One woman told the BBC her reaction to a hair dye was "severe" after she suffered from COVID, with her hairdresser saying other clients had also reported rashes and burns.
The NHS says that some people are prone to a skin reaction called contact dermatitis, which leaves their skin red, dry and inflamed when they come into contact with certain chemicals.
Many hair dyes contain a chemical called paraphenylenediamine (PPD), which is a known irritant and allergen, and is the main culprit for allergic reactions.