Recent discourse on synthetic pollutants has primarily centred on two categories: microplastics, which have been detected in environments ranging from Arctic snow to human blood, and PFAS, the so-called “forever chemicals” present in waterproofing materials, cookware, and food packaging. Both categories have attracted significant scientific and public attention.
However, a recent study published in Atmospheric Chemistry and Physics (2026) by researchers from universities in the Netherlands, Brazil, Lithuania, and China has identified a third category of synthetic pollutant that appears to be even more prevalent in ambient air and has received minimal public attention.
These pollutants, known as methylsiloxanes, are silicon-based synthetic compounds derived from silicone, a material commonly used in lubricants, personal care products, and industrial applications. Methylsiloxanes are ubiquitous in the atmosphere, present in urban, coastal, rural, and forested environments. Estimated inhalation exposures to methylsiloxanes surpass those of PFAS and microplastics by several orders of magnitude.
What Are Methylsiloxanes?
Silicones are synthetic organic compounds characterised by a backbone of alternating silicon and oxygen atoms, in contrast to the carbon backbone found in most organic molecules. Methylsiloxanes, a subset of silicones, feature methyl groups (CH₃) attached to the silicon atoms. These compounds are present in a wide array of products, including engine lubricants, shampoos, skin creams, defoaming agents, anti-freeze formulations, and waterproof coatings.
In 2025, global annual production of silicones and siloxanes was approximately 6.59 million metric tons, significantly exceeding the estimated 0.23 million tons of PFAS produced each year.
Historically, scientific research has focused on small, volatile methylsiloxanes, known as volatile methylsiloxanes (VMS), which readily evaporate from personal care products and enter the atmosphere as gases. In contrast, large molecular methylsiloxanes, such as polydimethylsiloxane (PDMS, also known as silicone oil), have remained largely unstudied and were essentially unknown until recently.
These larger molecules don’t evaporate. They attach directly to fine particles, the tiny airborne specks smaller than 2.5 micrometres that penetrate deep into human lungs and travel through the atmosphere in that bound state. Because they’re too large for conventional mass spectrometry to detect directly, they escaped notice for years. Researchers in this team developed a method to identify them by high-temperature thermal breakdown, which releases characteristic, smaller fragments that can then be measured.
Where Were They Found?
The research team collected fine particle samples from five contrasting environments across three countries:
- Urban Vilnius (Lithuania)
- Coastal Preila (Lithuania, near a harbour)
- Remote forest Rugsteliskis (Lithuania)
- Rural Netherlands (between Rotterdam, Amsterdam, and The Hague)
- Urban São Paulo (Brazil — one of the world’s largest cities)
The findings were both consistent and notable: methylsiloxanes were detected at every sampling location, including remote forest sites. The highest concentrations were observed in urban areas, with São Paulo exhibiting a median of 98 nanograms per cubic meter of air, whereas the lowest were recorded at the forest site (0.9 ng/m³). Nevertheless, these compounds remained detectable even in the most remote environments.
More significant than the absolute concentrations was the proportion of the total organic aerosol mass they represented: across all sites, methylsiloxanes accounted for 2.0% to 4.3% of the organic matter in fine particles. This is not a trace contaminant. It is one of the dominant categories of synthetic compounds in atmospheric particulate matter.
These proportions exceed those found in raw emissions from vehicles and ships, suggesting that methylsiloxanes are either more atmospherically stable than other co-emitted compounds or that additional sources (such as gas-to-particle conversion of smaller siloxanes) are contributing to the burden. Likely both.
Where Are They Coming From?
The researchers identified traffic emissions, primarily from vehicles and ships, as the major source of large-molecular-mass methylsiloxanes in the atmosphere.
The connection runs through engine lubricant oil. Modern engines use lubricants that contain polydimethylsiloxane (PDMS) as a defoaming agent and anti-wear additive. When engines burn oil, as all combustion engines do to some degree, they release not just hydrocarbons, but also the silicone components of those lubricants, directly into the exhaust as fine particles.
The study confirmed this link by simultaneously tracking two types of molecules: methylsiloxanes and long-chain hydrocarbons (C23–C38), the latter of which are well-established markers of lubricant oil combustion. In traffic tunnel samples in São Paulo, the two compound types were strongly correlated, indicating they were emitted together from the same source.
But here’s where the story becomes particularly interesting: when the researchers compared tunnel samples (close to the emission source) with ambient air samples collected across the city and in rural and forest environments, the long-chain hydrocarbons declined sharply with distance and dilution, as they were chemically broken down by atmospheric oxidation. The methylsiloxanes, by contrast, remained relatively stable.
