Silica and oesophageal cancer.

The growth of animal cells in culture can be stimulated very powerfully when they are allowed to extend upon a solid surface. In normal fibroblasts, the maximum is reached either on a plane surface with an area of 2500 micron 2 or on a narrow fibre with a length of 250 micron. This growth-stimulating effect of fibres could help to explain how asbestos causes cancer. All asbestiform minerals are complex mixtures of different lengths, but siliceous macrohairs with a uniform length are borne by several species of the grass genus Phalaris. Some of these species are common contaminants of the bread eaten in a part of Iran where oesophageal cancer has an unexplained high incidence. A pure preparation of 200 micron silica fibres from one of these species is a powerful promoter of cancer in the skin of mice. Similar fibres from millet (Seteria italica) are associated with the same disease in China, and plant silica has long been known to be associated the it in South Africa. In addition, a rare thoracic tumour, which normally only occurs after exposure to asbestos, has been detected among sugarcane farmers in the United States and in India; fine silica fibres are liberated into the air during the harvest.