Dense radiation fog blankets California's Central Valley as health concerns rise

A dense cloud of radiation fog has blanketed California's Central Valley for weeks, extending across roughly 400 miles and affecting more than 20 counties. The persistent fog, known locally as Tule fog, has coincided with reports of respiratory symptoms among residents in several communities, prompting questions about the air residents are breathing.

Gerald Deperalta, a Tracy resident, told Daily Mail that he and his family have been coughing and congested for about a week. "We've kept the kids' inhalers nearby, and we are trying to keep them indoors as much as possible," he said, sharing videos of his car blanketed in a thick white fog that he wiped off with a cloth. Comments on Deperalta's posts described similar symptoms in other households, including a mother who said her children's eczema has flared up since the fog, and another resident who reported multiple headaches in two weeks.

Health experts cautioned that fog itself does not cause illness, and other environmental or seasonal factors could be at play. Temperature inversions during dense fog can trap car exhaust and industrial pollutants near the ground, potentially worsening air quality and irritating the lungs, especially for people with asthma or other chronic conditions. Meteorologists say this autumn-to-winter Tule fog has formed into a major, continuous fog bank that stretches across the Central Valley and creates dangerous driving conditions due to severely reduced visibility. Forecasters have suggested a weather system could arrive around December 17, though it remains unclear whether it would be strong enough to disperse the fog.

Residents such as Michael Washington described persistent symptoms as the fog persisted. "I've been coughing for three weeks. antibiotics, inhalers, cough syrups and benzonatate ain't working," he said, reflecting the frustration of many who live in the fog-bound corridor. Others reported throat irritation and chest tightness, while some posted images of cars showing a white residue on exterior surfaces. Experts noted that natural radiation fog does not leave a white residue, since it is composed of microscopic water droplets; the white film observed on vehicles is more likely dust, smoke particles, or other pollutants suspended in the air.

The fog’s appearance can be dramatic, but its health impact is complex. When a thick inversion forms, a lid sits atop the fog layer, trapping pollutants closer to ground level. Tiny PM2.5 particles can accumulate in the trapped air and be drawn into the lungs, potentially triggering coughing, wheezing, and asthma flare-ups. Hospitals often see increased emergency-room visits for respiratory and heart problems on foggy, polluted winter days in the Central Valley. The World Health Organization has identified this kind of trapped air pollution as a major risk factor for early deaths worldwide, underscoring the broader health stakes of regional air-quality events.

Researchers at the University of California, Berkeley, noted in 2019 that California’s stricter clean-air rules have reduced the number of days with very thick tule fog by comparison with decades past. Study author Ellyn Gray, then a graduate student researcher at UC Berkeley, said that the southern part of the valley now experiences more fog in concert with higher pollution concentrations, highlighting a spatial pattern that complicates public health responses during dense fog events.

Separately, historical events linked to tule fog underscore the public-safety dimension of these conditions. In the past, dense tule fog has contributed to major traffic pileups on Central California highways. The most well-known example occurred on November 3, 2007, when visibility dropped to near zero south of Fresno, triggering a chain-reaction crash involving 108 vehicles and closing Highway 99 for more than 12 hours; two people were killed and more than 40 were injured. Earlier this year, a patch of tule fog contributed to a 40-car pileup in January, leaving several people dead or injured.

Officials emphasize that forecasting the fog’s end is tied to the progression of weather systems that can mix the atmosphere—cooling higher elevations while warming lower layers—and that any such system would need to be sufficiently strong to break the inversion. While a potential system around December 17 remains uncertain, it represents the best public signal that visibility and air quality could improve if conditions change.

For residents, the fog raises practical concerns about daily life, transportation, and health management. Authorities have urged people with preexisting respiratory or heart conditions to monitor air-quality reports, limit outdoor activity on dense days, and follow local guidance on protecting vulnerable populations. While the fog draws more attention due to surprising personal experiences and social-media reporting, public-health officials reiterate that the fog’s health effects are mediated by pollutants trapped near the ground, rather than by the fog itself.

As communities across the Central Valley endure this extended fog episode, officials will continue to track air-quality indicators and weather conditions, while meteorologists refine forecasts of when the inversion may break. The coming weeks will determine not only the fog’s persistence but also whether a stronger system will arrive to lift the veil and restore clearer skies for millions of residents.