Development of malignant mesothelioma
The development of malignant mesothelioma is thought to relate to the physical structure of asbestos. Asbestos occurs as two forms, serpentine chrysotiles, or straight, sharp, needle-like amphiboles (crocidolite, amosite, anthophyllite, tremolite and actinolite). Chrysotiles are not thought to be directly cause malignant mesothelioma, because their curled, flexible structure means they are likely to become trapped in the upper airways, and removed by usual physiological actions. Also, if they do become trapped in the lungs, the more soluble chrysotiles are gradually leached from the lung tissue. However, chrysotiles are commonly associated with malignant mesothelioma, probably because they are usually contaminated with amphiboles.

Amphiboles behave differently to chrysotiles, and are therefore much more carcinogenic. Their straight, stiff structure tends to align in the airstream, and they are then delivered deep into the lungs, where they cannot be easily cleared. They penetrate into the lung tissue and then to the pleura, where they initiate tumors by various chemical and mechanical processes.

Long, thin amphibole fibers are more injurious than short, thick ones, because short fibers are more likely to be cleared.

It is likely that asbestos fibers act both as a tumor initiator and a tumor promoter, because they adsorb potentially toxic chemicals (including tobacco smoke), which they then 'transport' with them into the body.