CARB Method 435

CARB Method 435 is the California Air Resources Board's laboratory method for measuring asbestos in serpentine and other ultramafic rock, gravel, aggregate and soil. The sample is crushed to a fine powder and analyzed by polarized light microscopy using a 400-point count, so results can be reported at levels well below 1 percent.

Origin

The California Air Resources Board adopted the method in the early 1990s to support its rules on naturally occurring asbestos in serpentine rock used for road surfacing. It has since been used for California's controls on construction, grading, quarrying and surface mining in areas with naturally occurring asbestos, and sometimes outside California.

Significance in Asbestos Context

Method 435 matters wherever naturally occurring asbestos is an issue: unpaved roads and driveways surfaced with serpentine gravel, quarries, construction sites and developments in serpentinite areas. California's rules for surfacing materials set a limit of 0.25 percent asbestos, which is far below the 1 percent threshold used for building materials, so a method able to measure low levels was needed. Results under Method 435 can show that rock or soil contained asbestos, which may support an environmental-exposure claim, but low percentages in rock can still release significant fibers when disturbed. The method uses light microscopy, so very fine fibers can be missed; some investigators add electron microscopy. Because the crushing step can change fiber sizes, experts sometimes dispute what the results mean for airborne exposure.

Example Sentence

Samples of the serpentine gravel from the driveway tested at 1.5 percent chrysotile under CARB Method 435.

Related Terms

naturally occurring asbestos (NOA), Point Counting, Polarized Light Microscopy (PLM), Serpentinite, Bulk Sample Analysis, Environmental Exposure, Ultramafic Rocks

Notes

• If you lived on a gravel road or worked at a quarry in a serpentinite area, tell your lawyer.
• Testing rock or soil should be done by a qualified consultant with proper chain of custody.
• Rules on naturally occurring asbestos vary by state and county.