Document DGyGQrYLKnm9wn97nweembdO5

INDUSTRIAL HYCIENE Free silica is uncombined silicon dioxide (SiOj) Sihcates contam sibcon and oxy gen combined with other elements in mme complex molecules The SiO reported in chemical analyses for mineral and geological reports is the total of the sibcon dioxide present, both the free silica (if present), and the silica combined m the mineral Such analyses are not reliable indications of the silicosis potential of the material, because it is the uncombined or free silica that is most important m industrial dust exposure. So that an exposure can be properly evaluated, the percentage of uncombined silica must be determined by petrographic analysis using a polarizing microscope or, preferably, by X-ray diffraction analyses and special ana lytical chemical procedures There has been some experimental evi dence that some dusts may tend to inhibit the aebon of silica on the body, but this in. hibitmg action is so slight and uncertain that it must be discounted m practice In fact, there is also evidence that the nonsihceous components of a dust mixture containing free silica may provoke a disabling condition more severe than that caused by the silica acting alone With the exception of asbestos and some talcs, the silicate dusts do not ordinarily cause a senous disabling lung condition sutm as is produced by free silica Much higher levels of silicate dusts than of free silica dust can be tolerated In many industries, men have worked with silicate dusts that con tained no free silica without development of disability or of nodulabcm in the lungs The X-ray ma> show shadows indicating dust deposits in the lungs, but the pneumoconiosis is essentially harmless However, partially disabling pneumoconioses have been reported where men have worked for long periods of time m very high concentrations of certain silicate dusts Disabling pneumoconioses from exposure to abnormally high concen trations of mica, tremohte talc, and kaolin dusts have been described in the literature The clinical signs are not the same for these silicate dusts as for free silica, but the symp toms can be marked The fine airborne fibers of asbestos can pass through the upper respiratory tract to the lower parts of the lungs to cause irrita tion and to form "asbestos bodies" where the fibers are encapsulated. This diffuse fibrosis probably begins as a "coDaF' about the terminal bronchioles. There is evidence that other minerals having a fibrous charac ter (except glass fiber) can produce a reac tion similar to that of asbestos. Following a study by the UJ> Public Health Service of the asbestos textile indus try, it was recommended that the dust con centration be kept at less than 5 mppcf to prevent asbestosis Evaluation of an expo sure to asbestos dust is based on the total amount of dust, since the concentration of injurious fibers will be kept within safe limits if the fine dust is kept below the suggested threshold limit Miscellaneous pneumoconioses Even though a dust is classified as harmless, amounts above the TLV can lead to trouble by causing a pneumoconiosis, mechanically irritating die walls of the respiratory system, or interfering with ordinary lung processes Mica dust and kaolin dost are two good ex amples of dusts that ordinarily are consid ered benign but amounts above the TLV can cause a troublesome pneumoconiosis. Mica pneumoconiosis has been observed in grind ing operations where mica dust, but no free silica was present There were marked changes m the X-ray pictures of the lungs and some disability The cases occurred where the dust exposures were massive over many years Toxic dusts and fumes Systemic reactions are caused by tone dusts and fumes of various elements and their compounds and by certain organic com pounds. All metalbc fumes are irritating, es pecially when freshly generated Industrially important metals and their compounds that can have a toxic effect when the dust or fumes are inhaled include arsenic, antimony, cadmium, chromium, lead, manganese, mer cury, selenium, tellurium, thallium, uranium, and a few others. Asbestosis Several minerals having a fibrous character are classed as "asbestos"-- hydrated sihcates of magnesium with vari able amounts of non, calcium, sodium, potas sium, and aluminum present as impurities U.S. Public health Service, Washington, D C. "A Study of Asbestosis in the Asbestos Textile Industry." Bulletin No 241 1938