Naturally Occurring Asbestos in California: 42 Counties Where the Ground Is Toxic

Serpentine rock formations containing asbestos run beneath 42 of California's 58 counties, and when construction crews, graders, or even homeowners with shovels disturb this ground, they release the same deadly fibers that killed industrial workers for a century—except these fibers come from the earth itself, not a factory product.^[1]

What Are the Key Facts About Naturally Occurring Asbestos in California?

Why Does California Have More Naturally Occurring Asbestos Than Any Other State?

California's geology is the product of tectonic plate collisions along the Pacific coast that pushed oceanic crust onto land, creating extensive belts of serpentine and ultramafic rock. These rock types form when deep-ocean mantle material is subjected to heat and pressure during subduction, incorporating chrysotile, tremolite, actinolite, and other asbestos minerals into their crystalline structure. The result is a geological inheritance that runs from the Oregon border to the Mexican border.

The USGS has mapped asbestos-bearing rock formations across the state and identified California as having more naturally occurring asbestos than any other state by a significant margin. The densest deposits follow three primary geological belts: the Sierra Nevada foothills from Plumas County to Mariposa County, the Coast Ranges from Trinity County to San Luis Obispo County, and the Klamath Mountains in the state's far northwest.^[1]

What makes California's NOA uniquely dangerous is the combination of geological prevalence and population density. Unlike remote asbestos deposits in other states, California's serpentine formations underlie suburban communities, school sites, highways, and recreational areas where millions of people live, work, and play.

"Most people think of asbestos as something in old building insulation or brake pads. In California, the asbestos is in the ground beneath your house, your kids' school, and the roads you drive on. When that ground gets disturbed, the fibers become airborne. You cannot see them, you cannot smell them, and by the time you are diagnosed, the exposure happened 20 to 40 years ago."

Which California Counties Have the Highest Naturally Occurring Asbestos Risk?

While 42 counties contain some level of NOA, the risk is concentrated in areas where serpentine rock sits at or near the surface and where human activity regularly disturbs it. The highest-risk counties include:

• El Dorado County: Home to the El Dorado Hills community where the EPA conducted its landmark NOA study. Serpentine formations underlie residential subdivisions, schools, and recreational areas. EPA sampling found that activities as basic as bicycle riding on unpaved trails generated measurable airborne asbestos.
• Lake County: Contains extensive serpentine deposits near the abandoned McLaughlin Mine and surrounding communities. Lake County has some of the highest mesothelioma incidence rates in rural California.
• Placer County: Rapid suburban development in the foothills east of Sacramento has brought residential construction into direct contact with serpentine formations. New housing developments have required asbestos management plans.
• Nevada County: Gold Country communities including Grass Valley and Nevada City sit within serpentine belts, with historical mining activity having further distributed asbestos-bearing material.
• Amador and Calaveras Counties: Sierra Nevada foothill communities with serpentine outcrops along major transportation corridors and near residential areas.
• Trinity County: Klamath Mountains region with some of the most extensive ultramafic rock formations in the state, affecting both communities and forest roads.

For information about occupational asbestos exposure across all industries and locations, see the WikiMesothelioma exposure database.

What Did the EPA Find in El Dorado Hills?

The EPA's investigation of El Dorado Hills in 2004 and 2005 remains the most comprehensive study of naturally occurring asbestos exposure in a residential community. The agency tested air quality during simulated normal activities on serpentine-rich soils and found that everyday actions generated asbestos levels significantly above background.

Key findings from the EPA study include detection of airborne asbestos during bicycle riding on unpaved trails, walking and running on gravel paths, children playing in unpaved areas, and vehicle traffic on unpaved roads. The fibers detected included both chrysotile and tremolite varieties, confirming that the serpentine formations in El Dorado Hills contain multiple asbestos mineral types.

The EPA study sparked significant community response and regulatory action. El Dorado County adopted a comprehensive Naturally Occurring Asbestos Ordinance requiring geologic assessment, dust control plans, and air monitoring for all construction and grading projects that disturb serpentine or ultramafic rock. The ordinance became a model for other California counties developing their own NOA regulations.

