Physicians at the 9th Nanoparticle Conference, that took place recently at ETH Zurich, highlighted, once again, the danger posed by minute particles in exhaust fumes. Large-scale US studies showed that cardiovascular and respiratory diseases with fatal consequences are directly linked to air pollution with nanoparticles.

Scientists are still investigating the question of how such fatal mechanisms happen in the body. One possible explanation is that nanoparticles cause small centres of infection in the lungs that contribute to the sclerosis of blood vessels. If these deposits then break away they build blood clots that can block coronary arteries and cause a heart attack, which, in the most serious case, can be fatal. A further hypothesis, however, is that aggressive nanoparticles could cause so-called oxidative stress, which the body cannot withstand for ever.

The blame for such suffering lies with motorised transport, especially with diesel engines that emit great quantities of polluting particles, such as soot particles (1). In Switzerland 3,300 people die prematurely every year from this kind of pollution. Tens of thousands of care days in hospital are necessary to mitigate the manifold kinds of health injuries.

No barriers for nanoparticles

There are hardly any barriers for ultra-fine particles. They overcome the natural barriers between the surface of the lungs and the underlying tissue. This is how nanoparticles reach other organs, such as liver or kidneys, as researchers at the University of Berne have proved.

In trials, rats are exposed to specific levels of titanium dioxide. This penetrated the lung tissue very quickly. The researchers found no sign of any membrane that might have encapsulated the mini-particles. Thesis: nanoparticles have access to all proteins, the enzymic systems and the DNA. If the nanoparticles enter the tissue, it seems that the body's immune system appears to fail. Phagocytes, such as macrophages, or dendrites, do not recognise the intruder as such–or only insufficiently–and let it through.

Lorries emit many dangerous tiny particles into the air. Efficient filters could help reduce them. (Picture: E. Ramseier) large

Nanoparticles, however, to not only enter the body via the lungs. Günter Oberdörster from the US University of Rochester, shows that they can also be absorbed via the mucous nasal membrane from where they can get to the brain or the central nervous system.

Filters for gross polluters

Engineers have a technical answer to the insidious contamination: particle filters. "New diesel engines with good filters trap the measurable particles very efficiently," says ETH Professor Konstantinos Boulouchos at the margins of the conference. The big question for toxicology, however, remains unanswered; namely, which particles are dangerous?

Switzerland plays a pioneer role in finding a solution to the problem. It was instrumental in developing a technique of measurement, said engineer Andreas Mayer who organised the conference. Such diesel particles were already investigated at ETH in 1970. This has led to the development of ever more sophisticated measuring techniques. "We have been able to measure diesel engine emissions for a long time," says Mayer. In fact, today, a spin-off company from ETH Zurich builds devices that are used in the EU for type tests.

Implementation challenges experts

From a technical point of view the route has been prepared. The big challenge for future, says Mayer, is with the political establishment and implementation. "Today we are able to filter out 99.99999 per cent of the nanoparticles in diesel exhaust gas and thus keep them from polluting the air," says the engineer. New diesel powered vehicles are serially fitted with such filters. The greatest problem lay with the adaptation of older diesel engines. The technical challenge was now to build efficient and reliable filters at low cost. However, even if they are still expensive at the moment, "the cost of adaptation can already be balanced today with six times higher benefit," says Mayer.