Mesothelioma and asbestos

What is mesothelioma?

Mesothelioma is a form of malignant cancer affecting the mesothelium, a protective lining that covers most internal organs of the body. The mesothelium has different names in different parts of body.

The disease’s most common forms are peritoneal mesothelioma and pleural mesothelioma. In the former, cancer cells develop in the peritoneum, the lining of the abdominal cavity.

In the latter, which is the most widespread form of mesothelioma, the affected site is the pleura, the membrane surrounding the lungs and lining the chest cavity.

Mesothelioma is a rare cancer, which in humans is almost invariably caused by exposure to asbestos, a material used in various sectors, in particular in the building industry. Most people (70-80 percent) who develop malignant mesothelioma have worked in jobs where they inhaled or were exposed to asbestos particles, asbestos fibres and dust.

As in all cancers, in both peritoneal mesothelioma and pleural mesothelioma cells multiply in excess and without control.

Mesothelioma prognosis is usually not good; it is a fatal disease, and death often occurs within twelve months after diagnosis.

Mesothelioma treatment exists, in the forms of surgery, radiation therapy, chemotherapy and immunotherapy, but it has not so far been successful. Mesothelioma is generally resistant to treatment.

Malignant mesothelioma and asbestos: what delayed recognizing the link?

The link between cancer and asbestos in humans became known on the basis of clinical studies in the early 20th century, so much so that the first lawsuits against asbestos manufacturers were in 1929.

Following this discovery, researchers extensively tried to induce cancer in laboratory animals by exposing them to asbestos. The results of animal experiments were disappointing, because the painful lesions which were produced in animals disappeared after asbestos was withdrawn, so in them, unlike in humans, the disease was not permanent.

The current prevailing paradigm of biomedical sciences is such that only tests on animals in laboratory conditions can confirm or disproof a hypothesis. This is what scientists have been trained to believe. So if, say, a correlation between a chemical substance and the development of a disease is observed in human subjects through clinical studies of patients or epidemiological studies (surveying large numbers of people), that is not considered scientific evidence until it is “validated” on some other animal species in the controlled conditions of a lab.

This reliance on animal research has had the effect that the biomedical establishment did not believe in the link between asbestos and human cancer for several decades.

In 1965 the Annals of the New York Academy of Science “reassuringly” wrote:

“…a large literature on experimental studies has failed to furnish any definitive evidence for induction of malignant tumours in animals exposed to various varieties and preparations of asbestos by inhalation or intratracheal injection”. [my emphasis]


But the human-based evidence continued to grow. Many epidemiological studies have over the years established an association between exposure to asbestos and the development of several conditions, including diffuse pleural thickening, lung cancer, carcinoma of the larynx, asbestosis, gastrointestinal tumours, peritoneal mesothelioma and pleural mesothelioma.

The link between asbestos and mesothelioma was finally accepted and led to legislation banning asbestos in many Western countries.

But it was only in the late 1970s and 1980s that this occurred. In 1989 the U.S. Environmental Protection Agency (EPA) banned all new uses of asbestos.

So animal tests delayed the introduction of these safety laws by several decades. This is a recurring pattern: something similar happened when animal experiments failed to confirm a connection between smoking and lung cancer in humans, and preventative measures in that area were delayed by many years.

Mesothelioma symptoms may appear as long as 30 to 50 years after exposure to asbestos. This explains why, despite a ban on asbestos use in the West, the incidence of malignant mesothelioma is still increasing.