Matta et al have now shown what we all should have seen coming: the chemicals of our most common and effective sunscreens do not just sit on the skin’s surface and soak up damaging ultraviolet rays before these rays can penetrate to deeper layers and injure nuclear DNA. No – these chemicals themselves can penetrate the skin’s barrier and enter the blood stream (and from whence they can theoretically go anywhere in the body). We should have seen this coming because: a) the permeability barrier is a biological construct, and as such, has its limitations and vulnerabilities; and b) previous studies have found some of these ingredients in human urine and breast milk.
While these compounds are not limited to sunscreens but can be found in many cosmetics and in other, non-dermatologic products, the present study clearly demonstrates that skin absorption is one route to our interior. Yet, the study sets a low bar for detecting ‘significant’ absorption. In the first place, they chose the “threshold of toxicologic concern” (defined here as “above the highest plasma level below which the carcinogenic risk of any unknown compound would be less than 1 in 100,000 after a single dose in foods”) as the level above which absorption would be deemed significant. Secondly, the quantities applied in the study (~30 gm/person four times a day) – although at the recommended level for optimal sun protection during an all day exposure – greatly exceed those routinely achieved in practice.
What this means for our health and safety – is entirely unknown. In an accompanying editorial, Califf and Shinkai provide background on the US regulatory issues that gave rise to the present state of affairs, in which chemical sunscreens are in widespread use despite little information on their long-term safety to humans and the environment.
In sum, we known that these sunscreens are effective in blocking UV light, and that their use can protect against the most common, sun-induced skin cancers – but we don’t yet know whether there are safety concerns that might counterbalance their benefits.
Some sunscreen ingredients have been shown in experimental systems to be endocrine disrupters – chemicals that interfere with the body’s endocrine system and may produce adverse developmental and reproductive effects – although their human impacts are unknown. Now that we know that these chemicals are indeed absorbed into our system underscores the importance of finally getting answers to these questions. But many of these studies concern long-term safety, and it could take years before we have definitive information.
In the interim, while we await the results of these studies, it makes no sense to stop protecting our skin from a known carcinogen – sunlight – out of fear of an unknown, and only theoretical, hazard to our health.
Fortunately, there are other options to consider. Titanium- and zinc-based formulations can be used instead of these chemical sunscreens. Although fortunately, these are considered safe, the quantities that must be applied to achieve very high levels of protection usually produce an undesirable, whitish cast to the skin. And in any case, sun protection strategies should never solely rely on the use of a sunscreen. Seeking shade, use of protective clothing (long sleeves, long pants, hats with a full brim) and avoidance of exposure during peak intensity hours of the day should always be part of the program.