Nobody Noticed Until Now: Car Engines Release Silicone Chemicals Into The Air

Scientists Say Exposure May Far Exceed PFAS

Every time a car engine turns over or a cargo ship moves through a harbor, tiny particles of silicone-based chemicals escape into the atmosphere. A new study has found these synthetic compounds, called methylsiloxanes, turning up in cities, forests, coastlines, and rural areas across three countries on two continents. Model estimates suggest that people in urban areas may be inhaling them at rates far exceeding estimates for better-known pollutants like “forever chemicals” and microplastics, though researchers stress the health effects remain poorly understood.

Methylsiloxanes are widely used in lubricants and silicone oils, built around a backbone of alternating silicon and oxygen atoms. Published in the journal Atmospheric Chemistry and Physics, the study found they made up roughly 2% to 4.3% of all detectable organic material in airborne particles collected from sites in the Netherlands, Lithuania, and Brazil. More than half of what researchers detected came from large, heavy versions of these molecules, a category that had gone almost entirely unnoticed until recently because standard instruments could not pick them up. Traffic emissions appear to be the dominant source identified so far, with engine lubricants the likely origin, though researchers note other sources cannot be excluded.

Tracking Methylsiloxanes in Air Across Multiple Environments

To find these chemicals, the research team collected air particle samples from a wide range of locations. A rural observatory in the Netherlands was sampled across all four seasons between 2011 and 2012. In Lithuania, researchers gathered winter samples at three sites covering urban, coastal, and forested environments. In Brazil, daily winter samples were collected in São Paulo, one of South America’s largest cities. Altogether, more than 100 individual samples were analyzed.

Using a specialized instrument that heats filter samples in stages, researchers could track what compounds came off the filters and at what temperature. Smaller methylsiloxane molecules are released at lower heat; the large versions only break apart at higher temperatures. That distinction allowed the team to separate compounds that drifted into particles from the surrounding air from those coming directly from engine lubricants. Concentrations were highest in urban areas, reaching 98 nanograms per cubic meter in São Paulo and 33 in urban Lithuania, and lowest at a Lithuanian forest site at just 0.9 nanograms per cubic meter.

Methylsiloxane Air Pollution: What Vehicle Exhaust Leaves Behind

Road tunnel air is essentially bottled vehicle exhaust. When researchers compared tunnel samples with open-atmosphere air in São Paulo, both showed the same chemical fingerprints from large methylsiloxanes, pointing squarely at vehicle emissions as a dominant source.

Researchers also tracked a companion compound released alongside methylsiloxanes: long-chain hydrocarbons from engine lubricating oil. In tunnel samples, the two types of compounds were tightly linked. In open air, however, the hydrocarbons dropped off sharply while methylsiloxane levels held relatively steady. That divergence points to an unusual chemical durability. Their unusual chemical structure appears to protect these molecules from the atmospheric reactions that break down most other pollutants.

Seasonal data from the Netherlands reinforced the picture. Concentrations peaked in winter and dipped in summer, but methylsiloxanes’ share of overall organic particle matter stayed relatively stable year-round, between about 1.7% and 3.3%. When winds arrived from the ocean and passed over major shipping lanes, the fraction of the largest molecular versions increased, consistent with ships producing even heavier versions than cars.

A Synthetic Pollutant Hiding in Plain Sight

Siloxanes are not obscure chemicals. Global annual production reached approximately 6.59 million metric tons in 2025, according to figures cited in the study, dwarfing the 0.23 million tons of PFAS produced globally each year. Yet while PFAS and microplastics have become household terms and subjects of intense regulatory focus, methylsiloxanes in airborne particles have received remarkably little scrutiny, partly because the large molecular versions simply could not be detected with standard equipment.

Model-based estimates in the study suggest an urban resident in Brazil could inhale around 1,480 nanograms of these compounds per day, potentially 10 to 10,000 times higher than estimated daily inhalation of PFAS or micro- and nanoplastics.

Researchers describe this as a preliminary assessment designed to serve as a basis for future toxicological studies, not a definitive risk measurement. The health effects of these large molecular versions remain almost entirely unknown. Some smaller, more volatile methylsiloxane types have been linked to effects on estrogen and the liver, but the large versions identified here belong to a separate category with almost no toxicological research behind them.

Researchers also raised the possibility that these compounds, known industrially for lowering surface tension, could theoretically affect how clouds form or how ice crystals develop if they coat atmospheric water droplets. They are explicit that this is a hypothesis grounded in known industrial properties rather than direct atmospheric observation, and that experimental confirmation is still needed.

Large methylsiloxanes are chemically stable, distributed across multiple environments through traffic emissions, and apparently inhaled daily at levels that may exceed exposure to pollutants already drawing serious public attention. How they behave inside human lungs, and what they may do to the atmosphere above, remain open and urgent questions.

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Disclaimer: This article is based on a peer-reviewed scientific study and is intended for informational purposes only. It does not constitute medical or environmental health advice. Readers with specific health concerns should consult a qualified medical professional.

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