"The El Dorado Hills study proved what environmental scientists had been warning about for years: that you do not need to work in a factory to be exposed to lethal levels of asbestos. Children playing outside, people walking their dogs, homeowners digging garden beds—all of these activities on serpentine soil can release fibers. I work with families in these communities who had no idea they were living on asbestos until the diagnosis came."

How Does California Regulate Naturally Occurring Asbestos Exposure?

California has developed the most extensive regulatory framework for naturally occurring asbestos of any state, reflecting the scale of the geological hazard. The three primary regulatory pillars are:

1. California Air Resources Board (CARB) Airborne Toxic Control Measure. The ATCM for Construction, Grading, Quarrying, and Surface Mining Operations requires any project that disturbs serpentine or ultramafic rock to implement dust mitigation measures including wet suppression, track-out prevention, vehicle speed limits, and air monitoring. Projects must obtain permits and demonstrate compliance with the ATCM before proceeding.^[10]

2. Cal/OSHA Asbestos Standards. Cal/OSHA enforces a permissible exposure limit of 0.1 fibers per cubic centimeter of air (f/cc) averaged over an 8-hour workday for all workers, with an excursion limit of 1.0 f/cc over 30 minutes. Construction and grading workers in known NOA areas must follow additional engineering controls and personal protective equipment requirements under the California Asbestos Standard (Title 8, Section 1529).^[6]

3. Natural Hazard Disclosure Act. California law requires property sellers to disclose the presence of naturally occurring asbestos when selling real property in areas mapped as containing NOA. This disclosure must be made on the Natural Hazard Disclosure Statement. Failure to disclose can create liability for the seller and potentially for real estate agents who knew or should have known about the hazard.

Despite these regulations, enforcement gaps remain. Small-scale grading, residential landscaping, and unpermitted construction can disturb serpentine soils without triggering ATCM requirements. Rural roads built through serpentine formations continue to generate dust. And the 20 to 50 year latency period between exposure and mesothelioma diagnosis means that people exposed decades ago, before modern regulations existed, are only now developing disease.^[3]

How Does Naturally Occurring Asbestos Cause Mesothelioma?

The mechanism by which naturally occurring asbestos causes mesothelioma is identical to industrial asbestos exposure. When serpentine or ultramafic rock is disturbed, microscopic asbestos fibers become airborne. These fibers, typically 3 to 20 micrometers long and less than 3 micrometers in diameter, are small enough to penetrate deep into the lungs when inhaled.^[3]

Once lodged in lung tissue, asbestos fibers migrate to the mesothelium—the thin membrane lining the lungs (pleura), abdomen (peritoneum), or heart (pericardium). The body cannot break down or expel these fibers. Over 20 to 50 years, the chronic inflammation and cellular damage caused by the persistent fibers triggers malignant transformation, leading to mesothelioma. The median survival after diagnosis is 12 to 21 months, making mesothelioma one of the most lethal cancers regardless of the exposure source.^[8]

The ATSDR confirms that there is no safe threshold of asbestos exposure. Even low-level environmental exposure from naturally occurring sources can cause mesothelioma, though the risk increases with both intensity and duration of exposure. Residents of communities built on serpentine formations face continuous low-level exposure every time the ground is disturbed—a fundamentally different exposure pattern from the concentrated industrial exposures of the 20th century, but one that produces the same disease.^[3]

"When I review a case from a California NOA community, I look for construction projects near the person's home, road grading, residential development, and school construction that may have disturbed serpentine soil. The exposure may have happened 30 years ago when their neighborhood was being built. Identifying that disturbance event and who was responsible is the key to a successful claim."

Can Residents File Legal Claims for Naturally Occurring Asbestos Exposure?

