I’m standing in Superdrug, holding two bars of soap and trying to decide which one my fragrance-averse husband might prefer. Taking out my phone, I hover the camera over the barcode on each. Ah, just as I’d hoped! One is rated “Poor” with a score of 42/100 and an orange dot, while the other is “Excellent” with a green dot and a score of 93/100. No prizes for guessing which goes back on the shelf.

I’m not alone in using Yuka to make up my mind in the cosmetics aisle. The app, founded in 2016 by the French brothers Benoît and François Martin and their friend Julie Chapon, might be best known for rating supermarket food, but 100 million beauty products are now scanned by its users each month. For many its traffic-light system has become gospel, dictating which brands they buy and which they abandon. Social media is full of videos of people rating their bathroom shelves via Yuka — and similar apps like Think Dirty and Inci Beauty — with titles like “Products I thought were clean”.

The business coach Tara Keyar Knowles, 34, from Bromley, will “only use Nala’s Baby products now” after getting endless red dots for her former favourites. Meanwhile, the Bridgerton actress Genevieve Chenneour, 27, tells me, “I don’t buy anything that isn’t ‘Excellent’ on the green scale. For work I can’t decide what goes on my skin, so in my private life I want to keep it simple and clean.”

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Yuka’s scoring system for cosmetics is straightforward: products get a coloured dot and a score based on the level of the highest-risk ingredient rather than on its effectiveness or how it’s used. “Bad” products (scoring 24/100 or less) receive a red dot, “Poor” (25 to 49/100) get orange, “Good” (50 to 74/100) are given yellow, and “Excellent” (75+) get green. Meanwhile, for the ingredients, a red dot signals “Hazardous”, orange “Moderate risk”, yellow “Low risk” and green “Risk free”.

“Each ingredient is assigned a risk level depending on its potential effects on health or the environment, such as endocrine disruption, carcinogenicity, allergenicity, irritation or pollution,” Chapon explains.

The problem? Many don’t scroll beyond the dot (guilty). “I’m very uninformed about the ingredients — I only look at the rating,” my friend Catherine, 42, admits. “It has led to me throwing out 90 per cent of my cosmetics.”

And while it’s easy to label entire brands as “Bad”, there is often little consistency. A leading bodycare lotion is “Poor” (11/100) but its body washes are “Excellent”. Another lotion from a household brand gets a dreaded 0/100 for containing a paraben preservative — though Yuka adds, as it often does, that “more studies are required”.

So, should we listen? After all, being told something contains a potentially “toxic” ingredient doesn’t scream “buy me”.

• Help! I’m addicted to Yuka, the app that rates all my food

“I understand the appeal of Yuka,” says the consultant dermatologist Dr Anjali Mahto, reassuring me about my soap purchase. “We all want simple answers: is something good or bad? The problem is that it flattens nuanced science into a traffic light system. What matters is not simply the presence of an ingredient but the dose, frequency of use and formulation. Many ‘red flag’ ingredients such as those labelled carcinogenic are only harmful in very high concentrations, which are strictly limited under UK regulation, or in animal studies that do not translate to human use. It can make safe, well-regulated products look alarming.”

Indeed, when I scan a bestselling hydrating cleanser, it pings up as “Poor” for containing phenoxyethanol — a preservative deemed safe in the UK and EU in concentrations up to 1 per cent. Except Yuka warns that “repeated exposure” may cause blood, liver and fertility issues. No wonder many shoppers simply choose something else.

Rani Ghosh, a registered toxicologist, cautions against treating Yuka “like a bible”. “A red dot doesn’t mean danger. It means the app’s algorithm flagged an ingredient,” she says. “It’s oversimplified and does not replicate the expert evaluations of toxicologists. Safety margins are built in to keep exposure limits well below harmful levels. This fear-based messaging can create unnecessary anxiety and erode trust in regulation.”

That lack of nuance, the experts say, can lead to product categories being demonised — such as sunscreen, despite chemical SPFs being safe (not to mention essential) when regulated. “Parabens are another example,” Mahto says. “We need preservatives in skincare to prevent it from giving us infections.”

She thinks the app is biased towards “natural” ingredients. “But those can be just as harmful to our skin, while a lot of synthetic ingredients have been around for decades and are safety tested,” she says. “Plants don’t always mean better.”

Chapon says that Yuka’s ingredients list is “updated continuously by our toxicologist” and that its ratings are based on authoritative sources such as the SCCS (European Scientific Committee on Consumer Safety). “The score is not gospel but a scientific tool to guide choices,” she adds. “Ultimately it’s up to consumers to decide how strict they want to be.”

One definite side-effect of Yuka mania is the smug factor. “There’s moralising and unspoken peer pressure, especially when your entire group of friends is using it but you’re not,” Mahto says. “It’s like, ‘Why don’t you care about what you put on your body?’ ” Or as my pal Catherine puts it, “I feel like a much better human now I know that I’m only using excellently rated products.”

As for the brands? Most of the cosmetics conglomerates I contacted to ask about the impact Yuka is having on their businesses did not respond. But there can be little doubt that Yuka is reshaping the industry. Chapon says many brands are reformulating products to meet its “green dot” standard, using Yuka’s specially created simulation tool to test scores prelaunch.

Still, Ghosh warns that by “using simplistic criteria to guide reformulation, products may lose efficacy, stability or even consumer safety if key ingredients are removed or replaced to chase a score, rather than being based on rigorous toxicology”.

The Cosmetic, Toiletry and Perfumery Association agrees. “Apps that claim to inform consumers about so-called harmful ingredients are misleading,” it says. “UK cosmetics law requires all cosmetic products to be safe and specifies strict processes that companies must follow.” It directs consumers to the Cosmile Europe database, saying that it offers “unbiased, scientific information” and cuts through “oversimplified scoring systems”.

Ultimately, Mahto says, “skincare comes down to personal preference. If you prefer botanical ingredients then use them, but don’t assume they’re morally better. Buy based on what suits your skin type and budget. Not because of a dot.”