Expert Commentary

Nanomaterials in Consumer Products: A Sunscreen Case Study

Nanomaterials are being used increasingly in consumer products. In fact, estimates from March 2011 rank personal care products as the largest single category of nanomaterials, with more than 267 products in the marketplace.1

October 2011

In particular, nanomaterials have gained extensive usage in many of the newer sunscreen formulations. For example, the opaque, milky white, "wipe-on," high sun protection factor sunscreens of the past have now been replaced by clear translucent products that can be applied via spray bottles. The sunscreen industry has embraced the transition from traditional ("bulk") chemicals to the nano form of chemicals, given that the nano forms of zinc (Zn) and titanium (Ti) oxide, which are widely used as active ingredients in sunscreen preparations, are transparent and easily aerosolized and thus improve the overall desirability of sunscreen products to the consumer.

Although sunscreen manufacturers have been hesitant to openly tout their products as nanotech-based, regulators are well aware of their nano constituents. In fact, the U.S. Food and Drug Administration (FDA) recently requested comments on a warning it is considering issuing to alert consumers to the potential risks of sunscreen sprays.

While the language used in the FDA Advanced Notice of Proposed Rulemaking does not mention the term "nano" specifically, the FDA requested product information about "the typical particle size distributions for sunscreen spray products," suggesting that nano-sized particles are its focus.2 In this article, we review the current state of science on the exposure and toxicity of engineered nanomaterials as used in sunscreens, as well as potential implications for litigation involving users of such sunscreens.

Nano-Sunscreen Study

Perhaps the strongest characterization to date of the exposure potential of engineered nanomaterials in sunscreen comes from a recent study by Australian investigators, who evaluated the skin penetration of zinc oxide (ZnO) nanoparticles under conditions of normal sunscreen use. Dr. Brian Gulson and colleagues tested two different sunscreens on 20 volunteers: a nano-sunscreen containing 19-nm ZnO nanoparticles and a non-nano sunscreen containing ZnO particles > 100 nm.3

A unique aspect of this study was the use of a stable isotope tracing technique to differentiate Zn from sunscreen from naturally occurring Zn normally present in the human body. By evaluating both blood and urine samples, these investigators provided the first evidence that Zn from ZnO particles in sunscreens can be absorbed through healthy human skin exposed to sunlight, although it is important to note that Zn was detected in blood and urine for both the nano and non-nano sunscreen exposures. Moreover, as emphasized by the study investigators, it could not be determined whether the Zn detected in blood or urine was absorbed as ZnO particles or as soluble Zn, meaning that the study findings do not necessarily reflect the skin penetration of ZnO particles.

In general, this study shows that, regardless of the form of Zn (i.e., nano or non-nano form), the total amount of dermally applied Zn detected in blood and urine is quite small compared to the amounts of natural Zn normally present in human body (i.e., specifically <0.001% of applied dose and ~1/1000 of total Zn blood levels). While additional research is needed, this study thus provides real-life evidence that sunscreen application, either in the form of nano or non-nano ZnO particles, results in minimal Zn exposure.

To characterize the human health risk of exposure to sunscreens, it is necessary to evaluate the hazard, or toxicity, of the chemical to which humans are potentially exposed. While such studies, as discussed below, provide evidence of the toxicity of nano-sized Zn particles, it is important to recognize that these studies should not be taken as evidence of risk absent a further consideration of exposure. Specifically, cell-based (or in vitro) studies that have compared the toxicity of various nanomaterials provide evidence that ZnO is capable of causing extensive cellular toxicity, which generally exceeds the extent of toxicity caused by nanomaterials such as silicon dioxide nanoparticles, carbon black, or single wall carbon nanotubes.4

Animal studies also suggest that nanomaterials used in sunscreen can cross the blood-brain barrier and cause neurological damage in mice exposed to ZnO nanoparticles.5-6 However, many of these studies involve routes of exposure (i.e., lavage, injection, nasal installation) that are of limited, if any, relevance to dermally applied or sprayed sunscreen; further, they often find effects at levels of exposure that would not be encountered by humans under normal use conditions (i.e., high-dose studies). Moreover, it is difficult to directly extrapolate the impact of nanoparticles on cells in culture or whole animals to typical human exposure conditions; cells in culture cannot fully mimic the response of the human body, and exposure levels in whole-animal studies are generally much greater than typical human exposures. Importantly, a recent U.S. EPA analysis that reviewed the available literature on nanoscale TiO2 in topical sunscreens suggested that there is little risk of adverse effects from human exposure to nanomaterials in sunscreen under intended use conditions.7

