Malfunction of the lungs caused by inhalation of dust. The condition is infrequent, and associated with certain occupations – it is an occupational disorder. The function of the lung deteriorates according to the quantity of substance inhaled, and the disorder is classified according to the nature of substance. The most common varieties are: silicosis (accumulation of silica or sand particles), and asbestosis (accumulation of asbestos fibres). Silicosis occurs in jobs such as quarrying, sand-blasting and grinding. The sand particles cause the formation of knots in the alveoli, resulting from the clustering of layers of connective tissue cells. As the disorder develops the knots of tissue form masses, a condition known as interstitial fibrosis, which gives a characteristic image on a chest X-ray. The knots show up as denser patches on the film, and the area around is brighter because of excess air in the lungs, the result of ephysema, wastage of the lung with expansion of the alveoli. Lung function tests show decrease in lung capacity and in the ability to breathe out quickly and powerfully. The disorder can be prevented by wearing good quality masks. Miners run a high risk of contracting pneumoconiosis because they inhale large quantities of coal dust.