Researchers at the Paul Scherrer Institute PSI have found photochemical processes inside the smallest PM2.5 particles in the air. At the same time, they found that additional oxygen radicals are formed in these aerosols in everyday life, which can be harmful to human health.

It is well known that PM2.5 particulate matter suspended in the air can pose a health hazard. Particles with a maximum diameter of ten micrometers can penetrate deep into the lung tissue and settle there. They contain reactive oxygen species called oxygen radicals, which can damage lung cells. The more particles in the air, the higher the risk.

From previous research, free radicals are formed in the human body when particles dissolve in the surface fluid of the respiratory tract. Particulate matter typically contains chemical constituents such as metals such as copper and iron and certain organic compounds. They exchange oxygen atoms with other molecules to 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 that can no longer serve as building blocks for cells. Doctors refer to such processes as pneumonia, asthma, and other respiratory diseases. Even cancer can be triggered, as free radicals can also damage the DNA of the genetic material.

Thanks to the bright X-ray light from the Swiss SLS light source, scientists were able to not only look at such particles individually with a resolution of less than one micrometer but even peered inside the particles while reactions were taking place inside them. To do this, they also used a new type of cell in which a wide variety of atmospheric conditions can be simulated. It can accurately regulate temperature, humidity, and gas exposure and has an ultraviolet LED light source that replaces solar radiation.

The researchers studied particles containing organic compounds and iron. Iron comes from natural sources such as desert dust and volcanic ash and emissions from industry and transportation. Organic components come from both natural and anthropogenic sources. These components combine to form iron complexes in the atmosphere, which then react to so-called radicals when exposed to sunlight. They, in turn, bind all available oxygen and thus generate free radicals.

Usually, on a humid day, most of the free radicals diffuse from the particles into the air. In this case, there is no more additional danger if we inhale particles that contain fewer free radicals. However, on a dry day, these radicals build up inside the particles and consume all available oxygen in seconds. And this is due to viscosity: solids can be solid, like rock or liquid, like water, but depending on temperature and humidity, they can also be semi-liquid.

It is especially alarming that the highest concentrations of free radicals are formed by the interaction of iron and organic compounds in everyday weather conditions: on average, below 60% and temperatures around 20 degrees Celsius, which is also typical for indoor environments.

“It used to be that free radical formed in the air, if at all when fine dust particles contain relatively rare compounds such as quinones. These are oxidized phenols that are found, for example, in plant and fungal pigments. It has recently become clear that there are many other sources of free radicals in particulate matter. These known radical sources can be greatly enhanced in perfectly normal everyday conditions as we have now determined. About one in every twentieth particle is organic and contains iron. ”

Peter Aaron Alpert, PSI researcher at the Paul Scherrer Institute

The same photochemical reactions are likely to occur in other fine dust particles. Scientists suspect that almost all suspended particles in the air in this way form additional radicals. If this is confirmed by further research, researchers urgently need to adapt models and critical air quality values. This finding could be an additional factor that can help explain why so many people develop respiratory disease or cancer for no specific reason.

In any case, free radicals have one positive side, as the study also suggests: they attack bacteria, viruses, and other pathogens present in aerosols and render them harmless. This link may explain why the SARS-CoV-2 virus has the shortest survival time in the air at room temperature and moderate humidity.