The Legal Importance of Studies—The Daubert Standard and Expert Testimony

From a legal perspective, one of the key takeaways about the increasing amount of research into potential toxicity of nanoparticles is that a body of science is building up that may provide a foundation for expert witnesses to render competent testimony in court. The U.S. Supreme Court established the standard under which an expert may testify in federal court in Daubert v. Merrell Dow Pharms., 509 U.S. 579 (1993). The court held that Rule 702 of the Federal Rules of Evidence governs the introduction of expert testimony and set forth five factors to be analyzed by the trial court. These are whether the theory:

  1. is testable from a scientific perspective,
  2. has been peer-reviewed and published,
  3. has a known or potential error rate,
  4. has standards and controls concerning its operation, and
  5. is generally accepted by a relevant scientific community.8

The Supreme Court has cautioned that the Daubert analysis is flexible and that an expert's testimony should not necessarily be excluded if one of the factors is not satisfied. In practice, the Daubert fight is often the linchpin of a case; without an expert to testify and explain the underlying science to a jury, a party's case cannot proceed. Consequently, an expanding body of scientific research brings with it the potential for expert witnesses to be able to testify in court regarding the toxicity of nanomaterials. And, while the Daubert test is usually an effective gatekeeper, there is always the possibility that dubious science will be allowed to reach the jury, with potentially disastrous consequences for insurers and insureds. Stakeholders should continue to monitor the state of nano-research very carefully and be prepared before the floodgates open and a wave of lawsuits is riding the tide of bad science.

*Members of Gradient, an environmental consulting company, assisted on the writing of this article, including Dr. Marc A. Nascarella, a toxicologist who specializes in comprehensive chemical evaluations, nanotoxicology, and human health risk assessment; Dr. Christopher M. Long, a principal scientist and an expert in exposure assessment, nanotoxicology, and human health risk assessment; and Dr. Barbara D. Beck, a toxicologist and risk assessor, who directs Gradient's nanotechnology, toxicology, and risk assessment practices.

1Project on Emerging Nanotechnologies, Woodrow Wilson International Center for Scholars and the Pew Charitable Trusts (PEN, 2011). Consumer Products Inventory Analysis. Updated 3/10/2011.

2Advanced Notice of Proposed Rulemaking (ANPRM). US FDA Fed. Reg. (2011) "Sunscreen Drug Products for Over-the-Counter Human Use; Request for Data and Information Regarding Dosage Forms." Accessed September 8, 2011.

3Gulson, B., M. McCall, M. Korsch, L. Gomez, P. Casey, Y. Oytam, A. Taylor, L. Kinsley, G. Greenoak. August 12, 2010. "Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin." Toxicological Sciences 40p.

4Yang, H., C. Liu, D. Yang, H. Zhang, Z. Xi. 2008. "Comparative study of cytotoxicity, oxidative stress and genotoxicity induced by four typical nanomaterials: The role of particle size, shape and composition." Journal of Applied Toxicology. doi:10.1002/jat.1385.

5Wang, J., C. Chen, Y. Liu, F. Jiao, W. Li, F. Lao, Y. Li, B. Li, C. Ge, G. Zhou, Y. Gao, Y. Zhao, Z. Chai. 2008a. "Potential neurological lesion after nasal instillation of TiO2 nanoparticles in the anatase and rutile crystal phases." Toxicology Letters 183:72–80.

6Wang, J., Y. Liu, F. Jiao, F. Lao, W. Li, Y. Gu, Y. Li, C. Ge, G. Zhou, B. Li, Y. Zhao, Z. Chai, C. Chen. 2008b. "Time-dependent translocation and potential impairment on central nervous system by intranasally instilled TiO2 nanoparticles." Toxicology 254:82–90.

7U.S. EPA (2010). "Nanomaterial Case Studies: Nanoscale Titanium Dioxide in Water Treatment and in Topical Sunscreen." EPA/600/R-09/057F. November 2010. 204 p.

8Importantly, not all states follow the Daubert standard, and, in cases involving scientific evidence, it is critical to become familiar with the law governing expert testimony in the jurisdiction in which the lawsuit is venued.

Opinions expressed in Expert Commentary articles are those of the author and are not necessarily held by the author's employer or IRMI. Expert Commentary articles and other IRMI Online content do not purport to provide legal, accounting, or other professional advice or opinion. If such advice is needed, consult with your attorney, accountant, or other qualified adviser.

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