At least 9 out of every 10 mesotheliomas is due to exposure to asbestos, although usually that exposure will have taken place many years before the cancer becomes apparent.

In most of the remaining cases there is usually no obvious cause. It is known that very low levels of exposure to asbestos may lead to the development of a mesothelioma, so it is possible that sometimes people who get the condition may not have had any obvious contacts with asbestos but may still have breathed in asbestos fibres without realising it.

Exposure to certain chemicals and radiation has also been linked to mesothelioma.

A small number of mesotheliomas have been linked to exposure to a type of radiation called thorium dioxide(thorotast). This was used in some x-rays up until the 1950's.

In some parts of the world mesothelioma has been linked to exposure to local minerals. A substance called erionite, which is found in rocks in parts of Turkey, is thought to account for a small number of cases.

A virus called SV40 (simian 40) has also been suggested as a cause for mesothelioma. However the evidence for this is uncertain and it is likely that this virus is not a direct cause of mesothelioma.

There is no evidence to link mesotheliomas with cigarette smoking and although the link to asbestos is clear other building materials, such as fibre glass, have not been found to carry any risk of causing mesothelioma.