By Ella Nasser
Whether you’re at the pool, beach, or even on the sidewalk with a piece of chalk in hand, it’s not a childhood summer day without hearing, “Don’t forget to put on sunscreen!” The protective properties of sunscreen have been so ingrained within us that many people would never think to question its ingredients. Yet, a glance at the back of a sunscreen bottle can open many eyes to the chemical-heavy product that we have been rubbing into our skin for years. While recent trends have promoted mineral sunscreens, even these formulas may hold the same genotoxic risk as the other ones, thus damaging DNA and causing genetic alterations.
A quick Google search before even grabbing a sunscreen bottle from the shelf immediately informs consumers to avoid the ingredient oxybenzone, or Benzophenone-3, at all costs. This UVA (Ultraviolet A) and UVB (Ultraviolet B) chemical filter is used in the majority of broad-spectrum sunscreens, and studies have found several of its metabolic derivatives in Americans’ urine, breastmilk, and semen due to an overwhelming amount of absorption through the skin. An in vitro study with human lymphocytes at the University of Turin explored the genotoxic effects of the BP-3 chemical. While they focused on low dosage consumption as opposed to dermal absorption, the researchers reported increased micronucleus formation even at extremely low concentrations, along with a distinct correlation between the amount of micronuclei and the dose tested. Along with the micronucleus assay, seven different chromosomal aberrations were detected at these low concentrations (less than 0.0125 µg/mL), which included gaps, chromatid and chromosome breaks, as well as dicentric chromosomes.
While the average person may only apply sunscreen to their skin, the dosage they apply is still relevant due to both absorption into the skin and the potential for the chemicals to wash off, contaminating not only the water we drink, but also the fish we eat. Furthermore, water filtering and treatment protocols do not require the removal of hazardous traces of oxybenzone, indicated by the prevalence of this substance in our urine. In fact, oxybenzone was detected in 97% of subjects tested for its presence.
Unfortunately, oxybenzone is not the only ingredient that makes sunscreen a potentially genotoxic substance. Another ingredient to avoid is ensulizole, or 2-Phenylbenzimidazole (PBI), a popular UVB chemical filter. An in vitro study utilizing both the bacteria Salmonella typhimurium TA102 as well as human skin keratinocyte cells demonstrated that when PBI absorbs solar light, it carries out light-induced reactions that cause genotoxic effects in humans, as well as significant cytotoxic effects in bacteria. A mere dose of 5 µM caused hundreds of the bacterial colonies to mutate. Perhaps more pertinent is that exposure of the human skin cells in an aqueous solution to PBI caused them to lose viability. Additionally, when exposed to UV light in combination with PBI, a comet assay produced tail moments that pointed to photo-induced DNA damage in the HaCaT keratinocytes. When PBI is UVB-irradiated, the chemical also generates reactive oxygen species and free radicals, which further contribute to DNA damage by modifying bases and causing strand breaks. Considering that UV rays are often deemed dangerous because of their ability to directly produce free radicals, it is concerning that the ingredient we have been slabbing on our skin to fight against this process is responsible for this same type of damage.
Before diving into safer sunscreen options, a quick look at the risks involved with the rest of the chemical ingredients in sunscreen is necessary. Octinoxate, another UVB filter, is easily absorbed by the skin and has been detected in blood samples at levels 16 times greater than the FDA has approved as safe. It is linked to endocrine disruption, targeting androgen and progesterone signaling as well as causing reproductive toxicity in both females and males. Homosalate is another common ingredient in sunscreens and it can interfere with bodily hormones, but it does not seem to be as concerning as the others when it’s concentrated below 15%. On the other hand, UVA filter, avobenzone, degenerates upon sunlight exposure, and aside from being a somewhat ineffective UV barrier, it is linked to more severe forms of endocrine disruption. Octocrylene is both a UVA and UVB filter that is found in many water-resistant sunscreens; it is also found in the human body at levels 14 times higher than the FDA’s cutoff. Plus, despite the chemical being safe at concentrations below 10%, it often becomes contaminated with the carcinogen benzophenone. Octisalate is the last common ingredient, but it does not seem to be as harmful as the others based on recent studies.
To put dermal absorption of these chemicals into perspective, a randomized control trial with 48 healthy individuals used four different formulations of sunscreens with the chemicals avobenzone, oxybenzone, octocrylene, homosalate, octisalate, and octinoxate. Blood samples pointed to systemic absorption and maximum plasma concentrations for each one, all greater than the FDA threshold of 0.5 ng/mL.
On the other hand, mineral sunscreens utilize inorganic ingredients, such as titanium dioxide (TiO2) and zinc oxide (ZO2). Both compounds serve as physical barriers to protect the skin from UV rays, causing the light to essentially bounce off and scatter. Nevertheless, with long-term exposure, nanoparticles can still be absorbed into the skin, specifically the stratum corneum, leading to similar genotoxicity concerns. A DNA illumination of human MRC-5 fibroblasts – both with and without using TiO2 sunscreen – detected DNA damage when exposed to sunlight (comet assay). Both titanium dioxide alone and sun rays alone caused less damage than the combination of the two, which induced the formation of free hydroxyl radicals and subsequent strand breaks.
A micronucleus assay on uncoated zinc oxide particles revealed slight genotoxicity, with the anatase form of titanium dioxide responsible for DNA cleavage. This study highlights the importance of mineral coatings, such as silica, to avoid the nanoparticle aggregation that can lead to such damage. Another study reaffirms the potential dangers of TiO2 when photo-irradiated, but instead posits that it is the formation of hydrogen peroxide, not hydroxyl radicals, that is responsible for the genetic damage. Utilizing [32P]-5′-end-labeled human DNA fragments, the research found that both the anatase and rutile forms of titanium dioxide caused DNA cleavage, mostly copper-mediated, at guanine and thymine residues of the DNA strands.
Recently, many consumers have been reaching towards these mineral-based sunscreens, and more specifically, their transparent formulas. However, considering that nanoparticle toxicity is correlated with decreasing particle size, the more white a sunscreen appears when it is applied, the safer it may actually be. Particles less than 100 nanometers in diameter allow for aesthetic application, but they are also linked to the risks described above. These small particles are also associated with neurotoxicity, including mitochondrial dysfunction, oxidative stress, and hippocampal cell apoptosis as discerned from in vitro and in vivo studies with mice.
At the end of the day, inorganic sunscreens prove to be the safer option, and normal bi-daily application should not be an issue. This regimen has been proven not to significantly penetrate the skin past the stratum corneum and pilosebaceous units, as revealed by an in vivo human volunteer study testing both intact epidermis as well as tape-stripped skin. On the other hand, it has been demonstrated time and again that one should avoid organic (chemical) sunscreens, especially those that contain oxybenzone, ensulizole, or octinoxate. While admittedly unpopular, the best option is to simply avoid over-exposure to sunlight. However, when a summer day is calling your name, you can physically scatter the UV rays by applying a mineral sunscreen.
These days, chemicals and harmful particles are often a part of routine aspects of our daily lives, so it is important to be wary of the adverse effects that some substances can have on our precious genetic material. Try to pick out a mineral sunscreen that does not boast about its transparency and invisible-like application, and one that highlights zinc oxide as the primary active ingredient. Apply the sunscreen with confidence knowing that you are protecting your body from not only the sun but also the chemical dangers of sunscreen.