Asbestos is the collective term for a group of six naturally occurring, fibrous silicate minerals. These minerals are composed of long, thin crystals that separate into microscopic fibers, giving the material unique properties. Historically, its resistance to heat, electricity, and chemical degradation made it highly desirable for commercial applications worldwide. Although its widespread use has declined due to health concerns, asbestos exists in the environment from two distinct sources: natural geological deposits and the remnants of human application.
Naturally Occurring Asbestos
Asbestos has a geological origin, forming within specific rock types deep beneath the earth’s surface. This form is referred to as Naturally Occurring Asbestos (NOA). The primary host rocks are serpentinites and ultramafic rocks, which undergo natural processes involving heat, pressure, and water to facilitate the growth of the fibrous minerals.
The six regulated types of asbestos fall into two mineral classes: serpentine and amphibole. Chrysotile, or white asbestos, is the sole member of the serpentine group and is the most common form, typically found in serpentine rock formations as flexible, curved fibers. The amphibole group includes crocidolite, amosite, tremolite, actinolite, and anthophyllite. Amphiboles tend to form straight, needle-like fibers and are associated with various metamorphosed rocks. These geological deposits remain inert until the host rock is disturbed or exposed to surface processes.
NOA is released into the environment through natural processes like weathering and erosion, which slowly break down the rock matrix. This geological material can become concentrated in soil, particularly where the host rock is prevalent. Human activities significantly accelerate this natural release, especially when they disturb the earth’s surface. Mining operations, quarrying for construction materials, and the development of land for housing can fracture the host rock and liberate substantial quantities of fibers. Furthermore, the use of crushed rock and gravel from these areas, such as for unpaved roads or construction fill, can spread NOA-containing material across a wider geographic area.
Anthropogenic Sources: Legacy Materials and Waste
Beyond its natural geological presence, human commercial activity introduced vast amounts of asbestos into the built environment, creating widespread anthropogenic sources of contamination. From the late 19th century through the 1970s, the material was incorporated into thousands of different products due to its strength and heat resistance. These legacy materials are now a primary source of environmental asbestos release as they age and degrade.
In the built environment, asbestos is commonly found in older structures. It was integrated into components like thermal insulation around pipes and boilers, vinyl floor tiles, and textured paints. Roofing and siding shingles, cement sheets, and various ceiling materials also frequently contain the fibers. As these materials become friable—meaning they can be crumbled or reduced to powder by hand pressure—they begin to release fibers into the surrounding air and soil.
Outdoor and industrial infrastructure also represents a substantial reservoir of anthropogenic asbestos. Asbestos-cement (AC) products were widely used for water pipes and sewer lines. The material was also a component in friction products, notably brake pads and clutch facings in older vehicles. The degradation of these pipes, the wear of friction products, and the eventual disposal of these materials contribute to environmental contamination.
The release of fibers from legacy materials is often concentrated during human activities such as renovation, demolition, and improper waste management. When older buildings are torn down or extensively remodeled, the disturbance of insulation, plaster, or other asbestos-containing materials (ACMs) can release high concentrations of fibers into the air. Similarly, materials improperly discarded at landfills or illegal dumping sites break down over time, allowing wind and water erosion to carry the fibers into the adjacent ecosystems. These sites of concentrated waste act as point sources for ongoing environmental contamination.
Environmental Pathways: Distribution in Air, Water, and Soil
Once released from natural rock or anthropogenic sources, asbestos fibers are highly durable and stable, allowing them to persist and travel through various environmental compartments. The distribution of asbestos in air, water, and soil dictates the potential for exposure. Because the fibers do not evaporate or dissolve, they become a persistent pollutant that cycles through the environment.
The atmosphere is a primary pathway for fiber distribution, as microscopic fibers can remain suspended in the air for extended periods. Fibers become airborne through the mechanical disturbance of NOA-containing soil, the demolition of asbestos-containing structures, or the erosion of waste piles. While ambient air concentrations in remote areas are generally low, localized high concentrations can occur near active disturbance sites. Examples include construction zones, quarries, or unpaved roads built with asbestos-laden gravel. Wind can then transport these airborne fibers over long distances before they settle out onto land or water surfaces.
Asbestos enters aquatic systems primarily through two mechanisms: the erosion of NOA-bearing rocks and the degradation of asbestos-cement water pipes. Runoff from contaminated soil or waste sites carries fibers into rivers, lakes, and reservoirs. The massive network of older AC water distribution pipes can also slowly release fibers into drinking water as the cement matrix degrades. The fibers are chemically stable in water and do not break down, allowing them to remain suspended or travel long distances until they settle.
Soil and sediment act as the ultimate sink for asbestos fibers, accumulating material transported by air and water. Topsoil can become contaminated directly from the breakdown of above-ground legacy materials or from the disturbance and transport of NOA-containing parent rock. In aquatic environments, suspended fibers eventually settle out and accumulate in riverbeds, lake bottoms, and ocean sediments. This accumulation creates long-term reservoirs of asbestos, which may be re-suspended into the water column if the sediment is disturbed by natural or human activities.