Microscopic pollutant particles given off by traffic and industry can enter the bloodstream and the brain after being inhaled, scientists have found.
By Alex Kirby
BBC News Online environment correspondent
The particles are known to cause lung damage in susceptible patients, and are implicated in cardiovascular disease.
Experiments on rats and humans have now discovered they can penetrate further into the body, with unknown results.
UK scientists are calling for vigilance over the finding, and over the possible effects of a new group of particles.
These objects are being created in the burgeoning field of nanotechnology, in which atoms and molecules in materials are manipulated to exploit novel and sometimes unusual properties.
Mindful of the potential health impact of such particles, the UK Government has asked the Royal Society and the Royal Academy of Engineering to review the current status of the science and assess if there is a need for new regulation.
Many of us are routinely exposed to particles from diesel vehicle fumes (these are normally known as PM10, from their size), which penetrate buildings and are ubiquitous in cities.
Some cooking stoves emit high levels of particles as well.
There is also occupational exposure for people making products like sunblock cream, inks, photocopier toners, and working with welding equipment.
Ken Donaldson, Professor of Respiratory Medicine at the University of Edinburgh, UK, said these sorts of particle were known to cause damage at the point of entry to the human body.
What was new, he said, was the discovery by researchers in Europe and the US that they "can get to areas that bigger particles cannot reach".
Patients who inhaled radioactive ultrafine carbon particles displayed traces of it in their bloodstream not long afterwards.
Experimental rats which inhaled similar particles showed a marked decline in particulate level in their lungs after six or seven days.
In the olfactory bulb and other parts of the brain, though, levels (although lower than in the lungs) remained relatively stable over the same period.
These very small pieces of matter are called nanoparticles, defined as anything smaller than 100 nanometres in size. A nanometre is one-billionth of a metre, 80,000 times smaller than a human hair.
Professor Donaldson said: "We are already exposed to nanoparticles of different kinds. We already recognise that there is some ill-health associated with these exposures.
"But they may also translocate away from their point of entry into the blood or the brain. We are not sure what the consequences of this are yet.
"The nanotechnology revolution may design particles that are very different chemically from the ones we are exposed to, and they might have very different properties that made them more harmful. We should be vigilant."
He told BBC News Online: "I think there could be an increased future risk for all of us, and also a higher risk for people exposed at present to nanoparticles at work, though it's impossible to say how much bigger their risk is.
"These particles are not things you can trap with a filter. But they do disperse rapidly, unlike asbestos."
Nanotechnology involves building working devices, systems and materials molecule by molecule, and exploiting the unique and powerful electrical, physical and chemical properties found at that scale.
It has developed from advances in microscopy, materials science, molecular-level manipulation, and the relationship between classical and quantum physics.
The UK's Royal Microcopical Society and the Institute of Physics are holding a conference on the health implications of nanoparticles on 13 and 14 January at the Daresbury Laboratories in northern England.