Nanotechnology & Sunscreens

When we began our sunscreen investigation at the Environmental Working Group, our researchers thought we would ultimately recommend against micronized and nano-sized zinc oxide and titanium dioxide sunscreens. After all, no one has taken a more expansive and critical look than EWG at the use of nanoparticles in cosmetics and sunscreens, including the lack of definitive safety data and consumer information on these common new ingredients, and few substances more dramatically highlight gaps in our system of public health protections than the raw materials used in the burgeoning field of nanotechnology. But many months and nearly 400 peer-reviewed studies later, we find ourselves drawing a different conclusion, and recommending some sunscreens that may contain nano-sized ingredients.

Consumer Reports (2007) testing showed that consumers can be protected from UV radiation using products free of nano-scale ingredients like zinc and titanium. We expected to find this as well, but we took our study further than Consumer Reports to be certain. We looked not only at whether or not products provide broad-spectrum UV protection, but also at which sunscreens break down in the sun, and at the full range of potentially hazardous sunscreen ingredients that can absorb through the skin and into the body to pose other risks. Our answers changed.

Our study shows that consumers who use sunscreens without zinc and titanium are likely exposed to more UV radiation and greater numbers of hazardous ingredients than consumers relying on zinc and titanium products for sun protection. We found that consumers using sunscreens without zinc and titanium would be exposed to an average of 20% more UVA radiation — with increased risks for UVA-induced skin damage, premature aging, wrinkling, and UV-induced immune system damage — than consumers using zinc- and titanium-based products. Sunscreens without zinc or titanium contain an average of 4 times as many high hazard ingredients known or strongly suspected to cause cancer or birth defects, to disrupt human reproduction or damage the growing brain of a child. They also contain more toxins on average in every major category of health harm considered: cancer (10% more), birth defects and reproductive harm (40% more), neurotoxins (20% more), endocrine system disruptors (70% more), and chemicals that can damage the immune system (70% more) (EWG 2007).

We also reviewed 16 peer-reviewed studies on skin absorption, nearly all showing no absorption of small-scale zinc and titanium sunscreen ingredients through healthy skin. In a 2007 assessment the European Union found no evidence of nano-scale particles absorbing through pig skin, healthy human skin, or the skin of patients suffering from skin disorders (NanoDerm 2007). Overall, we found few available studies on the absorption of nano-scale ingredients through damaged skin, but nearly all other sunscreen chemicals approved for use in the U.S. also lack these studies.

In contrast to zinc and titanium, the common sunscreens octinoxate and oxybenzone absorb into healthy skin — in large amounts according to some studies. These 2 sunscreens can cause allergic reactions, can lead to hormone-driven uterine damage, and can act like estrogen in the body, raising potential concerns for breast cancer.

On balance, EWG researchers found that zinc and titanium-based formulations are among the safest, most effective sunscreens on the market based on available evidence. The easy way out of the nano debate would be to steer people clear of zinc and titanium sunscreens with a call for more data. In the process such a position would implicitly recommend sunscreen ingredients that don't work, that break down soon after they are applied, that offer only marginal UVA protection, or that absorb through the skin.

If this were nano-containing eye shadow, blush, or body glitter our position would be different — if it's not protecting your health, don't use it. But sunscreen is meant to protect us from exposure to a known human carcinogen, UV radiation, responsible for some of the more than one million cases of skin cancer diagnosed in this country every year.

EWG conducted our sunscreen study because comprehensive sunscreen safety standards have not yet been set in this country. FDA has been drafting these standards for 31 years, and still has set no firm deadline for finalizing their latest proposed rule, issued in August 2007. FDA has also not yet evaluated sunscreen chemicals that are widely available in other parts of the world and that could potentially replace nanoparticles in sunscreen.

EWG has called for more safety studies for all sunscreens, nano or not. We've called for more data to understand when and in what amounts these ingredients penetrate the skin, and we've advocated for science-based assessments of health risks, so that everyone from consumers to health officials at FDA will know that we have the best possible products on the market. For nano-scale ingredients we have also called for full labeling so consumers can make informed choices.

