Originally posted on 2020 Science, 25/8/09:
Could using sunscreen lead to Alzheimer's, Parkinson's, or other neurodegenerative diseases? The association seems far-fetched - given the amount of sunscreens, creams and lotions used every day, surely someone would noticed a link by now if it existed! Yet a press release from the University of Ulster suggests the nanoparticles used in some sunscreens could potentially cause or exacerbate these diseases. Drawing on the release, a number of media outlets are now running stories along the lines of "Sunscreen could cause Alzheimer's" (this from The Daily Mirror in the UK).
This is a rather unfortunate case of a poorly conceived press release leading to sensationalist - and misleading - headlines. The press release is associated with new research funded under the umbrella of NeuroNano - a European project focused on developing nanoscale neuro-implants that will enhance the functioning of the brain. However this new project, being led by Professors Vyvyan Howard and Dr. Christian Holscher at the University of Ulster, is focusing on how nanomaterials inadvertently entering the brain could cause damage.
The basis of their research is actually quite reasonable. There is some evidence that exposure to specific types of nanometer-scale particles could lead to them entering the brain, either by traveling up the nerves connecting the nose to the brain, or by crossing over from the blood. If insoluble nanoparticles do get into the brain they are likely to stick around for a while, as there are limited ways in which the body is able to get rid of foreign material from here. While there, they could damage neurons by causing the release of reactive oxygen species (ROS) - highly active chemicals. And there is also research showing that some nanoparticles can cause the type of protein misfolding that has been associated with neurodegenerative diseases like Alzheimer's - although this was carried out outside the body, under conditions set up to favor misfolding.
These tantalizing snippets of information are like a red rag to a bull as far as scientists go - they suggest there is new knowledge waiting to be discovered; knowledge that could help prevent some forms of brain disease. Together, they form a sound reason for carrying out more research.
But in no way do they link sunscreens to Alzheimer's!
The sunscreen link comes about because a number of these lotions use insoluble nanoparticles as the active ingredient. The thought-process then goes something like this:
The nanoparticles of titanium dioxide or zinc oxide in a sunscreen could conceivably get into someone's body, by passing through the skin, being eaten, or being inhaled. Once there, they might be able to get into the blood. From there, there is a chance that they could pass over into the brain. Or they might even be inhaled and travel straight up the olfactory nerve and into the brain. And once there, they could cause vital proteins to misfold that then lead to diseases like Alzheimer's.
But while this makes an interesting story and a compelling grant proposal, it has little bearing on reality as we currently understand it:
Most research suggests nanoparticles in sunscreens don't pass through the skin. Even if you could snort sunscreens (a feat in itself), studies showing nanoparticles traveling from the nose to the brain have used rodents not humans - and human noses are very different; they don't offer the same opportunities for significant exposure through this route. It takes a very special type of nanoparticle to cross the blood-brain barrier - you can't get any old junk across it. And research into nanoparticle-induced protein misfolding is at a very preliminary stage - any associations between effects seen in test tubes and brain disease are little more than speculative.
More to the point, we are exposed to billions of nanoparticles each day in the air we breathe; from car exhausts, fires, even sea spray. If any nanoparticles are going to find their way to our brains in large numbers, it will be these - not those used in some sunscreens.
This is not to detract from the importance of this new research project. If there are links between nanoparticle exposure and neurodegenerative diseases, we need to know.
But linking sunscreens to Alzheimer's in the absence of any hard scientific data? Given what we currently know, that just seems irresponsible!
For more information...
Information on the NeuroNano program can be found here
Nanoparticles traveling from the nose to the brain: There have been a number of studies showing that this is possible in rodents, although little is known about how many particles are likely to get to the brain after being inhaled. Three useful papers are:
Oberdörster, G., Z. Sharp, V. Atudorei, A. Elder, R. Gelein, W. Kreyling and C. Cox (2004). "Translocation of inhaled ultrafine particles to the brain." Inhal. Toxicol. 16(6-7): 437-445.
Elder, A., R. Gelein, V. Silva, T. Feikert, L. Opanashuk, J. Carter, R. Potter, A. Maynard, J. Finkelstein and G. Oberdorster (2006). "Translocation of inhaled ultrafine manganese oxide particles to the central nervous system." Environmental Health Perspectives 114(8): 1172-1178.
Oberdörster, G., V. Stone and K. Donaldson (2007). "Toxicology of nanoparticles: A historical perspective." Nanotoxicology 1(1): 2 - 25.
For information on nanoparticles and protein misfolding, the following is a key paper:
Linse, S., C. Cabaleiro-Lago, W.-F. Xue, I. Lynch, S. Lindman, E. Thulin, S. E. Radford and K. A. Dawson (2007). "Nucleation of protein fibrillation by nanoparticles." Proc. Natl. Acad. Sci. U. S. A. 104: 8691-8696.
The Mexico City study mentioned in the University of Ulster press release is:
Calderon-Garcidueñas, L., B. Azzarelli, H. Acune, R. Garcia, T. M. Gambling, N. Osnaya, S. Monroy, M. R. DEL Tizapantzi, J. L. Carson, A. Villarreal-Calderon and B. Rewcastle (2002). "Air Pollution and Brain Damage." Toxicol Path 30(3): 373-389.
When it comes to crossing the blood brain barrier, there has been a lot of research on engineering nanoparticles to do exactly this - for delivering drugs. Most research has shown that fancy materials science and chemistry are needed to engineer nanoparticles to move across the barrier - it's pretty effective at keeping bad stuff out of the brain. However, there are indications that small quantities of very small nanoparticles could inadvertently cross over from the blood - more more research is needed to understand whether early findings have any significance though.
Less is known about the possibility of ingested nanoparticles getting into the bloodstream. Again, the barrier between the guts and the blood is a complex one, and it is unlikely that any old nanoparticle will be able to fool the body into getting where it isn't wanted. But this is an area where more research would be useful.
For more info on nanoparticles and sunscreens, check out Industry critics give nanotechnology sunscreens the thumbs up
For more papers on nanoparticles and the brain, check out the nanoEHS Virtual Journal