Document wqNY4Lkqd6DwqOm8va71w9jg4

INDUSTRIAL HYGIENE In very rare cases, enough dust has been inhaled to cause mechanical blockage of the air spaces. (Flour dust has been biown to cause this condition ) Some dusts may be essentially inert and remain in the lungs indefinitely with no recognizable irritation, and a few (like limestone dust) may be gradually dissolved and eliminated without harm Silicosis is the most important lung dis ease caused by the inhalation of mineral dust It is well-known m industries where crystalline free silica dust is present, such as foundries, glass manufacturing, granite cutting, mining, and tunneling in quartz rock It is found throughout the world, and in the past it has had many names, such as miner's asthma, grinder's consumption, miner's phthisis, potter's rot, and stone mason's disease The same occupational dis ease, however, is meant by all these names, and it is caused by dust from crystalline free silica, usually quartz Silicosis has been defined as "a disease due to breathing air containing silica char acterized anatomically by generalized fi brous changes and the development of mili ary nodulabon in both lungs, and chmcatty by shortness of breath, decreased chest ex pansion, lessened capacity for work, absence of fever, increased susceptibility to tubercu losis (some or all of which symptoms may be present), and by characteristic X-ray findings " Silicosis has been known to manifest it self after widely differing periods of expo sure to silica dust Apparently, develop ment of the disease depends upon 1 The amount and land of dust inhaled 2 The percentage of free silica contained in the dust 3 The form of the silica 4 The size of the particles inhaled 5 The duration of the exposure 6 The powers of resistance of the indi vidual concerned 7 The presence or absence of a complicat ing process such as infection Many theories have been advanced over the years to explain wby crystalline free silica acts as it does m the lungs It is now believed that the fibrosis produced is caused not by the hardness or sharpness of the particles, but by a combination of slight solubility with a physiochemical effect and an immunological effect--but no one is cer tain of the exact mechanism of the disease. Experimental work on the reasons for the development of silicosis is still going on in various parts of the world If the precise mechanism of silicosis could be determined, better medical preventive measures might be developed and possibly a cure could be found Amorphous free silica differs from cry stalline free silica in physical structure and in physiological effects In the amorphous state, molecules of silica are randomly ori ented and may be naturally converted to opal and diatomaceous earth (heselguhr) or artificially converted into such forms as silica gel, shea fume, and fused silica or quartz. If amorphous silica is heated to a high temperature, as in calcining, forms of cry stalline free silica called enstobahte and tndymite result, intermediate forms of amor phous silica are known as crypto-crystalline (ultra-microcrystalline) Inhalation of these crystalline forms can readily cause diatomite pneumoconiosis When diatomaceous earth is calcined, par ticularly m the presence of a trace of alka line flux, appreciable quantities are con verted to enstobahte As a result of studies made by the U.S Public Health Service, it has been recommended that the threshold limit value for crude or amorphous diatomite be placed at 20 mppcf (million particles per cubic foot), but that the atmospheric con centration for dust containing enstobahte be kept under 5 mppcf. Various commercial products containing particles of silica under 1pm size are available The physiological effects of these products have not been well defined Until more experience with human beings is avail able, thee products should be handled with care American Pubhc Health Association, 1790 Broadway, New York City. "Report (Joint) of the Committee on Pneumoconi osis and the Committee on Standard Prac tices in Compensation of Occupational Dis eases " Year Book, 1933 39-7