Sunscreen’s unintended consequence: skin-harming oxidants

Chlorine in swimming pools can strip the coating off titanium dioxide nanoparticles in sunscreens that protect against UV radiation, leaving them able to react with water and form compounds that can contribute to skin damage and cancer. The nanoparticles, which protect against harmful UV rays, are coated so they remain stable in sunlight. however, in a new study, researchers have for the first time shown that the protective coating can degrade to form free radicals. Free radicals are known to damage DNA, causing aging and potentially leading to cancer. Whether there is a human health effect from exposure to these compounds is unknown. But the study raises the question of whether sunscreens meant to protect people from sunlight are creating another risk that also can harm skin. A major risk factor for skin cancer is overexposure to the sun’s ultraviolet (UV) radiation. Sunscreens reduce the risk of skin cancer by protecting skin from UV radiation. In 2012, more than one million new cases of the most common forms of skin cancer, basal cell and squamous cell, are expected to be diagnosed in the United States (American Cancer Society 2012). In addition, more than 80,000 new cases of melanoma – the most deadly skin cancer – are predicted. Sunscreen lotions and personal care products protect skin from UV rays because they contain compounds that absorb and reflect the sun’s rays. Titanium dioxide (TiO2) particles are commonly used in these products. Recently, manufacturers have started using titanium dioxide nanoparticles instead. The very small particles can reflect UV radiation better than larger ones. However, the titanium dioxide particles and nanoparticles have a down side. In sunlight, they can react with water to form harmful chemicals known as oxygen-containing free radicals, or reactive oxygen species (Konaka et al. 1999). Free radicals can react in the body with lipids, proteins and DNA. Of particular concern is that the free radicals can damage DNA. When free radicals attack DNA they cause mutations that can lead to cancer. Furthermore, the signs of aging skin are attributed to the actions of the free radicals (Pan et al. 2009). To prevent the free radicals, the titanium dioxide nanoparticles used in sunscreens and personal care products are coated. The coated nanoparticles protect against the UV rays but do not directly contact water. There is limited research on the safety of these coatings, but previous studies suggest they are stable and stay intact (Auffan et al. 2010). During the present study, the researchers examined whether the chlorine added to disinfect swimming pools would degrade the aluminium hydroxide (Al(OH)3) coating on titanium dioxide nanoparticles in sunscreen lotions. The researchers made swimming pool water and disinfected it with sodium hypochlorite. Sodium hypochlorite is a chlorine compound found in bleach and commonly used to disinfect swimming pools. First, they checked if different concentrations of chlorinated pool water would damage the coating on the nanoparticles. A commercial sunscreen lotion (Neutrogena, SPF 30) with titanium dioxide nanoparticles was added to five different concentrations of pool water. The five chlorine levels varied from 0 to 7 parts-per-million (ppm). Pool water typically contains 1 – 3 ppm chlorine. After 45 minutes, the nanoparticles were analysed to determine if the coating had been altered. Next, they measured how much reactive oxygen formed when the coated nanoparticles in chlorinated pool water were exposed to sunlight. The particles were put in the pool water with 5 ppm chlorine for seven days. Then the water was exposed to artificial sunlight. The amount of hydroxyl radical (OH radical) – one form of reactive oxygen – was measured. The researchers discovered that the protective coating slowly degraded when the nanoparticles were immersed in chlorinated swimming pool water at levels that ranged from 0.2 to 7 ppm. Coating degradation was observed when the chlorine levels were greater than 0.4 ppm. The amount of coating removed was higher in water with higher chlorine levels. In swimming pool water without chlorine, the coating stayed intact. The study results suggest that chlorine was important in the removal of the coating. In addition, the authors suggest other components in the pool water – for example, calcium and phosphate – may contribute to the degradation. In addition, the study showed that one type of free radical – hydroxyl (OH) radicals – was formed when the coated nanoparticles were exposed to both artificial sunlight and chlorinated water. But in the absence of chlorine, OH radicals were not formed. Hydroxyl radicals are the most dangerous type of free radical. The findings suggest that chlorinated swimming pool water can remove the protective coating on titanium dioxide nanoparticles used in sunscreens that protect people against the sun’s UV rays. In the sunlight, the unprotected nanoparticles can react with water to generate harmful free radicals. While these free radicals can harm cell DNA in ways that lead to cancer and other diseases, the overall human health risks of the oxidants that form in this way from sunscreen nanoparticles are not known. The study is one of the first to show that the protective coating on the titanium dioxide nanoparticles can degrade in water. Previously, it was believed that the coating was stable, but these studies did not examine the influence of chlorine. It is well-known that uncoated titanium dioxide nanoparticles generate free radicals in the presence of sunlight and water. Free radicals are a human health concern since these chemicals have been linked to oxidative stress and DNA damage and may lead to cancers. The researchers used a laboratory prepared chlorinated swimming pool water for their experiments. This showed that the more coating was removed with higher chlorine levels. The chlorine levels tested in the study (0.2 to 7 ppm) are similar to those used in most chlorinated swimming pools (1 – 3 ppm chlorine), and therefore represent actual conditions. This is particularly relevant for human exposure because sunscreen lotions are typically applied prior to swimming, and many swimming pools are disinfected with chlorine. The findings also suggest that the protective coating will not be removed in non-chlorinated waters such as lakes, rivers or oceans. But the water chemistry of natural water bodies varies greatly and other factors contributing to particle coating stability and reactive oxygen formation were not investigated. In addition, the study demonstrated that the coated nanoparticles generate free radicals when exposed to sunlight, but only when the water contained chlorine. Presumably, the free radicals were generated only after chlorine removed the protective coating. The study investigated only one type of coating: aluminium hydroxide. Other materials used to coat the particles are polymers and silicon, magnesium and zirconium hydroxides. Additional research is needed to determine if other coatings degrade in chlorinated water. The findings present a challenge to green chemists to design a coating for titanium dioxide nanoparticles that is stable in all types of water – including chlorinated swimming pools – and does not generate reactive oxygens. The next research step will be to determine the risk to human health. The reactive oxygen species generated from the uncoated nanoparticles carry are a risk factor for cancer and skin damage. Their safety should be evaluated in the context of the benefits provided by sunscreens.

Environmental Health News, 5 October 2012 ; ;