A dense cloud of 'radiation fog' has blanketed California's Central Valley for weeks, coinciding with reports of respiratory symptoms among some locals.

More than 20 counties across a 400-mile stretch of the state have been affected by the low-lying fog, which forms when the ground cools rapidly after sunset, creating a thick layer of condensation close to the surface.

Gerald Deperalta, from Tracy, told Daily Mail that he and his family have been coughing and experiencing congestion 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 the fog, which appeared as a thick, white dust as he wiped it off.

Commenters on Deperalta's TikTok videos said they, too, have experienced similar symptoms over the past few weeks.

One mother shared that her children's eczema has flared up since the fog, while another local said they have had three headaches in two weeks, which they have never experienced in their lives.

However, health experts have cautioned that fog itself does not cause illness, and other environmental or seasonal factors could be at play.

During these fog events, temperature inversions can trap car exhaust and industrial pollutants near the ground, potentially worsening air quality and causing respiratory irritation, especially for people with asthma or other lung conditions. 

Radiation fog, also known as Tule fog, appears in California's Central Valley every autumn and winter, with a major, persistent event in late November to early December 2025 creating a 400-mile-long fog bank.

While unrelated to nuclear radiation, radiation fog can create extreme risks for drivers by severely limiting visibility. 

Ending the inversion will require a powerful weather system that can mix the atmosphere, cooling higher elevations while warming the lower ones. 

Forecasters said a system could arrive around December 17, but it is uncertain if it will be strong enough to clear the fog. 

California resident Michael Washington said: 'I've been coughing for three weeks. antibiotics, inhalers, cough syrups and benzonatate ain't working.'

Other locals said they have experienced sore throats and aches in their lungs. 

In Deperalta's TikTok videos, he wipes the exterior of his car, which leaves a white residue on his fingers.

However, natural radiation fog is not known to leave a white residue, as it is simply a cloud at ground level, consisting of millions of tiny, pure water droplets that evaporate as the sun warms the air.

The residue could be due to dust, pollution, smoke or chemical particles suspended in the air, which could be causing the mysterious health issues.

Last week, the fog became incredibly thick in the Central Valley's flat valleys, sticking around until the sun rises and burns it off over several hours.

'I have lots of relatives in Sacramento; that fog is nasty indeed... Driving anywhere in [Sacramento] is dangerous in tule fog; it is some of the thickest fog on earth!' another social media user commented. 

In past years, the radiation cloud has caused massive traffic pileups along Central California highways.

One of the worst chain-reaction accidents blamed on radiation fog involved 108 vehicles and closed the state's Highway 99 for more than 12 hours on November 3, 2007.

Visibility dropped to near zero just south of Fresno around 9am ET that morning, causing cars and big-rig trucks to collide, killing two people and injuring more than 40 others.

Earlier this year, two drivers were killed and nine were injured after a 40-car pileup in a patch of tule fog on January 6.

Even non-drivers can be seriously affected by radiation fog, which does not create pollution by itself, but acts like a lid on a pot that keeps pollution from being carried away by the wind.

When the fog forms, a layer of warmer air sits on top of the cold, foggy air near the ground.

This 'lid' traps car exhaust, smoke from fireplaces, factory fumes, and farm dust right where residents breathe it in.

The tiny particles (called PM2.5) get stuck in this trapped air and can transfer into a person's lungs and even their blood.

Breathing in the polluted air on fog days can make asthma attacks, coughing, and wheezing worse, especially for children and the elderly.

Hospitals typically see more emergency room visits for breathing problems and heart issues on foggy, polluted winter days in the Central Valley.

The World Health Organization (WHO) has said this kind of trapped air pollution is one of the top causes of early deaths worldwide, endangering millions of people every year.

However, researchers from the University of California, Berkeley found in 2019 that the state's tougher clean-air rules have removed a lot of the tiny dust and pollution particles from the sky.

Since fog needs those particles to form water droplets around, there are now fewer days of thick tule fog than there were 30 or 40 years ago.

Study author Ellyn Gray, a graduate student researcher at UC Berkeley, said: 'We have a lot more fog in the southern part of the valley, which is also where we have the highest air pollution concentrations.'