This chemical persistence, the property that makes silicones industrially valuable, also means that once large-molecular-weight methylsiloxanes enter the atmosphere, they remain there, potentially travelling long distances while remaining associated with fine particles.
In Lithuania, seasonal analysis confirmed this pattern in a different way: in winter, with cold temperatures and weak sunlight suppressing chemical reactions, both methylsiloxanes and lubricant hydrocarbons declined proportionally as air moved from polluted urban areas to cleaner forest sites, consistent with simple dilution. But in the Netherlands, where milder conditions allowed more atmospheric chemistry, the lubricant hydrocarbons broke down faster, while methylsiloxanes persisted. Near the Lithuanian coast, a higher proportion of particularly large methylsiloxane molecules, the kind more associated with ship engines, was detected, consistent with maritime shipping as a contributing source.
How Much Are People Inhaling?
This is where the numbers become genuinely alarming in context.
The researchers estimated daily inhalation of methylsiloxanes via fine particles across the sampled environments. In urban Brazil, the median daily intake was approximately 1,280–1,480 nanograms per person per day for children and adults, respectively. In Lithuania’s coastal and urban environments, intakes ranged from hundreds of nanograms per day. Even in forested areas, daily intakes were around 11–13 nanograms per person.
For comparison, the researchers then estimated daily inhalation of two other well-known synthetic pollutants:
PFAS (forever chemicals): median daily inhalation estimated at roughly 0.5 nanograms per person per day, three to four orders of magnitude lower than methylsiloxanes in urban environments.
Microplastics and nanoplastics: estimated at roughly 10–12 nanograms per person per day, one to two orders of magnitude lower than methylsiloxanes in cities.
To be clear: this comparison does not mean methylsiloxanes are necessarily more dangerous than PFAS or microplastics. The toxicology of large molecular methylsiloxanes is currently almost entirely unknown. Some smaller volatile siloxanes have been flagged as potential endocrine disruptors affecting estrogen signalling and liver function, and are under regulatory scrutiny in several countries. But the large, particle-bound forms studied here have barely been characterised.
That knowledge gap is precisely the problem. We are inhaling these compounds at substantial levels, and we do not yet know what they do to the human body over years or decades of exposure.
Beyond Health: What About the Climate?
The researchers also flag potential climate implications, though these remain poorly quantified.
Large-molecular-weight methylsiloxanes have very low surface tension, much lower than that of water, which means they can spread across the surfaces of airborne water droplets. In atmospheric science, this matters because surface tension affects whether droplets can grow large enough to become cloud droplets. If methylsiloxanes coat aerosol particles, they could reduce the energy threshold for cloud droplet formation, potentially altering cloud properties and rainfall patterns in ways that current atmospheric models, which assume aerosol droplets behave like pure water, do not account for.
Additionally, methylsiloxanes are used industrially as anti-freeze compounds. Their presence on atmospheric particles could potentially inhibit ice crystal formation in clouds, another process with implications for precipitation and climate that has not been studied in this context.
These are early-stage hypotheses that require experimental verification. But they add to the picture of an overlooked compound class with potentially broad environmental significance.
A Pollutant Category We’ve Been Missing
The study’s broader message is methodological as much as substantive: we have been systematically undercounting a major category of atmospheric pollutants because conventional detection methods couldn’t see them.
The fact that large molecular methylsiloxanes make up 2–4% of the organic mass in fine particles across environments as different as São Paulo, the Dutch countryside, and a Lithuanian forest suggests they have likely been present at these levels for as long as modern combustion engines have been in widespread use. We simply lacked the tools and the awareness to detect them.
Now that they’ve been identified and quantified, the research agenda is clear: systematic toxicological testing, wider geographic sampling (the current study is limited to Europe and South America), better atmospheric modelling of siloxane behaviour, and regulatory attention to the silicone content of lubricating oils.
For a compound produced in millions of tons per year, burned in engines globally, and now found in the lungs of city residents at levels exceeding PFAS exposure by thousands of times, the urgency is real, even if the specific health risks remain, for now, unknown.
Reference
Yao, P., Holzinger, R., Oyama, B.S., Masalaite, A., Paul, D., Ni, H., Noto, H., Materić, D., Andrade, M. de F., Huang, R.-J., & Dusek, U. (2026). Widespread occurrence of large molecular methylsiloxanes in ambient aerosols. Atmospheric Chemistry and Physics, 26, 5005–5018. https://doi.org/10.5194/acp-26-5005-2026