Asbestos fiber length influences toxicity and risk of mesothelioma

ATSDR, responding to concerns about asbestos fiber toxicity from the World Trade Center Disaster, held an expert panel meeting to review fiber size and its role in fiber toxicity in December 2002. The panel concluded that fiber length plays an important role in toxicity.

Fibers with lengths less than 5µm (1µm is about 1/25,000 of an inch) are essentially non-toxic when considering a role in mesothelioma or lung cancer promotion. However, fibers less than 5 µm in length may play a role in asbestosis when exposure duration is long and fiber concentrations are high. More information is needed to definitively make this conclusion.

In accordance with these concepts, it has been suggested that amphibole asbestos is more toxic than chrysotile asbestos, mainly due to physical characteristics that allow chrysotile to be broken down and cleared from the lung, whereas amphibole is not removed and builds up to high levels in lung tissue.

Some researchers believe the resulting increased duration of exposure to amphibole asbestos significantly increases the risk of mesothelioma and, to a lesser extent, asbestosis and lung cancer. OSHA, however, continues to regulate chrysotile and amphibole asbestos as one substance, as both types increase the risk of disease. EPA’s Integrated Risk Information System (IRIS) assessment of asbestos also treats mineralogy (and fiber length) as equipotent.

Evidence suggesting that the different types of asbestos fibers vary in carcinogenic potency and site specificity is limited by the lack of information on fiber exposure by mineral type. Other data indicate that differences in fiber size distribution and other process differences can contribute at least as much to the observed variation in risk as does the fiber type itself.

Counting fibers using the regulatory definitions does not adequately describe risk of health effects, as fiber size, shape, and composition contribute collectively to risks in ways that are still being elucidated. For example, shorter fibers appear to preferentially deposit in the deep lung, but longer fibers might disproportionately increase the risk of mesothelioma.