Crystalline silica Basic Principles is Occupational Hygiene 2010

43 4 EXAMPLES OF HAZARDOUS SUBSTANCESPROCESSES

4.1 Crystalline silica

Crystalline silica or quartz SiO 2 is the most widely occurring of all minerals and it is found in most rocks. The most commonly occurring form of silica is the sand found on beaches throughout the world. In the dry form, fine crystalline silica constitutes a toxic hazard since its inhalation as airborne dust could give rise to silicosis. Silicosis is a pulmonary fibrosis which is regarded as the most common and severe of all pneumoconioses. The risk of developing the disease depends on three factors, namely; dust concentration in the atmosphere; the percentage of free silica in the dust, and the duration of exposure. Silica is encountered during many processes which use minerals e.g. quarrying and mining, brick, tile and refractory manufacture, pottery and ceramic, sandblasting, Glass manufacture. At the beginning of this century, fatal cases of silicosis with a rapid evolution period 1-3 years were not uncommon among workers who inhaled enormous amounts of dusts containing a high quartz content. In many instances, death was due to the superimposition of tuberculosis. With the introduction of improved working conditions and modern methods of dust control, this rapidly evolving form of silicosis has virtually disappeared but has been replaced instead by the very slowly developing 15-30 year form of the disease. The initial stages of silicosis are asymptomatic and are only revealed by periodic radiological examination of workers exposed to free silica. The first symptoms of silicosis are loss of breath on exertion. In the serious cases, the symptoms occur even on very slight exertion or when the patient is at rest. As a rule there are no other subjective symptoms. Thus, the diagnosis of silicosis is largely by clinical examination and radiology. The ability to bring about lung changes is somewhat dependent upon the crystalline form that the silica can be in, and this is reflected in the current UK Workplace Exposure Limits, together with the particle size likely to be found. 44 Silica, Amorphous Total Inhalable Dust 6 mg.m -3 8 hour Time Weighted Average Respirable Dust 2.4 mg.m -3 8 hour Time Weighted Average Silica, Fused Respirable Dust 0.08 mg.m -3 8 hour Time Weighted Average Crystalline Silica Cristobalite, Tridymite Respirable Dust 0.1 mg.m -3 8 hour Time Weighted Average

4.2 Machine made mineral fibre MMMF