Apart from being an environmental polluter, a new study has revealed that soot particles present in the exhaust fumes of diesel vehicles, coal power stations and wood fires pose a serious health risk to humans.
Every time we breathe, we inhale these tiny particles in the atmosphere.
Now researchers have for the first time shown that more than half of all inhaled soot particles gets stuck in the lungs.
The figure is higher than for most other types of particles. For example "only" 20 per cent of another type of particle from wood smoke and other biomass combustion gets stuck in the lungs.
One explanation is that diesel soot is made up of smaller particles and can therefore penetrate deeper into the lungs, where it is deposited.
The study was made on diesel particles (which mainly consist of soot). Ten healthy people volunteered for the study.
"Findings of this kind can be extremely useful both for researchers to determine what doses of soot we get into our lungs out of the amount we are exposed to, and to enable public authorities to establish well-founded limits for soot particles in outdoor air," said Jenny Rissler, researcher in aerosol technology at Lund University's Faculty of Engineering and responsible for publishing the study.
In population studies, other researchers have been able to observe that people who live in areas with high concentrations of particulates are more affected by both respiratory and cardiovascular diseases. But since there is no conclusive evidence that it is precisely the soot that is to blame, the authorities have so far not taken any decisions on guidelines.
"Currently there is no specific limit for soot particles in the air, despite the fact that soot in the air is linked to both lung cancer and other diseases," noted Rissler.
But she thinks that in the future, limits on soot levels will also be set, with reference to the WHO's recent reclassification of diesel exhaust from "probably carcinogenic" to "carcinogenic".
Soot particles are not only connected to effects on health but may also contribute to a warmer climate. Paradoxically, other types of aerosol particles can partly be desirable, insofar as they have a cooling effect on the climate and thereby mitigate the warming effect of carbon dioxide.
"Soot particles are black and absorbs light, thus producing a warming effect. So it could be a double advantage to reduce it," she stated.
Rissler will next be studying individual variations in lung deposition and exposing cells to soot. She is also in the process of further developing methods to measure the surface area of the particles, as this has shown to be an important indicator of their harmfulness.
The study was recently published in the Journal of Aerosol Science.