The World Health Organisation (WHO) has classified diesel soot as a
lung carcinogen. Artur Braun, a physicist at Empa and an X-ray spectroscopy expert, has made crucial
contributions to analyzing the structure and composition of soot particles.
Soot particles are dangerous – there is nothing new in this knowledge. But what is it that makes fine
particulates dangerous? Is it only diesel soot from vehicle engines? Does the danger also come from woodburning
stoves in holiday chalets? Or even from grease-laden fryer fumes from the restaurant around the
corner? For a long time, these questions have been a hard nut for science to crack. Indeed, fine soot particles
were collected in filters and their chemical components were analysed. Yet the question remained: what
precisely is the source of the danger? Is it the soot particles themselves that make people ill? Or is it toxic
chemicals the soot carries with it – like a wet sponge?
Not all smoke is created equal
The Norwegian Institute of Public Health wanted to investigate this matter and asked Empa scientist Artur
Braun for support. Before joining Empa, Braun had worked at the University of Kentucky and there, in 2002,
he analyzed soot particles for the first time on a synchrotron using soft X-rays. Result: diesel particles that
have been "born" in the engine under high pressure and immense heat have a graphite structure – this is
clearly visible under X-ray light. In the case of soot particles from wood fires, which have been generated
under mild atmospheric conditions, this graphite structure is absent. The functional groups are also different:
diesel soot was found to contain carboxyl groups such as those occurring in formic and acetic acid
molecules; in the wood smoke, Braun found hydroxyl groups as in ethanol and methanol. There is thus a fine
difference between smoke and smoke.
Analyse separately, fight together
The Norwegian toxicologists then went a step further and asked Braun's colleagues at the University of North
Dakota to isolate the soot particles from the adherent chemical toxic substances using solvents. Braun then
analysed the components individually under X-ray light: first the "bare" soot particles, then the solution with
the suspected carcinogenic chemicals previously bound to the soot. Braun again found various functional
groups on the carbon structure and was able to compare them with the findings of his earlier work.
At the same time, the toxicologists tested the effect of the two soot fractions on human lung cells in culture.
For the first time separate investigations had been carried out to establish what is so dangerous in soot. The
study, which recently appeared in the journal "Toxicology Letters", is, in Braun's opinion, the first to combine
the methods of X-ray absorption spectroscopy (NEXAFS) with toxicological methods.
The WHO response
The results of the study were quite unambiguous: The "bare" soot particles triggered a genetic detoxification
mechanism in the cell cultures. The cells had therefore been under “toxic attack”. However, the washed out
substances previously adhering to the soot also exhibited an effect: they caused inflammatory reactions in
the cells and also acted as a cellular toxin. The World Health Organization (WHO) responded simultaneously.
A number of new studies – including those by Braun and his colleagues from Norway and the USA – had
indicated the carcinogenic effect of soot and sufficiently explained the underlying mechanisms.
It was now no longer possible to speak, as had been the case since 1988, of a probable risk of cancer
("probably carcinogenic to humans"). Reclassification followed on 12 June 2012. Diesel soot is now
considered a cause of lung cancer "based on sufficient evidence"; what’s more, there is a certain probability
that diesel soot also increases the risk of bladder cancer.
X-ray research at Empa – measurements in Berkeley and Stanford
Physicist Artur Braun – after his " assistance" in the field of health research – is resuming his duties as group
leader in Empa's High Performance Ceramics Laboratory, a position in which he also continues to work on
synchrotrons in the USA and in Europe. He is regularly at the ALS radiation source in Berkeley (California) and
at the Stanford synchrotron (SSRL) for measurements. For Empa, the expert uses synchrotron radiation
methods for materials research into energy storage devices and converters.
Currently, there is another publication in preparation on the subject of fine particulates from wood
combustion, to which Braun has also made crucial contributions. The cooperation between the disciplines will
not end there. According to Braun, "The medical scientific potential of synchrotron methods for analyzing the
biological interaction of cells with pathogenic substances is still far from being exhausted".
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