“Shampoo is definitely not a scam,” says Neil Harvey, chair of the Institute of Trichologists.

He is sceptical of the idea, claimed by the “no poo” movement, that hair can wash itself and that using a product to cleanse it makes it less healthy. In fact, he says that the scalp – with its tens of thousands of follicles and sebaceous glands – needs just as much attention (if not more) than other areas of the body.

“Imagine what would happen if you stopped washing your underarms – you’d be likely to develop a build up of sweat and dirt, which would smell awful,” Harvey says. “A proliferation of bacteria and yeasts would occur, as they thrive in oily environments.”

Why do some claim hair looks shinier after a few weeks off shampoo? “If someone’s using a harsh shampoo and then stops, the natural oils from the sebaceous glands in the scalp will coat the hair more. We want some of that – it protects the hair and gives it shine – but over time, it will build up and become unpleasant.”

Harsh shampoos include those containing higher percentages of ingredients like sodium lauryl sulphate, sodium chloride or perfume. “They can strip hair and cause irritation, whereas those with higher concentrations of natural ingredients like essential oils are gentler,” Harvey says. Other shampoos to avoid include those that promise to make hair “look” and “feel” healthier, rather than actually “be” healthier.

When it comes to how often you should lather up, that depends on scalp type, hair texture and personal preference. “For some people, overly frequently washing may cause hair damage and an itchy, dry scalp,” he says. “For others, not washing enough can make the hair look greasy and lifeless, and create an unhealthy scalp microbiome, the ecosystem of bacteria that protects the scalp and hair.”