Sunscreen ingredient may pose skin cancer risk, researchers find

May 8, 2012, Missouri University of Science and Technology

( -- As vacationers prepare to spend time outdoors this summer, many of them will pack plenty of sunscreen in hopes it will protect their bodies from overexposure, and possibly from skin cancer. But researchers at Missouri University of Science and Technology are discovering that sunscreen may not be so safe after all.

Cell toxicity studies by Dr. Yinfa Ma, Curators' Teaching Professor of chemistry at Missouri S&T, and his graduate student Qingbo Yang, suggest that when exposed to sunlight, zinc oxide, a common ingredient in sunscreens, undergoes a chemical reaction that may release unstable molecules known as free radicals. Free radicals seek to bond with other molecules, but in the process, they can damage cells or the DNA contained within those cells. This in turn could increase the risk of .

Ma also found that the longer zinc oxide is exposed to sunlight, the greater the potential damage to human cells.

"Zinc oxide may generate free radicals when exposed to UV (ultraviolet) sunlight," May says, "and those free radicals can kill cells."

Ma studied how human lung cells immersed in a solution containing nano-particles of zinc oxide react when exposed to different types of light over numerous time frames. Using a control group of cells that were not immersed in the zinc oxide solution, Ma compared the results of light exposure on the various groups of cells. He found that zinc oxide-exposed cells deteriorated more rapidly than those not immersed in the chemical compound.

Even when exposed to visible light only, the lung cells suspended in zinc oxide deteriorated. But for cells exposed to ultraviolet rays, Ma found that "cell viability decreases dramatically."

When exposed to ultraviolet long-wave light (ultraviolet A or UVA) for 3 hours, half of the lung cells in the zinc oxide solution died. After 12 hours, 90 percent of the cells in that solution died, Ma found.

Why does zinc oxide, an ingredient used in to help block harmful UV rays, cause cells to deteriorate when exposed to sunlight? According to Ma, when the zinc oxide nano-particles in the solution absorb the UV rays, the reaction releases electrons, which in turn may produce unstable free radical molecules in the zinc oxide solution. Those free radical molecules then bond with other molecules and act as parasites, damaging the other molecules in the process.

Ma and his colleagues at Missouri S&T published their initial research results in the January 2009 edition of the Journal of Nanoparticle Research. The paper, titled "Toxicity of nano- and micro-sized ZnO particles in human lung epithelial cells," was the first comprehensive study ever published on this subject. Lead author Weisheng Lin was Ma's Ph.D. student at the time. Other authors of the paper were Chuan-Chin Huang, a graduate student in biological sciences at the time, and two members of S&T's biological sciences faculty, Dr. Katie Shannon (assistant professor) and Dr. Yue-Wern Huang (associate professor).

Now, Ma is preparing to publish his latest research results in the journal Toxicology and Applied Pharmacology. A publication date has not been determined.

Ma's research on zinc oxide's effect on is still in the early stages, so he cautions people from drawing conclusions about the safety or dangers of sunscreen based on this preliminary research.

"More extensive study is still needed," May says. "This is just the first step."

For instance, Ma plans to conduct electron spin resonance tests to see whether zinc oxide truly does generate , as he suspects. In addition, clinical trials will be needed before any conclusive evidence may be drawn from his studies.

In the meantime, Ma advises sunbathers to use sunscreen and to limit their exposure to the sun.

"I still would advise people to wear sunscreen," he says. "Sunscreen is better than no protection at all."

Besides sunscreen, is used in many commercial products, including plastics, paints, ointments and sealants.

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4.7 / 5 (6) May 08, 2012
Testing the effects of ZnO nanoparticles and light on lung cells? How wrong is this?

Zinc Oxide is not normally inhaled.
Nanoparticles are known in many instances to cause DNA damage.
Lung cells are not normally exposed to UV radiation.

Why not test sun screen on skin cells where it is supposed to be used? This study is meaningless.
4.6 / 5 (5) May 08, 2012
Zinc Oxide is not normally inhaled.

