By Kenneth Portier, PhD
Recently, manufacturers have introduced new sunscreen products that use titanium dioxide, a typical ultraviolet (UV) radiation blocker found in many sunscreens, formed into tiny nanoscale particles. Why use nanoscale titanium dioxide? Because at this small size the particles do not block visible light, and therefore the sunscreen is invisible when applied to the skin and at the same time provides protection from cancer-causing UV radiation.
Titanium dioxide is an excellent UV-blocker, but there has been some concern about its safety because in dry powder form, titanium dioxide is highly toxic when inhaled. The International Agency for Research on Cancer (IARC) classifies the dry powder form of titanium dioxide as possibly carcinogenic to humans.
It's not yet clear, though, whether other forms of titanium dioxide, such as the nanoparticles used in sunscreen, also pose a risk.
What are the risks?
The issue of risk comes down to two questions:
- Can nanoscale titanium dioxide as used in sunscreen products penetrate human skin and enter our bodies?
- If penetration is possible, how much exposure to this kind of sunscreen might produce health effects?
Some background would help. Nanoscale particles are very, very small, less than 100 nanometer (nm) in size. To put that in perspective, household dust particles are typically 3,000-9,000 nanometers in size. There is concern that under the right conditions, nano-sized particles can find their way through human skin and into the bloodstream. While human skin is a highly effective barrier to most chemicals, chemicals that can make it through the skin are potentially more toxic because they do not get broken down by other bodily systems before entering the bloodstream.
This isn't a wild theory. In fact, a lot of recent research is focusing on formulating beneficial drugs into nanoparticles specifically because this allows better delivery of smaller doses directly to target tissues in our bodies. So the worry is that non-drug chemicals, when packaged into nanoparticles, could also pass through the skin, enter the bloodstream, and as a result make potentially negative health effects more likely.
What kinds of negative effects are we concerned about? Recent research has shown that titanium dioxide particles in the 2-5 nm particle size when injected in low dose under the skin of mice produce a significant, but reversible, inflammatory response. This could be a concern given what we are learning about the negative health effects of chronic inflammation.
So, can nanoscale titanium dioxide in sunscreens penetrate the skin? Unfortunately, testing on humans and human skin has produced mixed results. A number of studies have shown that the titanium dioxide in the sunscreen product does not get below the first layer of skin of test subjects. Other studies have shown that particles up to 6,000 nm in size are capable of collecting in skin pores and hair follicles. These and other openings in the skin provide logical pathways for nanoparticles to get under the skin, causing inflammation and potential health risks. Studies to confirm that penetration does indeed occur have not been performed so we are left with uncertainty in the answer to our first key question. And because we don't know the answer to the first question, scientists have not looked for an answer to the second question.
More about sunscreen and skin cancer:
What are the benefits?
But here's what we do know. Skin cancer is the most common cancer in the United States. One in every 3 cancers diagnosed worldwide is a skin cancer. UV radiation, through exposure to sunlight, is the major environmental factor responsible for basal cell carcinomas, squamous cell carcinomas, and for melanoma. Researchers at IARC estimate that from 65% to 95% of malignant skin melanomas are caused by excess sun exposure. The pathways through which UV radiation exposure produces skin cancer (photocarcinogenesis) are well mapped.
The research is very clear: use of sunscreens that absorb and/or block UV radiation results in reduced damage to the skin's DNA. Reducing DNA damage leads to reduced risk from skin cancer. Epidemiologic studies have demonstrated that consistent use of sunscreens results in lower likelihood of having a skin cancer.
The research is also very clear that titanium dioxide is an excellent broad-spectrum UV radiation blocker. But it is not the only broad-spectrum UV radiation blocker. Unfortunately, a number of the other chemicals used in sunscreens to block UV radiation have their own potential health risks. Many of these chemicals are also suspected of being able to penetrate the skin and damage DNA, possibly leading to cancer.
Where does this leave us?
The American Cancer Society and other agencies recommend use of broad-spectrum UV radiation blockers with SPF of 30 or greater whenever you are going to be in the sun and that you periodically reapply sunscreen to ensure continuous protection. They also recommend you avoid the sun during the middle of the day and that you cover up skin with clothing and hats made of tightly woven sun blocking materials. They recommend you wear sunglasses that wrap around your face to protect not only your eyes but the skin around your eyes and temple. Finally, they recommend you avoid using tanning booths and sunlamps.
The U.S. Food and Drug Administration recently changed labeling rules for sunscreens to help consumers better understand what is in a sunscreen product and how to best use it to protect yourself from the harmful rays of the sun. Warnings have also been added for many sunscreens to avoid using them on damaged or broken skin. At this time, product labels are not required to identify whether the titanium dioxide in the product is formulated into nanoparticles.
There are many sunscreen products that work and you should choose the product that works best for you. Most important is that you choose a product that you will feel comfortable wearing EVERY TIME you go out into the sun. If you are concerned that nanoscale titanium dioxide is a risk for you, choose a sunscreen product that does not have titanium dioxide as an ingredient. The important point is that you use SOME form of sunscreen along with other measures to protect your skin.
Dr. Portier is managing director of the statistics and evaluation center and an expert in assessing environmental carcinogens at the American Cancer Society.