We think people need to know what products to use while we all wait for FDA to finish finalizing their sunscreen safety standards. EWG will continue to advocate for better safety standards and more safety studies for sunscreens — from nano-scale up.

Ultimately, consumers make their own choices, and those wishing to avoid zinc and titanium can choose the "no nano" search option on our website to see the best of sunscreens that do not contain these small particles.

More about nanoparticles

Nano-scale particles are measured in nanometers (nm), or billionths of a meter. Relative to larger particles, nano-scale materials can be more chemically reactive and more easily absorbed into the body. A number of studies raise concerns about potential health risks when these particles are inhaled or are absorbed through the skin or gut. Nevertheless, they are already widely used in products, including sunscreens, with no requirement that their presence be disclosed.

Confusion about particle size

Nanoparticles are most commonly defined as particles having at least one dimension smaller than 100 nm (EPA 2006, ASTM 2006, SCCP 2006). Nanoscale titanium and zinc are clear, a significant aesthetic improvement from the white pastes formed with conventional particle sizes.

But safety concerns surrounding nanomaterials lead some manufacturers' choice to label their products as free of nanoscale zinc and titanium. EWG's 2009 analysis of sunscreens identified 16 manufacturers claiming to steer free of nanoparticles.

Consumers should view such claims with skepticism. FDA has neither set standards for nanoparticle claims nor defined the maximum dimensions of a nanoparticle. Products claiming to be nano-free do not divulge the size of the particles used instead. Friends of the Earth has called for consumers to avoid zinc and titanium-based sunscreens unless manufacturers specify that they use no nanoparticles (FOE 2007). A more reliable criteria is the color of the sunscreen: larger particles leave a white coating on the skin.

Sunscreen manufacturers have used nano-scale titanium dioxide since 1990 and nano-scale zinc since 1999. Now the typical size for titanium in sunscreens is 10-100 nm for titanium and 30 to 200 nm for zinc (Nohynek 2007). At these sizes both zinc and titanium are nearly transparent. An estimated 1,000 tons of nanoparticles were used in sunscreen worldwide from 2003 to 2004 (Börm 2006). Nano zinc and titanium are thought to be widely used in US sunscreens, although they are rarely labeled as such. The common label terms "micronized" and "ultra-fine" do not preclude the presence of nano zinc or titanium in sunscreen. Consumer Reports recently tested 8 mineral sunscreens and detected nanoparticles in each one (Consumer Union 2007).

The European Union and Australian cosmetic regulatory bodies have reviewed the toxicity of zinc and titanium nano-ingredients in sunscreen. In 2000, the EU approved nanoscale titanium for use in sunscreen, concluding that the chemical does not penetrate the skin or present risks for cytotoxicity, phototoxicity, or genotoxicity (SCCNFP 2000). In 2004 the same panel reviewed nano-scale zinc and found the evidence insufficient to support its use in sunscreen. The panel could not preclude the possibility that nanoscale zinc might penetrate the skin or damage human DNA (SCCNFP 2004). In 2005 the panel called for additional study by manufacturers to evaluate these concerns (SCCP 2005). The EU Scientific Committee on Consumer Products recently recommended a case-by-case risk assessment of all nanoparticles used in cosmetics, particularly those particles that are insoluble and biopersistent, with the potential to build up in body tissues (SCCP 2007).

In 2006 the Australian Therapeutic Goods Administration concluded that the weight of evidence showed no penetration of nanoscale zinc and titanium to viable skin cells (Australia TGA 2006). The US has not evaluated the safety of nanoscale zinc and titanium in sunscreen. The FDA considers their approval of zinc and titanium as sunscreens to encompass all particle sizes.

Zinc oxide and titanium dioxide offer moderate to strong UVA protection. In the nano size range UVB protection increases and UVA protection decreases. These 2 chemicals are among only 4 US-approved sunscreens providing significant UVA protection. The remaining two, Avobenzone and Mexoryl SX, provide UVA-I protection. However Avobenzone can be quite unstable and Mexoryl is not widely sold in the U.S. Two alternative UVA blockers, Tinosorb S and Tinosorb M, have been used in Europe since 2000 but have not yet been approved by FDA.

