Black lung disease also known as Coal workers' pneumoconiosis, is a legal term describing a preventable, occupational lung disease that is contracted by prolonged breathing of coal mine dust. There are two forms: simple CWP and complicated CWP, which also involves progressive massive fibrosis (PMF). Coal dust provides a sufficient stimulus for the macrophage to release various products, including enzymes, cytokines, oxygen radicals, and fibroblast growth factors, which are important in the inflammation and fibrosis of CWP. The centres of dense lesions may become necrotic due to ischemia, leading to large cavities within the lung. There is no proven effective treatment for Black lung disease, although complications can be treated. Avoiding further exposure to the dust is the only one of treatment. Black lung disease can be prevented by controlling dust and having good ventilation in the workplace. Lung cancer continues to be the leading cause of death in both men and women in the US, with over 158,900 deaths in 1999. Worldwide, lung cancer kills over 1 million people a year. Work to investigate the relationship between respirable dust exposure and coal worker's pneumoconiosis was carried out in Britain by the Institute of Occupational Medicine. This research was known as the Pneumoconiosis Field Research (PFR).