Signs and symptoms of mesothelioma vary depending on where the cancer occurs.

Pleural mesothelioma, which affects the tissue that surrounds the lungs, causes signs and symptoms that may include:

• Chest pain
• Painful coughing
• Shortness of breath
• Unusual lumps of tissue under the skin on your chest
• Unexplained weight loss

Peritoneal mesothelioma, which occurs in tissue in the abdomen, causes signs and symptoms that may include:

• Abdominal pain
• Abdominal swelling
• Nausea
• Unexplained weight loss

Other forms of mesothelioma

Signs and symptoms of other types of mesothelioma are unclear, since these forms of the disease are very rare.

Pericardial mesothelioma, which affects tissue that surrounds the heart, can cause signs and symptoms such as breathing difficulty and chest pains.

Mesothelioma of tunica vaginalis, which affects tissue surrounding the testicles, may be first detected as swelling or a mass on a testicle.

In general, cancer begins when a series of changes (mutations) happens in a cell's DNA. The DNA contains the instructions that tell a cell what to do. The mutations tell the cell to grow and multiply out of control. The abnormal cells accumulate and form a tumor.

It isn't clear what causes the initial genetic mutations that lead to mesothelioma, though researchers have identified factors that may increase the risk. It's likely that cancers form because of an interaction between many factors, such as inherited conditions, your environment, your health conditions and your lifestyle choices.

Asbestos exposure: The primary risk factor for mesothelioma

Most mesotheliomas are thought to be related to asbestos exposure. Asbestos is a mineral that's found naturally in the environment. Asbestos fibers are strong and resistant to heat, making them useful in a wide variety of applications, such as in insulation, brakes, shingles, flooring and many other products.

When asbestos is broken up, such as during the mining process or when removing asbestos insulation, dust may be created. If the dust is inhaled or swallowed, the asbestos fibers will settle in the lungs or in the stomach, where they can cause irritation that may lead to mesothelioma. Exactly how this happens isn't understood. It can take 20 to 60 years or more for mesothelioma to develop after asbestos exposure.

Most people with asbestos exposure never develop mesothelioma. This indicates that other factors may be involved in determining whether someone gets mesothelioma. For instance, you could inherit a predisposition to cancer or some other condition could increase your risk.

Factors that may increase the risk of mesothelioma include:

• Personal history of asbestos exposure. If you've been directly exposed to asbestos fibers at work or at home, your risk of mesothelioma is greatly increased.
• Living with someone who works with asbestos. People who are exposed to asbestos may carry the fibers home on their skin and clothing. Exposure to these stray fibers over many years can put others in the home at risk of mesothelioma. People who work with high levels of asbestos can reduce the risk of bringing home asbestos fibers by showering and changing clothes before leaving work.
• A family history of mesothelioma. If your parent, sibling or child has mesothelioma, you may have an increased risk of this disease.

Radiation therapy to the chest. If you had radiation therapy for cancer in your chest, you might have an increased risk of mesothelioma.

As pleural mesothelioma spreads in the chest, it puts pressure on the structures in that area. This can cause complications, such as:

• Difficulty breathing
• Chest pain
• Difficulty swallowing
• Pain caused by pressure on the nerves and spinal cord
• Accumulation of fluid in the chest (pleural effusion), which can compress the lung nearby and make breathing difficult