A new study shows that a specific type of silicone, the so-called methylsiloxanes, are widely present in the atmosphere across diverse environments. Also, concentrations appear to be much higher than expected. According to the researchers, this raises concerns about their potential, yet poorly understood, effects on human health and the climate. Methylsiloxanes, are commonly used in industry, transportation, cosmetics and household products. The study was supervised by Utrecht University and the University of Groningen, and the results were published today in Atmospheric Chemistry and Physics .
Synthetic pollutants, such as PFAS and microplastics, are notoriously pervasive in the environment. One class of these synthetics, a water-repellent lubricant named methylsiloxanes, received little attention. Whenever it was detected in the atmosphere, researchers assumed it came from the evaporation of methylsiloxanes in personal care and industrial products. However, a few years ago, researchers found that ships and vehicles emit a substantial amount of a different variant of methylsiloxanes, consisting of large molecules that do not evaporate.
High concentrations
In this new study, researchers discovered that these large molecular methylsiloxanes are commonly present in the atmosphere, not only in areas close to traffic, but across diverse environments, including urban, coastal, rural, and forest sites.
“The findings also suggest that concentrations of methylsiloxane in the atmosphere are much higher than expected”, says Rupert Holzinger , associate professor at Utrecht University who co-supervised the study. In fact, large molecular methylsiloxanes appear to be one of the most concentrated synthetic compounds in the atmosphere, making up 2 to 4.3 percent of the total mass of organic aerosols. Atmospheric PFAS concentrations are typically more than three orders of magnitude lower than those observed for methylsiloxanes.
Cities and forests
Concentrations were highest in urban areas, with 98 nanograms per cubic meter in samples taken from the large metropolitan area of São Paolo, Brasil. Concentrations were lowest in forest sites, with 0.9 nanograms per cubic meter in Rugsteliskis, Lithuania. In the Netherlands, samples were taken at Cabauw, a small rural village. Here, researchers measured concentrations of 2 nanograms per cubic meter.
The air samples came from various regions in the Netherlands, Lithuania, and Brazil. This combination provides a reasonably broad geographical and socioeconomic representation, because it spans different climatic regions and hemispheres, and includes both developed and emerging economies.
Unknown health risks
Since methylsiloxanes appear to be omnipresent in the atmosphere, the study implies that humans are continuously exposed to them and inhale considerable amounts. Whether there are any health risks involved, is largely unknown. “However, we estimate that the daily inhalation dose of methylsiloxanes may exceed that of other synthetic compounds, such as PFAS and micro- and nanoplastics”, says Holzinger. “We therefore underscore the urgent need for the evaluation of these health impacts.”
In addition to potential effects on human health, large molecular methylsiloxanes in the atmosphere may influence climate change and environmental sustainability. They can alter aerosol properties, which in turn affects aerosol behaviour and their climatic impact. For example, methylsiloxanes can modify aerosol surface tension, influencing the role they play in the formation of clouds. They may also interfere with ice nucleation, further affecting cloud formation and atmospheric processes.
Long distance transport
Over half of the detected particles of large molecular methylsiloxanes originate from traffic emissions. Further research suggests that the exact source is lubricant-related, such as engine oil. The researchers came to this conclusion because they found that large molecular methylsiloxanes have a similar dispersion pattern as long-chain hydrocarbons, which are commonly present in engine oil. These similarities strongly suggest that the compounds share the same emission source.
Interestingly, the concentration of these long-chain hydrocarbons does decrease strongly during atmospheric transport and dilution, whereas the concentration of methylsiloxane does not. In fact, a substantial fraction persists as large molecular methylsiloxanes. According to the researchers, this underscores that large molecular methylsiloxanes are chemically very stable and likely to be transported over long distances.
[box] How is engine oil emitted?
Methylsiloxanes used as lubricant additives are generally assumed to function solely in lubrication and are not intended for combustion. However, during engine operation, components such as pistons require lubrication, making it inevitable that a fraction of engine oil enters the combustion chamber. Due to their thermal stability and resistance to complete combustion, methylsiloxanes can survive the high-temperature conditions and be emitted with the exhaust gases.
Atmospheric Chemistry and Physics
Data/statistical analysis
Not applicable
Widespread occurrence of large molecular methylsiloxanes in ambient aerosols
16-Apr-2026