The Central Valley has been shrouded in a 400-mile stretch of radiation (Tule) fog that social media users say contains odd black-and-white particles and a sticky residue. Scientists from UC Santa Cruz and UC Berkeley say the droplets act like scavengers, concentrating agricultural, industrial and biological material — not radiation — and can leave slimy deposits. Experts advise washing outdoor produce and covering open water; an incoming storm should clear the fog, though it may return if high pressure resumes.
400-Mile Fog Chokes California’s Central Valley — Scientists Say Pollution, Not Radiation, Explains the Weird Residue
Northbound drivers on Interstate 5 near the Grapevine encounter tule fog Dec. 9.(Myung J. Chun / Los Angeles Times)
A 400-mile blanket of dense fog has settled over California’s Central Valley for weeks, prompting viral social media posts and local concern about strange-looking particles and a sticky residue left behind on cars and plants. Researchers say the phenomenon is explainable and, while unsettling, is the result of ordinary pollutants and biological material concentrated by long-lived fog — not radioactive contamination.
What Happened?
Atmospheric conditions — an early wet season, cold temperatures and a stagnant high-pressure system — have created ideal conditions for persistent radiation fog (often called Tule fog in this region). The fog’s droplets remain suspended for extended periods, allowing them to collect gases, soot and microscopic particles from the valley’s heavy traffic, agriculture and industry.
Social Media Reaction
On platforms such as X (formerly Twitter), Instagram and TikTok, users posted photos and videos showing white and black specks in fog condensate and describing a gelatinous residue after the moisture settled. Some viewers speculated about asbestos or other dramatic contaminants.
“What is this s— right here? There’s something in the fog that I can’t explain ... They got asbestos in there,” an online clip quoted a local resident as saying.
What Scientists Say
Experts from UC Santa Cruz and UC Berkeley say those reactions are understandable but that the likely explanations are far more mundane.
Peter Weiss-Penzias, a fog researcher at UC Santa Cruz, explained that fog droplets have large surface area and can remain airborne for days or weeks, during which they absorb gases and particles — essentially acting as tiny scavengers of airborne pollution. He listed plausible contributors including agricultural chemicals, industrial emissions, vehicle exhaust and wood smoke, as well as biological material such as fungal spores, pollen and algae.
Because fog droplets can gather biological material, the residue left on surfaces can appear slimy or gelatinous once it dries. Weiss-Penzias noted that while inhalation is the primary exposure route, the relatively large size of fog droplets means they tend not to penetrate deep into lung tissue the way fine particulate matter does on dry, smoggy days. A greater practical concern is ingestion if fog deposits contaminate produce or open water containers.
Advice: Wash vegetables and any food left outdoors before consuming. Cover outdoor cisterns or water collection points if possible.
Dennis Baldocchi, a fog researcher at UC Berkeley, said a storm system forecast to arrive this weekend will likely disperse the fog and clear the valley temporarily, but warned that the pattern could return if high pressure re-establishes.
Bottom Line
The fog’s strange appearance and residues are consistent with a mixture of pollutants and biological material concentrated by persistent radiation/Tule fog. While unsettling to see, there is no evidence presented that the fog contains radioactive material; routine precautions like washing produce and avoiding drinking from uncovered outdoor water sources are sensible steps.
Originally published in the Los Angeles Times.
