Do sunscreens become carcinogenic as they age?

Sunscreens are considered one of the most effective means to protect the skin from the harmful effects of sun rays. These rays cause various types of cellular damage, including DNA mutations, alterations to structural proteins such as collagen and elastin, and skin inflammation. Over time, these alterations accelerate the skin ageing, and above all, increase the risk of skin cancers. In response to this threat, public health agencies recommend protecting the skin from the sun by avoiding prolonged direct exposure, especially between 11am and 4pm, wearing long clothing and to use a sunscreen with a high SPF. Despite these benefits, some concerns persist and sunscreens are sometimes suspected of becoming carcinogenic in the long term.

Current scientific data do not support that sunscreen use increases skin cancer risk; they suggest the opposite.

A 2014 meta-analysis pooled results from 21 studies including 7,150 malignant melanoma cases. Using regression analysis, researchers demonstrated that sunscreen use did not increase melanoma risk, including long-term use. Studies show a significant protective effect of sunscreens against skin cancers. A robust piece of evidence comes from a randomised controlled trial conducted in Australia. Over 4.5 years, 1,621 adults were followed to assess the impact of daily use of a broad-spectrum SPF 15+ sunscreen.

In this protocol, volunteers were divided into two groups: one applied sunscreen every day, while the other used it at their discretion or not at all. At the end of the trial, the first group showed a 40 per cent reduction in squamous cell carcinoma incidence compared with the control group. The protective effect persisted after the intervention concluded. Eight years after the protocol ended, the first group continued to show a 40 per cent reduction in squamous cell carcinoma, reflecting that more participants in this group maintained sunscreen use after the trial.

Far from raising the risk of skin cancer, regular and rigorous application of sunscreen is an effective prevention tool.

When discussing the possible toxicity or carcinogenic potential of sunscreens, the ingredient implicated in most cases is octocrylene, a chemical filter that blocks UV rays. In addition to its suspected carcinogenicity, its ability to undergo a retro-aldol reaction and its conversion into benzophenone is concerning. This reaction is favoured by heat and light and takes place over time during storage at room temperature.

The benzophenone formed in this way is a concern. It is classified as a suspected carcinogen (category 2 in Europe) and a potential endocrine disruptor.

Studies have confirmed the accumulation of benzophenone in products containing octocrylene. For example, a chemical analysis carried out on 28 products containing octocrylene revealed the presence of benzophenone in all products, often at concentrations above 10 ppm (0.001%). Moreover, benzophenone levels rose over time and with temperature, suggesting ongoing degradation of octocrylene in formulations.

Benzophenone has been studied for its potential to influence tumour development. A 2008 study showed that exposing human lung cancer cells to low concentrations of benzophenone induced an epithelial-to-mesenchymal transition, a process that gives cells more aggressive behaviour, facilitating migration, invasive capacity and, ultimately, the formation of metastases. In mice, other studies highlighted a role for benzophenone-3 in the progression of mammary tumours: increased cell proliferation and reduced apoptosis of tumour cells were observed. These findings were obtained at doses and under experimental conditions that differ from realistic skin exposures in humans.

Besides benzophenone and octocrylene, other UV filters are under scrutiny for their potential carcinogenicity, including ensulizole, homosalate, octinoxate, and octisalate. However, current scientific data are insufficient to confirm that these filters are carcinogenic. A recent review examined these six sunscreen filters. It analysed data from several dozen studies conducted in vivo, in vitro and in silico and performed an assessment of exposure margins, defined as the difference between the dose at which a toxic effect is observed and the dose actually absorbed by humans. The review concluded that none of these filters is genotoxic and that there is no evidence suggesting a carcinogenic mode of action Relevant to humans. Systemic exposure levels measured in humans remain below the concentrations at which any biological activity is observed in laboratory settings.

Although further research is needed to fully assess the carcinogenic potential of UV filters, current evidence does not support concluding that they render sunscreens carcinogenic. The benefit–risk balance favours daily sunscreen use to prevent skin cancers.

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