How far to hold a hair dryer from your hair: about 15 cm (6 inches). In the one lab study that measured it, the air just above the hair was 47°C at 15 cm and 95°C at 5 cm, and the closest setting did the most cuticle damage.

We found no professional lightener directions that include shampoo, conditioner or water, and nobody we found has measured what retail shampoo, conditioner or water does to lift. Makers' route to a milder mix is a lower developer and a ratio inside their range.

A perm works in two chemical steps: a lotion breaks some of hair's disulfide bonds while it is wound on rods, then a neutraliser re-forms them in the new shape, though not completely. Here is what manufacturers and lab studies say about how long it lasts, timings, damage and safety.

Crazy Color says to leave it on for around 15-30 minutes after application, or 45-60 minutes for a more intense result. Here is its full method, from patch test to rinse, and what it says about pale shades, dark hair and fading.

Schwarzkopf says IGORA ROYAL Natural shades give up to 100% white coverage on their own, up to level 9; Wella says Koleston Perfect needs a Pure Natural added from 30% grey. Here is each brand's rule for mixing, developer, timing and retouches, what science says about 'resistant' grey, and the safety steps first.

Each maker prints its own mixing ratio and development time. Here is what Schwarzkopf, Wella and L'Oréal say for Igora Royal, Koleston Perfect, Majirel, INOA and Color Touch, including heat, when timing starts, and where the instructions say nothing.

Permanent, demi and semi are trade labels, not legal categories. Here's what Wella and L'Oréal call Koleston Perfect, Illumina Color, INOA, Dia Color and Dia Light, and what that means in the bowl.

We found no published study comparing a finished heat protectant with using nothing at blow-dryer temperatures, but distance from the hair has measured data. Here is what the studies and the manufacturers actually say.