Nanoparticles in sun creams can stress brain cells
Tiny grains send cells into potentially dangerous overdrive.
Tiny particles used in some sun creams have the potential to cause neurological damage, researchers in the United States have found1.
The research does not necessarily imply that these microscopic grains, which are also used in consumer products such as some toothpastes and cosmetics, are harmful in the human body. But it adds to a growing body of evidence that suggests that their safety cannot be taken for granted simply because larger particles of the same substance have no ill effects.
Bellina Veronesi of the US Environmental Protection Agency's research laboratories in North Carolina and her co-workers have studied the effect of nanoparticles of titania (titanium oxide) on cultures of mice cells called microglia, which protect neurons in the brain from harm.
They find that the particles provoke the cells to manufacture chemicals that are protective in the short term but potentially damaging when released in the prolonged manner seen in the experiments.
Günter Oberdörster, a specialist in nanoparticle toxicity at the University of Rochester in New York, stresses that it is too early to say whether the findings reveal a real health hazard. "These are valuable results," he says, "but you have to be very careful about extrapolating them to live organisms."
Into the brain
Nanoparticles are fragments of a material just a few nanometres (millionths of a millimetre) in size. Titania is the white pigment used in paints, and is generally considered non-toxic. It has long been used as a fine powder in many sun creams because of its ability to absorb ultraviolet light.
Some of these creams use titania nanoparticles, which are so small that they appear transparent rather than white. This means that applying the creams on skin does not leave it looking pallid.
The chemicals industry has tended to assume that if large grains are safe, smaller ones will be too. But that assumption is coming under increasing scrutiny, and is not necessarily always valid. "In most cases nanoparticles are unlikely to be dangerous," says Oberdörster, "but we need to look at it on a case-by-case basis."
Scientists working with nanoparticles have known for a long time that size matters: at these very small scales, the properties of materials can change. For one thing, the chemical reactivity of powders depends on their surface area, which increases as the particles get smaller.
But the behaviour of small particles can also be altered by more exotic influences. Quantum-mechanical effects make the colour of light-emitting nanoparticles change with their size, for example.
Nanoparticles may also travel around natural environments, including the human body, in different ways to bigger particles. In particular, they can enter the brain from the bloodstream, whereas big particles cannot. "The blood-brain barrier is normally very tight," says Oberdörster, but nanoparticles can slip through. Many researchers now think that the safety of such particles should be examined as if they were completely new chemicals.
That caution seems to be warranted for titania nanoparticles. Previous studies have suggested that they might be toxic to various types of cell, such as skin, bone and liver cells. Veronesi says that nothing previously was known about their effects on brain cells, however.
Burst or bust?
The researchers used commercially available titania nanopartices about 30 nanometres across, which they added to cultures of mouse microglia. These cells protect neurons in the brain by engulfing foreign particles and releasing a burst of chemicals known as reactive oxygen species (ROS) to 'burn up' the invading substances.
This is a risky strategy, because ROS are also potentially damaging to neighbouring cells. It's a bit like releasing poison gas in a room containing invaders and hoping that it won't seep out into the rest of the building.
Veronesi and colleagues found that titania nanoparticles are swallowed by microglia and that they trigger the release of ROS not as a burst but in a prolonged manner, over an hour or more. That could subject the brain to so-called oxidative stress, which is thought to be the underlying cause of some neurodegenerative diseases such as Parkinson's and Alzheimer's.
It's too early to know how worrying the findings are. No one knows whether nanoparticles applied on the skin, inhaled or ingested can find their way to the brain, or at what concentrations. Effects seen for cultured mice cells might not be duplicated in living mice, let alone in the human body. And there is no firm evidence that this oxidative stress could damage neurons, although Veronesi says they have preliminary results showing that titania nanoparticles can trigger cell death in neurons.
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- Long T. C., et al. Envir. Sci. Technol, doi:10.1021/es060589n (2006).
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