Scientists have identified surprisingly high levels of a little-known silicone pollutant in the atmosphere, raising new questions about possible risks to human health and the climate. The chemicals, known as methylsiloxanes, are commonly used in cosmetics, industrial products, transportation, and household items. Researchers found these compounds across a wide range of environments, from major cities to rural villages and forests.
The study was led by researchers from Utrecht University and the University of Groningen and published in the journal Atmospheric Chemistry and Physics.
Hidden Silicone Pollution Found Worldwide
Pollutants such as PFAS and microplastics are already known for their widespread presence in the environment. But methylsiloxanes, a class of water-repelling silicone compounds often used as lubricants, have received far less attention.
For years, scientists believed the methylsiloxanes detected in the atmosphere mainly came from evaporation from personal care products and industrial materials. More recently, however, researchers discovered that ships and motor vehicles release a different form of methylsiloxanes made up of much larger molecules that do not easily evaporate.
The new research shows these larger methylsiloxanes are not limited to traffic-heavy locations. Scientists detected them in urban, coastal, rural, and forest environments, suggesting the chemicals are widespread in the atmosphere.
“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.
According to the researchers, these large molecular methylsiloxanes account for between 2 and 4.3 percent of the total mass of organic aerosols in the atmosphere, making them among the most abundant synthetic compounds detected in airborne particles. By comparison, atmospheric concentrations of PFAS are typically more than a thousand times lower.
How Engine Oil Releases Methylsiloxanes
Methylsiloxanes added to lubricants are intended to assist with lubrication rather than combustion. However, during engine operation, moving parts such as pistons require constant lubrication, making it unavoidable that small amounts of engine oil enter the combustion chamber.
Because methylsiloxanes are highly heat resistant and do not fully break down during combustion, some survive the intense temperatures inside engines and are released into the atmosphere through exhaust gases.
Highest Levels Found in Cities
The highest concentrations were measured in urban regions. Samples collected in the São Paolo metropolitan area in Brazil reached 98 nanograms per cubic meter. The lowest levels appeared in forest locations, including Rugsteliskis, Lithuania, where concentrations measured 0.9 nanograms per cubic meter.
Researchers also collected air samples in Cabauw, a small rural village in the Netherlands, where concentrations reached 2 nanograms per cubic meter.
The measurements came from sites in the Netherlands, Lithuania, and Brazil, giving researchers data from different climates, hemispheres, and economic regions, including both developed and emerging economies.
Possible Health and Climate Effects
Because methylsiloxanes appear to be present almost everywhere in the atmosphere, researchers say people are likely inhaling them continuously. Scientists still do not know what long-term health effects this exposure could have.
“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.”
Researchers also warn that the chemicals could influence climate and atmospheric processes. Methylsiloxanes can change the properties of aerosols, which play an important role in cloud formation and climate behavior.
For example, the compounds may alter aerosol surface tension, affecting how clouds form. They could also interfere with ice nucleation, an important step in atmospheric cloud processes.
Vehicle Emissions Linked to Pollution Source
The study found that more than half of the detected large molecular methylsiloxane particles likely come from traffic emissions. Researchers believe the source is connected to lubricants such as engine oil.
Scientists reached this conclusion after finding that methylsiloxanes spread through the atmosphere in a similar way to long-chain hydrocarbons commonly found in engine oil. The matching dispersion patterns strongly suggest they come from the same source.
Interestingly, the long-chain hydrocarbons decreased significantly as they traveled through the atmosphere and became diluted. Methylsiloxanes, however, remained far more stable. Large amounts continued to persist in the atmosphere even after long-distance transport.
According to the researchers, this stability means the compounds can likely travel great distances through the air.