Document 1yN9mmQ48dJ7MypmrnE9w4Y6d

-9- 20 alllioa, which equals 3 million (good practioe)| 30%, with average "total dust concentration of 60 million, would equal times 50, or 15 million 9 (unsatisfactory). This formula is not applicable to any dust containing less than B% free silica." Theodore Hatch in his article "Some Fundamental Data on Mechanical Dust Traps" (Tech, Pub.No.637-A. 60, Amer. Inst. Min. 4 Met. Engrs., Aug., 1935) gives these i Safe Dust Concentrations. Impinger Samples; (Light Field, Low Power Counts.) Kind of Dust Maximum Permissible Concentration Containing no Silica. ............................... Containing any small amount of silioa, free combined............................. Containing 20 to 40 per cent. free Silica .................................... Greater than 40 per cent. free Silica .................................... , 50 millions per cubic foot . 30 " "" " ii nn n g ti tt ti tt (Preferably as much less as possible) Dr. R. R. Sayers, Medical Offioer in Charge, Industrial Hygiene and Sanitation, TJ,S. Public Health Servioe in answer to a question as to permissible dustiness writes* "During recent years, it has been customary to recommend threshold or permissible dust limits ranging from five to ten million partioles per cubic foot as determined by Public Health Servioe teohnique, especially with dust in whioh the percentage of free silioa is above five per oent. There is no basis for the application of this criterion to all dusts, but it is generally considered a safe practice and one whioh can be attained in most industries by exhaust ventilation, wetting, or other methods of dust control." As pointed out by Drinker and Hatch ("Industrial Dust" - 1936) it is unfortu nately impossible to evaluate dust exposures with the arithmetical nicety that we should have liked yet it is oommonly olaimed - by laymen - that prolonged exposure to low concentrations is just as serious as short exposures to heavy concentrations. This claim is contrary to a fundamental law of physiology. In discussing the sub ject Clark and Drinker ("Industrial Medioine" - 1935) remark that ".........a sub-thres hold stimulus (dust inhalation) for a long time produces no reaction whereas a rela tively brief super-threshold stimulus may cause a reaction". If workers are exposed to 6udden heavy concentrations, the threshold values suggested previously would be correspondingly lowered. However, most threshold values are weighed -- an average of the ranges. V - HOW MAY THE CONCENTRATION AND CHARACTER OF INDUSTRIAL DUSTS BE DETERMINED? The properties of a dust which determine its harmfulness have been shown to be its composition, concentration (the quantity suspended in the industrial atmosphere), its particle sire. The methods used for the determination of the concentration and oharacter of industrial dusts vary from a routine dust count for the purpose of evaluating the efficiency of dust control methods to a detailed study of both the concentration and the character of the dust in connection with medical and engineer ing surveys. E 01499