Nobody is keen on having patch tests – they require an extra trip to the hairdresser, and more often than not, there’s no discernible reaction. But it’s worth remembering that that’s the ideal case scenario, and that should a severe reaction occur, the difference between having applied the dye to a whole head versus a small patch of skin can quite literally be lifesaving.

In general, the best place to apply a patch test is behind the ear, swiping the formula that’s going to be used on the hair directly onto the skin. You should do this if you’re colouring your hair in a salon or at home, and repeat the process every time you’re using a different formula.

Once on, if your skin becomes dry, red, or itchy, you may be differing from contact dermatitis as a result of colour. If this is the case, ask to try a different kind of dye as while this often isn’t life-threatening, it is uncomfortable.

A stronger reaction can include anything from hives to swelling of the face and airways. If you suspect you’re having an adverse response to your patch test, it’s best to seek medical advice immediately.

In terms of the chemicals more likely to be responsible for having an allergic reaction after a patch test, para-phenylenediamine, aka PPD, is quite widely known to be the cause of some reactions, despite it being a safe addition to dyes for the majority of people.

If you know that PPD provokes a response in you, don’t feel the need to eschew colouring your hair altogether – there are plenty of synthetic and natural alternatives available, including semi-permanent and lead-based dyes, as well as henna.

If you’re unsure of what’s causing your reaction, head to an allergy clinic where they can narrow it down, and then remember the all-important patch test when selecting another way to colour your hair.