Exposure to asbestos can cause a variety of diseases, both malignant and benign. Malignant diseases are those that involve neoplasms, or cancer growths, that can metastasize, or spread to other organs or body systems. Benign diseases are those that are not cancerous. "Benign" in this sense certainly does not mean harmless, as these conditions can be life-threatening themselves.
Asbestos is the name given to a collection of naturally occurring minerals that occur in fibrous forms. The very tiny fibers of asbestos cause disease when inhaled. They lodge in the lungs and can overwhelm the ability of the body to protect itself. In the lungs, the nearly-indestructible fibers cause long-term irritation, inflammation and scarring and can eventually result in cancer.
There are two major types of asbestos fibers: chrysotile, the "serpentine" fibers, which are long and curled, and amphiboles, which are long straight fibers. Either type can cause disease. The body's immune system attacks the fibers with cells called macrophages that engulf foreign substances and "eat" them. When the fibers are longer than the size of a macrophage, about 20 micrometers, they are incompletely destroyed, so longer fibers are more dangerous to the body and more likely to cause disease.
Asbestosis is the name given to the benign disease condition of scarring and fibrosis that occurs when embedded asbestos fibers irritate the lung tissue over a long period of time. It usually progresses slowly, and in some patients has relatively minor symptoms. It may take 20 to 40 years after asbestos exposure to become noticeable. As the lung tissue becomes scarred, it loses mobility and is unable to expand as well during breathing. Symptoms usually include shortness of breath and coughing. Smoking increases the severity and rate of development. People with asbestosis are at a higher risk from viral and bacterial infection. They may not recover well from pneumonia, for instance. They also have higher risk of low blood oxygen levels and heart failure.
The pleura, which is a thin membrane that surrounds the lungs, is particularly susceptible to damage caused by asbestos fibers. There are two pleural layers, one immediately surrounding the lung tissue, and another lining the chest wall. Healthy lungs have a thin layer of fluid between the two layers to allow the pleura and thus the lungs to move easily.
There are four types of benign pleural disease caused asbestos: benign pleural effusion, pleural plaques, diffuse fibrosis, and rounded atelectasis.
Benign pleural effusion: A pleural effusion is an accumulation of excessive fluid between the two layers of pleura. This is the most common reaction to asbestos exposure to appear within 5 to 20 years after exposure. Pleural effusions can vary from completely asymptomatic to active inflammatory pleuritis (infection of the pleura) with pain and accumulation of bloody pleural fluid. They may grow larger or smaller over time, disappear or reappear. They are benign and do not develop into malignancy. However, pleural effusions are also one of the first signs of mesothelioma, a highly aggressive form of asbestos cancer caused by asbestos exposure, so they cannot be assumed to be benign in people with a history of asbestos exposure without further diagnostic investigation.
Pleural plaques: Pleural plaques are localized areas of fibrosis of the pleura. This is the most common pulmonary response to asbestos inhalation overall. Plaques are present in approximately 50% of people with a history of heavy exposure to asbestos, and the presence of plaques is considered proof of prior asbestos exposure. Most plaques are asymptomatic. There is no evidence that they develop into cancers. They may be barely visible or they may involve large areas. These are found more in males and females, with an increasing probability with advancing age. Plaque formation progresses at an extremely slow rate. They can become calcified over time. When extensive, they can restrict lung movement, resulting in functional impairment such as shortness of breath.
Diffuse pleural fibrosis: Diffuse pleural fibrosis appears as scarring and thickening of the pleural layers all over their surfaces. This can also occur in the visceral pleura, which is a similar layer of membranes in the abdomen. A rind of scar tissue may form around the lungs and prevent the lungs from expanding properly, leading to increasing shortness of breath. There can even be fusion of the pleural layers. Diffuse pleural thickening can cause significant functional impairment.
Rounded atelectasis: Rounded atelectasis is a unique kind of pleural thickening. It occurs when an area of pleural fibrosis extends into the lung tissue and makes a portion of the lung airless. It is sometimes called "folded lung". It is much less frequent than focal plaques or diffuse fibrosis. Rounded atelectasis is usually asymptomatic but if it becomes large it can cause symptoms.
There are two main types of malignant disease caused by asbestos exposure: bronchogenic carcinoma (lung cancer) and diffuse malignant mesothelioma.
Lung cancer: Bronchogenic carcinoma is the medical term for the lung cancer we commonly associate with cigarette smoking. Asbestos exposure increases the risk of developing small-cell and non-small-cell bronchogenic carcinoma. Nonsmokers who are exposed to asbestos are five times more likely to develop lung cancer than the average nonsmoker. Heavy smokers who are exposed to asbestos are up to sixty times more likely to develop lung cancer than the average nonsmoker.
Diffuse malignant mesothelioma: Mesothelioma is an aggressive tumor of the mesothelial cells, most commonly developing in the pleura of the lungs. It can also develop in the membranous layers around the abdominal organs or around the heart, each causing a different type or the disease: pleural mesothelioma, pericardial mesothelioma, or peritoneal mesothelioma. It is an extremely rare cancer among the population in general, but for people exposed to asbestos, the lifetime risk of developing mesothelioma is high. It has a very long latency period and can take 20 to 60 years to develop, with an average latency period of about 30 years. The prognosis is very poor and death will usually occur within one to two years of diagnosis, although longer survival rates are documented.
Other malignancies have been linked to asbestos including cancer of the esophagus, throat, kidney, gallbladder, stomach and intestines. Past research also suggests that asbestos exposure may be associated with the development of several autoimmune diseases, such as rheumatoid arthritis, lupus, and scleroderma. The method by which asbestos affects the autoimmune response is not known.
Most cases of asbestos diseases have a strong exposure history, but some cases develop after only minimal exposure. There is a long latency period between exposure to asbestos and development of disease, so even in countries like the US where asbestos exposure is now regulated and diminished from previous levels, these diseases are still developing. Globally, the incidence of disease is expected to peak 30 to 40 years after the period of peak usage.