Safety concerns of nanoparticles

The two key factors that govern the safety of nanoparticles in consumer products are the amount of human exposure and the effects of nanoparticles inside the body.

Potential for human uptake of nanoparticles

Skin absorption. EWG reviewed all scientific publications and government safety assessments on the penetration of zinc and titanium nanoparticles through skin. The current weight of evidence suggests that these nanoparticles do not penetrate through the thickness of the outer stratum corneum and epidermis to the living tissue below.

The concern of skin penetration was raised a decade ago by a pilot study of 13 Australian patients scheduled for skin surgery. Patients applied an 8% nano-titanium sunscreen for 2 to 6 weeks before surgery. Researchers found higher titanium levels in the epidermis of these patients than in that of a comparison group, but only after excluding a person in the comparison group who also showed high titanium dioxide levels in the skin (Tan 1996). Studies performed since have failed to detect similar evidence of penetration. (Table 1)

The European Union-funded NanoDerm project conducted a series of experiments over 3 years and found no evidence of dermal penetration in human and pig skin using a variety of analytical techniques, titanium types, and test conditions. NanoDerm focused on titanium penetration since zinc is not approved for use in European sunscreens. They observed that nano-scale titanium particles often aggregated into larger masses on skin, and penetrated deepest in hair shafts. The project also performed absorption studies on skin samples from several patients with psoriasis, which has been a particular concern because skin affected by this condition lacks a protective barrier. Titanium particles penetrated nearly to the level of living skin cells (keratinocytes), but researchers found no evidence that the particles reached the bloodstream. None of their 11 publications found evidence that nano-scale titanium reached "vital tissues" (NanoDerm 2007; Kiss 2008).

EWG separately reviewed results from 16 academic experiments representing a variety of skin types (mouse, pig or human skin) and including nano-scale titanium and zinc. All of the studies examiing human skin or pig skin (the most suitable surrogate for human skin) conclude that very few particles, if any, reach living skin cells (Cross 2007; Dussert 1997; Gamer 2006; Gottbrath 2003; Kiss 2007; Lademann 1999; Landsdown 1997; Mavon 2007; Menzel 2004; NanoDerm 2008; Pflücker 2001; Pirot 1996; Schulz 2002; Tan 1996; Wu 2000; Zvyagin 2008). The only signs of penetration come from studies of hairless mice, whose skin is much more permeable than human or pig skin (Kuo 2009; Wu 2009).

One recent study found no titanium particle penetration and limited zinc particle penetration (1.5 to 2.3%), though the study is difficult to interpret because of apparent background contamination of laboratory materials by zinc (Gamer 2006). Hair follicles make up 0.1% of the skin surface and can be potential openings for deeper movement of nanoparticles into skin. Penetration studies for nano zinc and titanium note accumulation of the particles in follicles, but no movement into deeper tissues (Lademann 1999, 2005, 2006, 2007; NanoDerm 2007).

The available research, including studies of psoriatic skin tested by NanoDerm, does not completely address the potential for penetration in damaged skin. Sunburned skin might be more permeable, as might skin of children or the elderly, or thinner skin that occurs in some areas of the body. The NanoDerm assessment concluded that sunscreen containing titanium nanoparticles should not be applied to open wounds, and called for more study of psoriatic skin, which has a damaged outer protective barrier.

Two studies find direct evidence of skin penetration to hairless mice (Kuo 2008, Wu 2009). Hairless mouse skin is less than half as thick as human skin and is a poorer barrier to absorption (Kuo 2009). The European Union's Scientific Committee on Cosmetic Products considers it to be a poor proxy for human exposure (SCCP 2006).

In one study researchers coated nano zinc particles with acidic coatings to increase the amount that permeated the skin. The acidic solutions acted as chemical enhancers to alter skin lipids, increasing fluidity and permeability. Sunscreens are not typically coated in acids, so any concerns about zinc study findings are outweighed by the number of studies showing little concern for penetration in more typical sunscreen formulations.

