Why does Typology not use hydroquinone?

Hydroquinone (CAS No. 123-31-9) is an aromatic compound belonging to the phenol family, also known by the chemical names 1,4-benzenediol, p-dihydroxybenzene or 4-hydroxyphenol. Used for several decades, it was initially employed as a hair dye before becoming established as a reference depigmenting active ingredient in dermatology. It is particularly used to reduce various forms of hyperpigmentation, such as melasma, lentigines or post-inflammatory marks.

The effectiveness of hydroquinone is based on a well-known mechanism of action. This active substance works by inhibiting tyrosinase, an enzyme involved in melanogenesis. More specifically, tyrosinase is responsible for the conversion of tyrosine, an amino acid, into melanin, the pigment that gives skin its colour. By limiting this production, hydroquinone gradually reduces the pigmentation of the treated areas. It can also interfere with oxidative processes within melanocytes, helping to reduce pigment formation.

Hydroquinone is considered one of the most effective active ingredients for lightening pigmentation spots, with results generally visible after a few weeks of use.

However, despite this efficacy, the use of hydroquinone is now strictly regulated. In Europe, it has been banned from cosmetic products intended to lighten the skin since 2001 because of uncertainties regarding its safety. It remains authorised for certain very specific uses, such as in artificial nails, at very low concentrations (≤ 0.02%) and for professional use only. In the United States, hydroquinone is regarded as a medicine when used as a depigmenting agent, and is available only on prescription.

Beyond its depigmenting effectiveness, hydroquinone is associated with several well-documented cutaneous adverse effects, which partly explains the caution surrounding its use. The reactions observed may be acute, meaning they appear rapidly after application, or they may develop more gradually over the course of repeated use. Among the early side effects, the most common are skin irritation, with redness, burning sensations, discomfort and sometimes desquamation (skin peeling). The literature reports a highly variable frequency of these irritative reactions, ranging from 0 to 70% in monotherapy, and reaching 10 to 100% when hydroquinone is used in combination with other active substances, particularly in more intensive depigmenting protocols.

Other acute reactions have also been described, such as contact dermatitis or pigmentary disorders, whether in the form of post-inflammatory hyperpigmentation, resulting from skin sensitisation caused by hydroquinone, or even hypopigmentation in certain areas. In practice, this means that even though hydroquinone is effective on dark spots, its use is not without consequences: it can weaken the skin, particularly when used repeatedly, at high concentrations, or on skin that is already sensitised.

Other adverse effects associated with prolonged use of hydroquinone are also a cause for concern.

Among these, exogenous ochronosis is the best-documented complication. It is a paradoxical hyperpigmentation characterised by a grey-blue discolouration of the skin, sometimes associated with papules or a granular appearance on sun-exposed areas. This condition generally appears after prolonged use of hydroquinone, often over several months or even several years, and seems to be promoted by high concentrations or repeated applications without medical supervision.

Several hypotheses have been put forward to explain its origin. Hydroquinone may disrupt certain enzymatic pathways involved in phenol metabolism, notably by inhibiting homogentisate oxidase, which would promote the accumulation of metabolites similar to those observed in endogenous ochronosis. Moreover, its repeated oxidation in the skin could lead to the formation of brown-black polymerised pigments, which gradually deposit within the dermis. This pigment accumulation would account for the persistent and poorly reversible nature of ochronosis.

Other chronic effects have also been described. Nail discolouration, for example, is thought to be related to the oxidation of hydroquinone into reactive quinones, which are capable of polymerising and binding to nail keratin. This process leads to the appearance of brownish or blackish shades, often progressive, particularly in the event of repeated contact with the product. More rarely, ocular involvement has been reported, mainly in occupational settings with exposure to high concentrations of hydroquinone in the form of vapours or particles. These manifestations include conjunctival melanosis, corresponding to pigment deposits in the eyes, and corneal alterations, possibly linked to local oxidation phenomena and oxidative stress.

Overall, these data highlight that repeated exposure to hydroquinone can lead to lasting pigmentary alterations, which may sometimes be opposite to the initially intended effect.

These chronic effects, although variable according to the conditions of use, raise questions about the ability of hydroquinone to penetrate the body and about its fate once absorbed. Indeed, when applied to the skin, hydroquinone is able to cross the cutaneous barrier and partially reach the systemic circulation.