And even if inhaled - it seldom gets exposed to sunlight in the lungs
(If it does you've got other problems to worry about - like finding the guy that just shot you).
not rated yet May 08, 2012
Are lung cells easily obtainable, or are they what is commonly used to test toxicity? Because if not, I cannot comprehend why, as earlier comments have pointed out, they would be selected for this research.
5 / 5 (2) May 08, 2012
Love the second comment, not so sure about the first... The lung cells are probably used simply because they are less hardy, we know zinc oxide in sunscreen doesn't kill all skin cells it touches even with heavy usage for a year. but that doesn't really tell us if it isn't killing or causing some cells to become cancerous. So like the Dr. said these studies are preliminary, but they do show a tendency for zinc oxide exposed to light to kill cells more readily than light alone. If Ma's hypothesis is correct that it is due to free radicals being created, than we can also state with certainty that zinc oxide exposed to light may actually cause skin cancer in some cases. of course all of these cases would currently be contributed to sun exposure, instead of the sun screen.....

if Zinc Oxide is to volatile in the sun and is actually causing cancer as well, than there are other albeit currently more expensive forms of sunblock that would be better to use.

5 / 5 (2) May 08, 2012
The study is hardly meaningless, it tells us that cells can die or possibly become cancerous if mutilated but live. It told us in a very cost effective manor, if nothing turned up in the lung cells, than great! we wouldn't have to study further, but when something does show up we can now spend additional resources to figure out what's going on.

let's say fish are dying in a stream at a faster rate than in other streams, fish always die in that stream, they always have, there is a farm next to the stream and we have a suspicion that something chemical (or biological) on the farm might be causing it.

do we whip out the mass spectrometer and take samples of the water and everything surrounding it right from the start.. or do we walk down with a strip of litmus paper and see if the ph is off first.
not rated yet May 08, 2012
It may be the "nano" bit that causing the problems. The properties of materials can change quite radically when they are finely divided. Old fashion ZnO sunscreen might be a lot safer.
not rated yet May 08, 2012
of course all of these cases would currently be contributed to sun exposure, instead of the sun screen.....

However it is easily demonstrable that without sunscreen the cancer rate is far higher.

Are lung cells easily obtainable, or are they what is commonly used to test toxicity
They are more sensitive to toxic substances and thus give you a better signal when doing tests
Lung cells need to be highly permeable to gases because they allow us to exchange O2 and CO2 with the blood. Skin cells are far less permeable (the outer skin layer is mostly dead cells, anyhow. these cells are entirely unaffected by radicals or other toxic stuff. Can't get more dead than dead.)

It's only after stuff goes deeper than the dead surface layer that you get an effect. Whether radicals penetrate that deep is something that still needs to be tested. I'd hazard that you'll get some kind of logarithmic curve of absorption by depth.

3 / 5 (2) May 08, 2012
I am wondering why they chose lung cells as well seeing as human skin cells for in vitro studies are readily available for under $1000. In fact you can obtain about any kind of human cell you want from companies like

So why in the world would they chose lung cells that never see sunlight?

The function of lung cells is entirely different that skin cells. Many things that would cause cancer in the lungs would never cause cancer on the skin.

I am betting they also used skin cells but saw no difference between subject group and the control group and so then went to lung cells to show something for their study. A study with nothing to show is not very interesting after all.
not rated yet May 09, 2012
"In addition, clinical trials will be needed before any conclusive evidence may be drawn from his studies."

Shouldn't they have done clinical trials *before* they put out a product with these claims? Aside from a small % of people who lack adequate amounts of melinin (sp?) in their skin, I have always been skeptical of the advantages of sun screen.
not rated yet May 09, 2012
Shouldn't they have done clinical trials *before* they put out a product with these claims?

No, because sunscreen is not a medical product.
The cancer preventing effect of sunscreen is well documented. There's two mechanisms at work here.
UV can alter the genome (because a UV photon has enough energy to ionize some atoms - e.g. it can split hydrogen bonds)
But there is also an indirect mechanisms. The body contains/creates at all times cancer cells. In places where the cell doesn't need to divide an individual cancer cell is also not much of a problem.

The problem arises when you get something like sunburn (which sunscreen prevents to a point). Sunburn means that living skin cells die (and get shed). They get replaced by other skin cells dividing and taking their place. Unfortunately cancer cells tend to be faster at dividing - so when the trigger comes to replace skin cells the cancer cells multiply/spread faster than others.

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