Sunscreen and nano time, again (part 1)

Before launching into another discussion about the safety of sunscreens with titanium dioxide and/or zinc oxide nanoparticles, I’m going to mention uncertainty and risk. First, the notion of uncertainty that I”m using is proposed in a theory coined Knightian Uncertainty and comes from the field of economics. Here’s a definition from an article by Peter Dizikes at the Massachusetts Institute of Technology News Office,

Frank Knight was an idiosyncratic economist who formalized a distinction between risk and uncertainty in his 1921 book, Risk, Uncertainty, and Profit. As Knight saw it, an ever-changing world brings new opportunities for businesses to make profits, but also means we have imperfect knowledge of future events. Therefore, according to Knight, risk applies to situations where we do not know the outcome of a given situation, but can accurately measure the odds. Uncertainty, on the other hand, applies to situations where we cannot know all the information we need in order to set accurate odds in the first place.

Making a distinction between uncertainty and risk is very helpful in trying to understand some of the less pleasant possibilities of nanotechnology-enabled products. As more research about titanium dioxide and zinc oxide nanoparticles in sunscreens is published, there’s more information that can be used to assess risk.

With regard to health, two categories of risk are commonly discussed, absolute risk and relative risk. Here is a definition excerpted  from the Patient UK website,

Absolute risk of a disease is your risk of developing the disease over a time-period. We all have absolute risks of developing various diseases such as heart disease, cancer, stroke, etc. The same absolute risk can be expressed in different ways. For example, say you have a 1 in 10 risk of developing a certain disease in your life. This can also be said a 10% risk, or a 0.1 risk – depending if you use percentages or decimals.

Relative risk is used to compare the risk in two different groups of people. For example, the groups could be ‘smokers’ and ‘non-smokers’. All sorts of groups are compared to others in medical research to see if belonging to a group increases or decreases your risk of developing certain diseases. For example, research has shown that smokers have a higher risk of developing heart disease compared to (relative to) non-smokers.

In these scenarios (heart diseases and smoking vs. non-smoking) there’s enough information to make an informed guess even though a degree of uncertainty still exists. By contrast, the situation with nanosunscreens doesn’t lend itself to an analysis of absolute or relative risk as basic information is still being gathered.

Meanwhile the civil society/activist group, Friends of the Earth (FOE) has renewed their call to remove sunscreens with titanium dioxide and/or zinc oxide nanoparticles from the market. Andrew Maynard at 2020 Science has very carefully analysed the six studies that FOE has published on their nanosunscreen page. From Andrew’s posting,

As these are evidence-based statements, I thought it would be worth while going through them, and taking a look at the evidence they are based on:

FoE: “Manufactured nanomaterials used in sunscreens (such as zinc oxide and titanium oxide) can Damage human colon cells: A study from the University of Utah showed that nano zinc oxide is toxic to colon cells even in small amounts. The scientists called for more research and warned that the evidence is especially concerning for children who are more likely to accidently ingest sunscreen. The colon is vital because it eliminates food waste and absorbs important nutrients.”

This was a study that looked at interactions between zinc oxide (ZnO) particles and cells derived from the human colon, and was carried out in vitro (i.e. in a cell culture rather than in animals or people). It did indeed indicate that nanometer scale ZnO particles were around twice as potent as larger ZnO particles in their ability to kill these cells under idealized conditions. But the research also emphasized that direct contact with the cells was needed for a nanoscale particle-related effect. In fact, the title of the paper was “ZnO Particulate Matter Requires Cell Contact for Toxicity in Human Colon Cancer Cells,” emphasizing this point above the higher potency of the more finely structured particles.

The research was interesting, but did not resolve whether zinc oxide particles could survive long enough in the gut to come into contact with cells lining the colon, whether interactions like those observed in the laboratory are plausible under real-world conditions, and what levels of exposure would be needed to cause significant harm. The research also indicated that larger particles of zinc oxide – similar to particles that have been used in sunscreens and other topical creams for decades – were toxic to cells under the conditions of the study.

So regular and nanosized particles of zinc oxide are both toxic to colon cells with the nanosized particles being 2 x more toxic. That’s not great but we’ve been using regular sized zinc oxide particles  for quite some time and, as far as I know, no one claims that it causes colon cancer.  Also, these tests were conducted with cultures (i. e. not inside the human or any other type of body) so there’s no proof that zinc oxide of any size survives long enough to enter the colon. If there were an absolute risk associated with using regular sized zinc oxide particles, theoretically we could infer an absolute risk for nanosized zinc oxide particles.

More later today.

2 thoughts on “Sunscreen and nano time, again (part 1)

  1. Pingback: Sunscreen and nano time, again (part 2) « FrogHeart

  2. Pingback: Not enough data to assess risk for nanoscreens? « FrogHeart

Leave a Reply

Your email address will not be published. Required fields are marked *