaking. Water boils at 100 °C. If a hot curling iron is put on to wet hair, it boils the water inside the hair. The boiling water softens the keratin of the cortex; then the steam from the boiling water expands and forms tiny bubbles inside the hair. Eventually the hair breaks off, either at or somewhere near a bubble.
    Every woman who has used curling irons knows that they work better on some days than others. One day she may have them just a little hotter than usual: that might be just enough to cause bubble hair, and for the affected hair to break off.
    Although the sufferer usually claims that her hair was perfectly normal until she changed the hair product she was using, further enquiry always uncovers a history of increased or excessive cosmetic treatments. Usually bubble hair happens to people who are doing a lot of styling to their hair at home. And it is always, always associated with the use of heated appliances on damp hair.

Hair spray accumulation

This photograph was - quite wrongly - diagnosed as head lice!

An electronmicrograph of a droplet of hair spray

    In fact it shows a droplet of hair spray which has dried on a single hair forming a perfectly smooth cylinder around it. It has not touched another hair at all.

A spot weld between two hairs produced by a droplet of a modern hair spray

    In this photo you can see a perfect replica of the cuticle of the second hair, here.

This kind of replica may be seen in heavily sprayed hair which is not washed often enough

    What has happened here was that the lady concerned was washing her hair about once a week, and applying hair spray frequently and generously between shampoos. By the end of each week, significant amounts of hair spray had built up on her hair. Fortunately, modern shampoo technology is excellent at removing even a week's accumulation of hair spray, grease and dirt.