Fine dust - more dangerous than thought

It is well known that fine dust can endanger health. The particles with a maximum diameter of 10 micrometers can penetrate deep into the lung tissue and settle there. They contain reactive oxygen compounds (ROS), also called "oxygen radicals," which can damage the cells of the lungs. The more particles suspended in the air, the higher the risk. Particles also enter the air from natural sources such as forests or volcanoes. But human activities, such as in factories and traffic, multiply the amount so that concentrations of concern are reached. The potential of particulate matter to carry oxygen radicals to or generate them in the lungs has already been studied for various sources. Researchers at the Paul Scherrer Institute (PSI) have now gained important new insights into this.

Previous research has shown that some ROS are formed in the human body when particulate matter dissolves in the surface fluid of the respiratory tract. Particulate matter usually contains chemical constituents, such as metals like copper and iron, and certain organic compounds. These exchange oxygen atoms with other molecules, producing highly reactive compounds such as hydrogen peroxide (H2O2), hydroxyl (HO) or hydroperoxyl (HO2), which cause so-called oxidative stress. For example, they attack the unsaturated fatty acids in the body, which can then no longer serve as building blocks of the cells. Doctors attribute pneumonia, asthma and various other respiratory diseases to such processes. Even cancer could be triggered, since the ROS can also damage the genetic material DNA. 

New insights thanks to unique device combination

It has been known for some time that certain ROS species are already present in atmospheric particulate matter and enter our bodies via the air we breathe as so-called exogenous ROS, without having to form there first. It now turns out that we have not yet looked closely enough: "Previous studies have used mass spectrometers to analyze what fine dust consists of," explains Peter Aaron Alpert, lead author of the new PSI study. "But that doesn't give you any information about the structure of the individual particles and what's going on inside them."

Alpert, on the other hand, took advantage of PSI's capabilities for a more precise look: "With the brilliant X-ray light from the Swiss Synchrotron Light Source SLS, we were not only able to view such particles individually with a resolution of less than a micrometer, but even to look inside them while reactions were taking place." To do this, he also used a novel cell developed at PSI that can simulate a wide range of atmospheric environmental conditions. It can precisely regulate temperature, humidity as well as gas exposure, and a UV LED light source mimics solar radiation. "This combination - high-resolution X-ray microscope and cell - exists only once in the world," Alpert says. The study was therefore only possible at PSI. He worked closely with Markus Ammann, head of the surface chemistry group at PSI. He was also supported by researchers led by atmospheric chemists Ulrich Krieger and Thomas Peter at ETH Zurich, where additional experiments were conducted with particles held in suspension, and by experts led by Hartmut Hermann from the Leibniz Institute for Tropospheric Research in Leipzig.

How dangerous compounds are formed

The researchers examined particles with organic components and iron. The iron comes from natural sources such as desert dust or volcanic ash, but is also present in emissions from industry and traffic. The organic components also result from natural and man-made sources. In the atmosphere, these components combine to form iron complexes, which then react under solar radiation to form so-called radicals. These in turn bind all available oxygen and thus produce the oxygen compounds.

Normally, a larger part of these ROS would diffuse out of the particles into the air in the warmth of the sun and no longer pose a danger when we inhale the particles, which then contain less ROS. If the conditions are right, however, the radicals accumulate inside the particles and consume all the available oxygen there within seconds. And this is due to the so-called viscosity: fine dust can be as solid as stone or as liquid as water - but depending on temperature and humidity, it can also be as viscous as syrup, chewing gum or Swiss herbal sugar. "This state of the particle, we have found, ensures that the ROS remain trapped in the particle," Alpert says. And no additional oxygen gets in from the outside either.

What is particularly frightening is that the interaction of iron and organic compounds causes the highest concentrations of ROS to form in everyday weather conditions: at average humidity of 50 percent and temperatures around 20 degrees, such as those found indoors. "It used to be thought that ROS in the air were formed - if at all - only when particulate matter contained comparatively rare compounds such as quinones," Alpert says. These are oxidized phenols found, for example, in dyes of plants and fungi. Recently, it has become clear that many other sources of ROS are present in particulate matter. "As we now found, these known ROS sources can be significantly amplified under completely everyday conditions." About one in twenty particles is organic and contains iron.

But that's not all: "We assume that the same photochemical reactions also take place in other fine dust particles," says research group leader Markus Ammann. "We even suspect that almost all airborne particles form additional radicals in this way," adds Alpert. "If this is confirmed in further studies, we urgently need to adjust our models and limit values with regard to air quality. We may have found an additional factor here for the fact that so many people fall ill with respiratory diseases or cancer seemingly without any specific cause."

After all, the ROS - especially in times of the COVID-19 pandemic - also have their good side, as the study also suggests: They also attack and render harmless bacteria, viruses and other pathogens that sit on top of aerosols. This connection could explain why the Sars-CoV-2 virus survives the shortest in air at room temperature and medium humidity.