However hairless mice exposed to uncoated titanium dioxide for 60 days exhibited a concerning list of effects. Mice exposed to 4 and 25 nm particles had skin damage (lipid peroxidation, decreased collagen and keratinization), increase organ weights (liver and spleen), and decreased body weight. One form of titanium (Degussa P25-rutile/anatase) was detected in the mouse brains. The larger forms of titanium were not as toxic. The 60 nm titanium was associated with skin but not organ effects. The 90 nm size (described by the researchers as "normal size") showed no effects in any of the outcomes studied (Wang 2009).

The implications of this study for human sunscreen users are not clear. Mouse skin, is a less desirable proxy than pig skin for studies of human dermal exposures (SCCP 2006). In addition to greater permeability of hairless mouse skin, only uncoated titanium was used, which is more reactive and potentially more toxic than the forms in sunscreen. The same study reported no penetration to the dermis of pig skin, which is the animal whose skin considered most similar to humans'. Particles penetrated to the pig skin epidermis in-vivo, but did not reach the dermis. The 10 nm particles penetrated at a greater rate than the 60 nm sized particles.

Dermal penetration of other nanoparticles

Several studies using other types of nanoparticles explore conditions that may affect penetration of nano sunscreens. One reports that another nano metal, silver passes more easily into damaged skin reaching the outermost surface of the epidermis (Larese 2009). Others find that quantum dots reached deeper tissues when the skin was exposed to UV rays, yet there was still limited penetration of the epidermis and dermis (Mortensen 2008). Carbon-based fullerines and dextran beads penetrate deeper when skin is flexed, similar to the movement of a wrist (Tinkle 2003; Rouse 2006).

Otherwise the types of nanomaterials shown to penetrate skin tend to be very different than zinc and titanium: lipids, carbon-based structures and other biodegradable nanoparticles (Puglia 2008; Ryman-Rasmussen 2006; Sheihet 2008; Sanna 2007; Scafer-Korting 2007; Kohli 2004).

Oral exposures to nanoscale zinc and titanium

Oral intake is a potential concern for nanoparticles in sunscreen when they wash off consumers and enter swimming pools or reservoirs, and the ingestion of nanoparticles used in lip products. Our database includes dozens of lip balm products with zinc oxide and/or titanium dioxide.

Several studies have investigated the effects of ingestion large doses of zinc or titanium (Wang 2006, 2007b). Nano Titanium dioxide (25 or 80 nm sizes) was more likely to penetrate organs and cause damages when single large doses were administered by mouth to test animals (Wang 2006). Scientists found altered enzyme levels (liver, kidney and heart) and pathological changes (to liver and kidney) indicating titanium passed out of the gastrointestinal system to provoked organ damages at this dosage.

An identical dose of nano zinc (5 grams per kilogram bodyweight) agglomerated in mouse intestines causing severe effects like lethargy, vomiting, diarrhea and death. Researchers determined that nano zinc aggregated and blocked the intestines (Wang 2007b). Larger, or “microscale” zinc particles provoked more dramatic impacts to liver and kidney enzyme levels than the nano or conventional size zinc particles. However nano zinc caused kidney lesions, indicating they also damage kidneys at this high dosage. Nanoparticles also caused severe anemia. Slight stomach and intestinal inflammation were detected in both groups. (Wang 2006)

The potential harms of ingesting lower concentrations of nano zinc and titanium from lip products with UV protection are less clear.

Inhalation exposures to nanoscale zinc and titanium

Powdered mineral sunscreens or sprays can be inhaled. Some of these products are advertised for use on the face and for use on children's skin. Inhalation of nano-sized zinc and titanium can provoke an inflammatory response due to the high surface area (and therefore high reactivity) of small particles. Furthermore, titanium is recognized by the International Agency for Research on Carcinogens (IARC) as a "possible carcinogen" based on evidence from animal inhalation studies (IARC 2006). This determination was made for conventionally sized particles, and at exposures higher than would likely be experienced by a consumer using powdered or spray-type sunscreens.

Nanoparticles are believed to have reduced clearance from lungs relative to larger particles (Geiser 2008) and can have ingreased transfer into the bloodstream (Oberdörster 2004). Inhaled nanoparticles can be absorbed through the nose or mouth.

Smaller particle sizes are more potent lung carcinogens in rat studies (Dankovic 2007). Micro- and nano-scale particles induce DNA damage and cytotoxicity to lung tissue, likely due to the generation of oxidative stress (IARC 2006; Wang 2007a).

