A new class of airborne chemical compound is turning up everywhere on Earth, and scientists have little idea what it might be doing to human bodies or the climate. The culprit is methylsiloxanes, a family of silicone-based substances used in cosmetics, lubricants, industrial fluids, and household products that researchers say now rank among the most abundant synthetic chemicals in Earth's atmosphere.
An international team from Utrecht University and the University of Groningen detected remarkably high concentrations of these compounds across urban centers, coastal zones, rural villages, and remote forests. The findings, published in Atmospheric Chemistry and Physics, suggest that methylsiloxanes have become a pervasive atmospheric pollutant whose health and climate effects remain largely unknown.
"We therefore underscore the urgent need for the evaluation of these health impacts," said Rupert Holzinger, the Utrecht researcher who helped lead the study. The team's measurements revealed that large-molecular methylsiloxanes constitute between 2 and 4.3 percent of the total mass of organic aerosol particles suspended in air, making them vastly more abundant than PFAS, the infamous forever chemicals that scientists have spent years documenting and worrying about.
The source of the pollution traces back to a surprisingly mundane place: engine oil. Methylsiloxanes added to lubricants in vehicles and ships are designed to improve lubrication performance. But during combustion, these heat-resistant compounds survive the extreme temperatures in engines and slip out through exhaust as vapor and particles.
Because methylsiloxanes are remarkably stable once airborne, they do not degrade quickly the way many other pollutants do. This persistence means they linger in the atmosphere and travel across vast distances. The research team found evidence that more than half of the large-molecular methylsiloxanes detected in their samples originated from traffic emissions, a conclusion they drew by matching the chemicals' atmospheric dispersal patterns to those of engine oil hydrocarbons.
The concentration measurements painted a stark picture of exposure variation. Air samples from the São Paulo metropolitan area in Brazil reached 98 nanograms per cubic meter, far higher than levels recorded in forests near Rugsteliskis, Lithuania, which measured just 0.9 nanograms per cubic meter. A rural Dutch village fell somewhere between at 2 nanograms per cubic meter. The wide geographic spread of the research, spanning multiple hemispheres and economic regions, suggests the problem is genuinely global.
People are almost certainly breathing these chemicals every day, yet science has barely begun to assess the consequences. The inhalation dose of methylsiloxanes may actually exceed daily exposure to other synthetic contaminants including PFAS and micro- and nanoplastics, yet regulatory agencies and medical researchers have paid scant attention to their potential toxicological effects.
The climate implications may be equally troubling. Methylsiloxanes can alter the surface properties of aerosol particles, which play crucial roles in cloud formation and precipitation. By changing surface tension and potentially interfering with ice nucleation, these compounds could subtly shift atmospheric processes in ways that affect weather patterns and climate behavior. The full scope of those effects remains unknowable without further study.
The discovery highlights a recurring pattern in environmental science: industrial chemicals achieve global ubiquity before anyone seriously examines whether they belong in the atmosphere or inside human lungs. Methylsiloxanes had been detected in air samples for years, but the scientific community largely assumed they came from the straightforward evaporation of personal care products. The realization that vehicles and ships emit larger, more persistent forms of these compounds suggests the problem is more entrenched and harder to solve than previously assumed.
Author Jessica Williams: "We've built a world where lubricants in car engines end up as persistent atmospheric particles measured on multiple continents, yet we're only now asking whether that's a problem."
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