Particles are more dangerous than previously thought

Markus Ammann on one of the devices used to test fine dust. Credit: Paul Scherrer Institute / Markus Fischer

Researchers at the Paul Scherrer PSI Institute first observed photochemical processes within the smallest particles in the air. At the same time, they discovered that additional oxygen radicals are formed in these aerosols in everyday conditions, which can be harmful to human health. They report on their results today in a magazine Nature Communications.

It is well known that particles in the air can pose a danger to human health. The particles, with a maximum diameter of ten micrometers, can penetrate deep into the lung tissue and settle there. They contain reactive oxygen species (ROS), also called oxygen radicals, which can damage lung cells. The more particles float in the air, the greater the risk. Particles enter the air from natural sources such as forests or volcanoes. But human activities, for example in factories and transport, multiply quantity so that concentrations reach a critical level. The potential of particles to bring oxygen radicals into the lungs or create them there has already been tested for various sources. Now PSI researchers have gained important new insights.

From previous research, it is known that some ROS are formed in the human body when particles dissolve in the surface fluid of the respiratory tract. Particles usually contain chemical components, such as metals such as copper and iron, as well as certain organic compounds. They exchange oxygen atoms with other molecules and form highly reactive compounds, such as hydrogen peroxide (H2O2), hydroxyl (HO) and hydroperoxyl (HO2), which cause so-called oxidative stress. For example, they attack unsaturated fatty acids in the body, which can then no longer serve as a building block for cells. Doctors attribute pneumonia, asthma and various other respiratory diseases to such processes. Even cancer could be triggered, because ROS can also damage the DNA of genetic material.

New insights thanks to a unique combination of devices

It has been known for some time that some reactive types of oxygen are already present in the particles in the atmosphere and that they enter our body as so-called exogenous ROS through the air we breathe, without having to form there first. As it now turns out, scientists have not yet observed carefully enough: “Previous studies have analyzed particles using mass spectrometers to see what it consists of,” explains Peter Aaron Alpert, the first author of the new PSI study. “But it doesn’t give you any information about the structure of individual particles and what happens in them.”

In contrast, Alpert took advantage of the possibilities offered by PSI to take a closer look: “With bright X-ray light from the Swiss SLS light source, we could not only individually view such particles with a resolution of less than one micrometer, but even look at particles while in them. In order to do that, he also used a new type of cells developed in PSI, in which a wide range of atmospheric environmental conditions can be simulated. It can precisely regulate temperature, humidity and exposure to gases, and it also has an ultraviolet LED light source that represents solar radiation. “Combined with high-resolution X-ray microscopy, this cell exists in only one place in the world,” says Alpert. The study would therefore only be possible at PSI. He worked closely with the head of the Surface Chemistry Research Group at PSI, Marcus Ammann. He also received support from researchers working with atmospheric chemists Ulrich Krieger and Thomas Peter at ETH in Zurich, where additional experiments with suspended particles were conducted, as well as from experts working with Hartmut Hermann of the Leibniz Institute for Tropospheric Research in Leipzig.

Particles are more dangerous than previously thought

Peter Aaron Alpert analyzes the processes in the finest particles in air. Harmful substances are already being created in the atmosphere, not just in the human body. Credit: Paul Scherrer Institute / Markus Fischer

How dangerous compounds are formed

The researchers examined particles that contain organic components and iron. Iron comes from natural sources such as desert dust and volcanic ash, but it is also found in emissions from industry and traffic. Organic components also come from natural and anthropogenic sources. In the atmosphere, these components combine to form iron complexes, which then react to so-called radicals when exposed to sunlight. They in turn bind all available oxygen and thus produce ROS.

Typically, on a wet day, a large proportion of these ROS would diffuse from the particles into the air. In that case, it no longer poses an additional danger if we inhale particles that contain less ROS. On a dry day, however, these radicals accumulate inside the particles and consume all available oxygen there within a few seconds. And that’s because of the viscosity: Particles can be solid like stone or liquid like water – but depending on temperature and humidity, they can also be semi-liquid like syrup, dried chewing gum or Swiss herbal throat drops. “This state of the particles, we found, ensures that the radicals remain trapped in the particle,” says Alpert. And no extra oxygen can enter from the outside.

It is especially alarming that the highest concentrations of ROS and radicals are formed by the interaction of iron and organic compounds in everyday weather conditions: with an average below 60 percent and temperatures around 20 degrees C., also typical for indoor conditions. “ROS used to be thought to form only in the air – if at all – when fine dust particles contain relatively rare compounds like quinone,” says Alpert. These are oxidized phenols that occur, for example, in plant pigments and fungi. It has recently become clear that there are many other sources of ROS in particles. “As we have now established, these well-known radical sources can be significantly strengthened in quite normal everyday conditions.” Approximately every twentieth particle is organic and contains iron.

But that’s not all: “The same photochemical reactions probably take place in other fine dust particles,” says research group leader Markus Ammann. “We even suspect that almost all suspended particles in the air create additional radicals in this way,” adds Alpert. “If this is confirmed in further studies, we urgently need to adjust our models and critical values ​​with respect to air quality. We may have found an additional factor here that will explain why so many people develop respiratory disease or cancer without any specific cause.”

At least ROS have one positive side – especially during the COVID-19 pandemic – as the study also suggests: They also attack bacteria, viruses and other pathogens present in aerosols and make them harmless. This relationship may explain why the SARS-CoV-2 virus has the shortest survival time in air at room temperature and medium humidity.

Superoxide produces hydroxyl radicals that decompose soluble organic matter in water

More information:
Photolytic radical stability due to anoxia in viscous aerosol particles, Nature Communications, DOI: 10.1038 / s41467-021-21913-x

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Citation: Particles are more dangerous than previously thought (2021, March 19) downloaded March 19, 2021 from

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