Titanium particles inhaled through the nose can reach the brain and provoke oxidative stress (Wang 2008a, 2008b). Interestingly of 2 types of titanium particles tested, the larger, anatase form (155 nm) was more penetrating and reactive than the smaller, rutile form (80 nm). This finding is supported by other inhalation studies (Warheit 2007). Both types of titanium crystals are used in sunscreen, and it is difficult or impossible for consumers or researchers to determine whether anatase or rutile-type titanium is used in a given product. Inhalation risks for zinc oxide are less studied. However intratracheal instillation of 1 or 5 mg/kg of nano or fine zinc particles provoked, “potent but reversible inflammation” which resolved within one month of exposure (Sayes 2007).

For these reason we strongly discourage the use of powder or spray sunscreen, particularly those with titanium and zinc.

Toxicity of nano-scale zinc and titanium

The primary toxicity concern of nanoparticles is damage caused by free radical generation, which can provoke intense oxidative stress, inflammation and cell damage (Nel 2006). All sunscreen chemicals provoke a degree of oxidative stress, but nano-size titanium dioxide and zinc oxide have been shown to be more potent than conventional size particles in laboratory tests (Kang JL 2008; Sayes 2007). Interestingly, the level of oxidative damages caused by several types of nano-scale titanium is minimal compared to the amount generated by UV radiation on bare skin (Popov 2009) underscoring the importance of comparing UV filters to each other and to the effects of sunlight on unprotected skin.

The exact particle size and surface coatings of nanoparticles affect reactivity. The anataste form of titanium is much more reactive than the rutile form, whether it be to oxidize the paint on steel roofs or generate cellular stresses in skin cells.

Titanium and zinc nanoparticles used in sunscreen are commonly coated - titanium with magnesium, silica, alumina or zirconium and zinc with dimethicone. Coatings greatly reduce or eliminate the UV reactivity of nano-scale titanium (Wakefield 2004; Tsuji citing Mills 1997; Pan 2009). In a study of 3 types of titanium nanoparticles, researchers found that uncoated anatase and rutile titanium caused cell damage and oxidative stress, while particles with polymer coatings did not (Pan 2009).

Despite this, both zinc and titanium dioxide, including forms extracted from sunscreen, have been shown to provoke oxidative stress and a variety of cell damage (Hidaka 2006; Dunford 1997; Uchino 2002; Lu 2008; Kang SJ 2008; Brezová 2005). Some forms of titanium dioxide are more reactive when catalyzed by UV light (Sayes 2006), though other studies report no UV-catalyzed damages (Theogaraj 2007).

Zinc is generally less reactive than titanium, and also appears to be less catalyzed by UV exposure (Dufour 2006). However, a recent study exposing epidermal cells found markers of oxidative stress and DNA damage after exposure to uncoated nano zinc particles. The particle size was generally 30 nm, which aggregated into clusters about 165 nm in diameter (Sharma 2009). This study indicates than nano-scale zinc could have toxic effects in the absence of skin penetration.

The NanoDerm project tested the toxicity of nano-scale titanium dioxide to skin cells and found a variety of impacts to cell viability, proliferation, apoptosis and differentiation (Kiss 2008). Fibroblasts and melanocytes took up titanium, which resulted in altered calcium levels, a key regulator of cell mechanisms in these types of cells. Titanium also reduced cell growth in all cell types and induced apoptosis in some cells. These and other effects noted may result in abnormal barrier function of skin cells. These impacts were deemed of minimal concern for sunscreen products given the conclusion by the same group that titanium nanoparticles do not penetrate to living tissues. Nevertheless, researchers raised cautions about the application of nano-scale titanium to broken or psoriatic skin where it might have direct access to living tissues (NanoDerm 2007; Kiss 2008).

Scientists seeking to study nanoparticle behavior and toxicity have injected nanoparticles directly into test (Takeda 2009; Liu 2009, Chen 2009). These studies examine worst-case exposures to high doses of nanoparticles in the bloodstream. Titanium particles have been found to accumulate in the liver, kidneys, spleen, lung, brain and heart (Liu 2009). Particles can also penetrate the reproductive organs and developing fetus causing reproductive and brain disturbances in offspring (Takeda 2009).

