As the 2011 deadline for the Pleural Plaques Former Claimants Scheme is reached on 1 Aug, there is certain to be confusion surrounding the differences between pleural mesothlioma, pleural plaques, pleural thickening and pleural effusion.

Pleural mesothelioma is the most common type of mesothelioma, an aggressive cancer that develops in the mesothelium, i.e. the membrane that lines many of the body’s organs and cavities.

The membrane called the pleura is defined as two separate protective linings, the outer layer – called the parietal layer – lines the entire chest cavity and the diaphragm, while the inner layer – or visceral layer – covers the lungs.

As with all mesothelioma cancers, pleural mesothelioma is solely caused by exposure to asbestos, which mainly occurred due to a lack of asbestos awareness by countless thousands of workers employed in major UK engineering, manufacturing and construction industries throughout the greater part of the twentieth century.

The disease begins to develop after asbestos fibres, which have been breathed in as asbestos dust become trapped in the spaces between the mesothelial cells. Despite the cancer developing typically in one layer, it can ‘metastasize’, i.e. spread, to the other layer.

However, due to the unusually long gestation period, it may take between 15 to 40 or 50 years before the first mesothelioma, or asbestosis symptoms, will appear and at a very late stage of the disease. This may often mean that the victim may only have up to 18 months left to live.

Consequently, asbestosis treatments, such as radiation or surgery may delay the spread of the disease by a very short period only. There is a three year deadline by which to submit a mesothelioma claim from confirmed diagnosis.

Pleural plaques are localised scars, called fibrosis, consisting of collagen fibre deposits, and are the most common sign of exposure to asbestos. Unlike other airborne particles, asbestos fibres are small enough to avoid the lungs’ natural filtering system and imbed themselves permanently in bodily tissues, where they cause inflammation and scarring.

Around 50 per cent of those who are exposed to asbestos over prolonged periods of time develop pleural plaques – normally found in the parietal pleura, i.e. on the inside of the diaphragm – which also first appear between 15 to 40 years after initial exposure

Pleural plaques can form from even low-dose, intermittent exposure but are themselves benign, i.e. non-cancerous, cannot become malignant, are not necessarily a sign of an asbestos lung disease nor are associated with any symptoms. However, pleural plaques are almost always present in patients with asbestosis and are often present in patients suffering with mesothelioma.

Pleural effusion is the accumulation of fluid between the membranes lining the lungs and the chest cavity. Many diseases and medical conditions can cause pleural effusions, but it can also occur as a result of asbestos exposure causing inflammation and subsequent diseases.

Cells contained within the membranes surrounding the body’s major organs produce small amounts of fluid which behave as lubricant and allow the organs to rub alongside each other without causing damage.

The fluid that fills the space between the visceral pleura and parietal pleura is called pleural fluid and normally, is easily absorbed by capillaries and lymph nodes. Pleural thickening, asbestosis and malignant mesothelioma, can cause the overproduction of fluid, called pleural effusion.

Pleural thickening is a common side effect and an early warning sign of exposure to asbestos. The pleural membranes thicken as a result of the chronic irritation and inflammation their needle-like fibres cause when they become embedded in the delicate tissue.

The condition occurs in conjunction with other consequences of pleural disease, such as effusion or fibrosis, i.e. the excessive amounts of connective tissue which occur as the lungs attempt to repair the damage caused by asbestos fibres.