Colour-treated and bleached hair: safer dyeing and aftercare

Before you dye: test first

The NHS says hair dye reactions are fairly common, usually mild, and can take up to 72 hours to appear. Many permanent and some semi-permanent dyes contain PPD (paraphenylenediamine), which can irritate skin or cause allergy, and you may be more at risk if you've ever had a black henna tattoo.

The NHS advice:

• Test the dye on a strand of hair at least 48 hours before you want to use it, following the packet instructions.
• Buy a patch test kit to use at home if you think you might be allergic.
• Don't use the dye if you feel unwell or react after a patch or strand test.
• Wear gloves, and don't leave dye on longer than the instructions say.
• If you've reacted to a permanent dye before, choose a non-permanent one.

Why bleached hair behaves differently

Bleaching works by oxidising the hair, and it makes the fibre much more porous — lab studies show the number of pores rises sharply within minutes. More porous hair soaks up water and product quickly, tangles more and breaks more easily; a hair-science review describes chemically treated hair as "highly sensitive and prone to breakage".

The American Academy of Dermatology (AAD) suggests:

• Stay within three shades of your natural colour — going lighter than that needs stronger peroxide and causes more damage. Going darker is generally kinder than going lighter.
• One chemical service at a time. If you want a perm or relaxer as well as colour, do that first and wait two weeks before colouring.
• Stretch the time between touch-ups where you can.

Aftercare

• Condition every wash, with a richer mask now and then — damaged hair takes up more conditioning protein (see proteins).
• Go easy on heat (safe heat styling).
• Protect hair from the sun — the AAD says sun makes coloured, bleached or permed hair weaker, drier and more faded.
• Clarifying shampoos can fade colour faster; use them sparingly.
• Swimming: chlorine can lighten and roughen hair, and copper in pool water can tint blonde or damaged hair green — see swimming.