Nano-scale zinc and titanium life-cycle and environmental risks

Lifecycle concerns about nanotechnology include occupational exposures during production, and ecotoxicity of disposed waste. There are significant unaddressed safety concerns for workers handling nano materials. Few rules govern the use of protective equipment and other controls to limit nanoparticle inhalation or ingestion during product formulation. There are indications that workers are not currently receiving adequate protection from nanoparticle exposures (Maynard 2005). Therefore nano-scale zinc and titanium risks should be more carefully assessed for occupational exposures, which can be orders of magnitude more intense than consumer exposures (Schulte 2008).

Both zinc oxide and titanium dioxide nanoparticles may impact the environment when they wash off of skin. Each metal has strong antibacterial properties, which are heightened in smaller particles (Nair 2008) and potential to provoke oxidative stress in the ambient environment (Huang 2008; Tsuang 2008; Jones 2008; Reddy 2007).

Nano-scale particles may aggregate into larger masses, reducing potential antimicrobial activity (Franklin 2007). Overall, nanoparticle impacts to living systems have not been sufficiently assessed. As a 2006 nanotechnology review concluded, "No studies to date have been done on protists, fungi, plants, birds, reptiles, or amphibians, and the only mammalian studies have been carried out using laboratory species" (Börm 2006).

Nano-scale zinc and titanium may absorb to sediments in the water column, potentially impacting microorganisms and filter-feeders who would take in suspended nanoparticles. Nano-scale titanium particles bind with cadmium in water sediments and lead to increased cadmium absorption for fish. One study found a 146% increase in cadmium accumulation over a 25-day exposure period (Zhang 2007). A separate study found that nano fullerenes deplete antioxidant defenses in fish (Oberdörster 2004), leading to lipid peroxidation in the brain and liver (Zhu 2006).

The variety of potential negative environmental impacts of nano-scale and conventional zinc and titanium should be carefully studied, and also weighed against the environmental impacts of alternative UV blockers. A recent publication documented "rapid and complete" coral bleaching in response to extremely low concentrations of non-mineral sunscreens such as ethylhexyl-methoxycinnamate (octinoxate), benzophenone-3 (oxybenzone) and 4-methylbenzylidene camphor (Danovaro 2008). Sunscreen chemicals make corals more susceptible to viral diseases, leading to bleaching (loss of symbiotic algae on which the corals depend for their survival) and eventual death of the entire coral. These observations were confirmed on coral reefs in four distinct ocean regions, leading scientist to conclude that up to 10% of the world reefs is potentially threatened by sunscreen-induced coral bleaching (Danovaro 2008).

Non-mineral UV filters such as octocrylene and 4-methylbenzylidene camphor have been found to accumulate in surface waters and in fish (Buser 2006; Giokas 2007) and are linked to hormone disruption in both fish and amphibians (Kunz 2004; Kunz 2006; Weisbrod 2007) as well as in mammals (Schlumpf 2004).

For all sunscreens, including nano-scale zinc and titanium, there is an urgent need to carry out thorough environmental assessments so that regulators have the data needed to control hazards associated with widespread use of these and other chemical ingredients in personal care products.

Conclusions

Our assessment of the comparative benefits of zinc and titanium sunscreens that might contain nanoparticles is obviously not meant as an endorsement of all nano-scale products nor the manufacturing processes. But we find nano-scale zine and titanium to be reasonable choices for use in sunscreen, particularly given the known hazards of UV exposure, and the limited choices for UV protection in the United States. We are concerned about the potential for nanoparticle inhalation with powder or spray forms of mineral sunscreens, particularly given marketing claims promoting their use on faces and on children's skin. EWG urges consumers to avoid mineral-based sunscreens sold in powder or spray forms, and for manufacturers of these products to avoid using nano-scale particles. Consumers can expect a wider range of safe and effective products when FDA finalizes comprehensive sunscreen standards and reassesses the safety of all susncreens to ensure they they are effective and that they are safe for people and the environment alike.

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