Legal claims for naturally occurring asbestos exposure in California are viable under several legal theories, though they differ from traditional occupational exposure lawsuits in important ways:

• Developer/contractor negligence: Claims against developers or construction companies that disturbed serpentine soil without implementing proper dust controls, particularly when they knew or should have known about the NOA hazard
• Government entity liability: Claims against municipal or county governments that approved construction on known NOA sites without requiring adequate mitigation, or that maintained roads through serpentine formations without dust control
• Mining company claims: In areas near historical or active mining operations that distributed asbestos-bearing materials into surrounding communities
• Failure to disclose: Claims against property sellers who failed to disclose known NOA conditions as required under California's Natural Hazard Disclosure Act
• Asbestos trust fund claims: If the exposure involved products from bankrupt asbestos companies (for example, vermiculite insulation containing Libby Montana asbestos), trust fund claims may also be available

The landmark 2025 verdict in Weaver v. Vanderbilt Minerals—$12.25 million in New York for environmental proximity to a talc mine—established the first known U.S. liability finding for environmental proximity exposure, creating important precedent for California NOA cases.

For families affected by secondary exposure—including household members of workers who brought NOA-contaminated dust home on clothing—additional legal theories may apply.

What Should California Residents in NOA Areas Do to Protect Themselves?

Residents of communities in the 42 affected California counties should take specific precautions to minimize NOA exposure:

• Know your geology: Check the USGS mineral resource data system to determine whether your property sits on or near serpentine or ultramafic rock formations
• Wet before disturbing: Keep soil wet during any ground-disturbing activity including gardening, landscaping, and home improvement projects to prevent fiber release
• Avoid unpaved areas during dry conditions: Dust from unpaved roads, trails, and construction sites in serpentine areas can contain asbestos fibers. Avoid dusty conditions and keep car windows closed on unpaved roads
• Monitor construction: If construction or grading occurs near your home, verify that the contractor has obtained proper permits and is complying with CARB ATCM requirements
• Document potential exposures: If you lived in an NOA area during construction, grading, or mining activity, document the dates, locations, and activities. This information may be critical if a health condition develops years later
• Seek medical screening: Residents with known exposure to NOA should inform their physicians and discuss appropriate monitoring, particularly lung function testing and periodic imaging

If you or a family member has been diagnosed with mesothelioma and lived in a California NOA community, take our free mesothelioma compensation quiz to evaluate your eligibility for legal claims. Environmental exposure cases require specialized investigation to identify when and how the exposure occurred. Call 1-800-692-8608 for a free, confidential evaluation from attorneys who understand California's asbestos litigation landscape.

"California's naturally occurring asbestos problem is not going away. The serpentine formations have been there for millions of years, and every new housing development, road project, and construction site in these 42 counties creates a new potential exposure. My role is to help the families who have already been affected get the compensation they deserve, while raising awareness that this hazard exists beneath the surface in communities that look perfectly safe."

References

1. 1. USGS Reported Historic Asbestos Mines in California — U.S. Geological Survey (2025)
2. 2. Learn About Asbestos — U.S. Environmental Protection Agency (2025)
3. 3. Toxicological Profile for Asbestos — Agency for Toxic Substances and Disease Registry (2024)
4. 4. SEER Cancer Statistics Explorer: Mesothelioma — National Cancer Institute (2025)
5. 5. Mesothelioma Mortality in the United States — Centers for Disease Control and Prevention (2025)
6. 6. OSHA Asbestos Standards — Occupational Safety and Health Administration (2025)
7. 7. U.S. Federal Bans on Asbestos — U.S. Environmental Protection Agency (2024)
8. 8. Mesothelioma Treatment (PDQ) — National Cancer Institute (2025)
9. 9. VA Asbestos Exposure Eligibility — U.S. Department of Veterans Affairs (2025)
10. 10. NIOSH Asbestos Topic Page — National Institute for Occupational Safety and Health (2025)
11. 11. Occupational Exposure Index — WikiMesothelioma
12. 12. Secondary Exposure — WikiMesothelioma
13. 13. Mesothelioma Quick Facts — WikiMesothelioma