This systemic passage, even if only partial, explains why hydroquinone has been the subject of numerous toxicological evaluations, particularly concerning its mutagenic and carcinogenic potential.

Several studies, particularly in vitro, have indeed suggested that hydroquinone may induce DNA damage, notably via the production of reactive oxygen species or the formation of reactive metabolites such as benzoquinone. Studies on isolated human cells have shown an increase in certain genotoxicity markers, such as micronucleus formation or DNA strand breaks, following the addition of hydroquinone to the culture medium. These findings have contributed to growing concern, all the more so because hydroquinone is also a metabolite of benzene, a compound that increases the risk of leukaemia.

In a similar vein, certain epidemiological data have also suggested an association between exposure to hydroquinone and some haematological cancers. An analysis based on health data from more than 130,000 patients showed that individuals exposed to hydroquinone had a significantly increased risk of developing lymphoma or leukaemia compared with unexposed subjects. These findings suggest a possible link, although the underlying mechanisms remain poorly understood.

However, these data must be interpreted with caution.

The results are heterogeneous and vary between studies. Moreover, several studies have shown that these effects can be attenuated, or even neutralised, in the presence of enzymatic or antioxidant systems, suggesting that the organism possesses mechanisms capable of limiting this damage. Furthermore, although some studies have demonstrated an increase in certain tumours, particularly renal tumours in rats at high doses, these effects appear to depend on the route of administration (often oral or injectable) and on a species-specific susceptibility. Conversely, other studies have not shown a significant increase in tumour risk.

The International Agency for Research on Cancer (IARC) classifies hydroquinone in Group 3, that is, as not classifiable as to its carcinogenicity in humans, due to a lack of sufficient evidence. This classification reflects ongoing uncertainty: signals do exist, but they do not allow a formal conclusion to be drawn regarding a carcinogenic risk associated with hydroquinone.

Hydroquinone is not only controversial because of its negative effects on the skin and on health, but also due to the environmental issues it raises.

Like other phenolic compounds, it is considered highly toxic to aquatic environments, sometimes at relatively low concentrations. Studies have notably demonstrated marked toxicity in various species used as ecotoxicological indicators, such as Daphnia magna, certain fish, rotifers, and photosynthetic micro‑organisms. In Daphnia magna, for example, a 48‑hour EC50 value of around 0.15 mg/L has been reported. The EC50 (median effective concentration) corresponds to the concentration of a substance required to produce a biological effect in 50% of the test organisms. The lower this value, the more toxic the substance.

This toxicity is not limited to visible aquatic organisms. Hydroquinone also affects microorganisms that are essential to ecosystem functioning, particularly in water and soils. Cyanobacteria, for example, are especially sensitive to it: their photosynthetic activity can be impaired, which disrupts primary production at the base of aquatic food chains. In soils, exposure to hydroquinone has been associated with a reduction in the number of cultivable microorganisms as well as with the inhibition of certain key enzymes, such as dehydrogenases and β-glucosidases, which are involved in carbon and organic matter cycles.

These effects of hydroquinone reflect a disruption of microbial metabolism, which is likely to slow down the degradation of organic matter and alter local biological equilibria.

It is, however, important to qualify this observation: hydroquinone is not regarded as a particularly persistent pollutant. Certain bacteria and fungi are capable of biodegrading it, sometimes quite efficiently, by gradually transforming it into intermediate compounds and then into simpler metabolites. Under aerobic conditions, some bacteria can directly cleave the aromatic ring of hydroquinone, while other microorganisms first convert it into intermediates such as 1,2,4-trihydroxybenzene before complete degradation. Fungi are also able to incorporate it into their metabolic pathways. This biodegradation helps to limit its persistence in natural environments.

That said, the fact that a compound is biodegradable does not mean that it is harmless. A substance may be degraded relatively quickly and nevertheless, before it disappears, cause marked toxic effects if it locally reaches sufficient concentrations or if discharges are repeated. In the case of hydroquinone, it is precisely this combination of high intrinsic toxicity and limited, but not negligible, persistence that justifies a cautious approach. In other words, the natural biodegradation capacities of microorganisms partly mitigate the problem, but are not sufficient to eliminate ecotoxicological concerns related to hydroquinone.

It is therefore both for regulatory reasons and as a precautionary health and environmental measure that we do not use hydroquinone at Typology. In our depigmenting treatments, we prefer to use vitamin C, niacinamide, tranexamic acid and liquorice extract.

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