Tiny particles used in some sun protection creams have the potential to cause nerve damage, researchers in the US say. It may 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, Nature magazine said reporting the latest findings.
But it also warned that a growing body of evidence suggests their safety cannot be taken for granted simply because larger particles of same substance have no ill-effect.
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 found 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. Gunter Oberdwrster, a specialist in nanoparticle toxicity at University of Rochester in New York, said it was too early to say if 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," he was quoted as saying in Nature. Nanoparticles are fragments of a material just a few nanometers (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 the sun protection 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, Nature said, 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," said Oberdwrster, "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, the report said.
The chemical reactivity of powders depends on their surface area, which increases as the particles get smaller. But the behaviour of small particles, the magazine said, can also be altered by more exotic influences. For instance, quantum-mechanical effects make the colour of light-emitting nanoparticles change with their size.
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," Oberdwrster said, 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.
However, nothing was previously known about their effects on brain cells, however, Veronesi said.
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