INDISTI{IM, IIY(;iK\E MFCHOSCOI'Y 



lung is mainly below 5 microns in size, the 

 largest not exceeding 10 microns. 



Information obtained as a result of micro- 

 scopic investigation is usually reported to 

 the Medical and Safety Departments of the 

 company involved. In the case of conditions 

 exceeding the Threshold Limit Value, an 

 exhaust system to remove the dust from the 

 worker's breathing area may be recom- 

 mended. If an exhaust is already present, 

 results may indicate that it is inadequate in 

 size or not operating efficiently. 



Qualitative and Quantitative Analysis 



Identification of the dust is not always 

 necessary since in many industrial opera- 

 tions, the nature of the product is known. 

 However, many materials such as abrasives, 

 polishing and buffing compounds are sold 

 under trade names with little information as 

 to their chemical constituents. 



Prerequisite to microscopic examination, 

 spectrographic analysis is often of value for 

 determination of the elements present. For 

 example, if it is found that the sample is 

 largely silicon, there is the possibility that 

 it is primarily silicon dioxide. Confirmation 

 as to whether the material is silicon dioxide 

 and its molecular form can be made with the 

 microscope. Determination of molecular 

 form is important since investigation has 

 served to indicate that crystalline forms of 

 silicon dioxide such as cjuartz, cristobalite 

 and tridymite are more toxic than crypto- 

 crystalline and amorphous forms. 



Preliminary microscopic examination 

 should be made with the petrographic mi- 

 croscope (Fig. 1) or with a laboratorj^ mi- 

 croscope equipped with polarizing accessories 

 (cap analyzer and disc polarizer). Examina- 

 tion of the sample should be made with the 

 analyzer and sub-stage polarizer in a crossed 

 position. Particles observed will appear 

 bright or dark on a dark background indi- 

 cating whether they are single refracting 

 (isotropic) or double refracting (anisotropic). 

 Fig. 2 shows two forms of silicon dioxide as 



Fig. 1. Petrographic microscope. 



Fig. 2. Mixture oi {[uartz (briglit particles) 

 and opal (dark particles). 



observed with the petrographic microscope. 

 Amorphous opaline silicon dioxide appears 

 dark due to the fact that the ray of light 



401 



