Document 15Vy2Q70oGB9ZvErVN5MYyqBo

154 CHAPTER 8 1957 Guide v particles in the 0.1 micron vicinity may have influenced the determination . ' of average particle size. The lower limit of particle size visible to the naked eye cannot be stated definitely. It depends not only upon the individual eye, but also upon the shape and color of the particle, intensity and quality of the light, and na ture of the background or opportunity for contrast. Under ideal condi tions a particle of 10-micron size might be recognized, while under less favorable conditions it may be impossible to distinguish a particle smaller than 50 microns. The lower limit, of visibility probably ranges from 10 to 50 microns. Dusts, powders and granular materials are frequently classified by refer ence to the size of screens used for separation. Particles above 40 microns are said to be the screen sizes and those below, the sub-screen or microscopic sizes. Approximate or theoretical sizes of particles corresponding to the mesh scale of the U. S. Standard Sieve Series are given in Table 1. Microscopic examination of screened dust indicates that the average diameter of a sample of irregular particles may be substantially larger Table 1. Relation op Scbeen Mesh to Particle Size U. S. Standard Sieve Mesh... 400 325 200 140 100 60 35 18 Nominal Sieve Opening in 37 44 74 105 149 250 500 1000 . than the openings of the screen through which it has passed, if the particle shapes deviate considerably from the spherical form.2 The smallest di mension of many such particles will correspond with the maximum per missible distance between the wires of commercial screens made to ASTM Standard Specifications. Screening does not give sharp separation into size groups, and accordingly, such a classification is statistical rather than absolute. AIR POLLUTION BY SMOKE, ASH AND CINDERS Total airborne solids settling in urban areas are usually reported as soot ' fall in tons per (square mile) (month). Such data published for the cities , in this country range from 20 to 200 tons per (square mile) (month). To the air conditioning engineer this information may indicate the effective ness of smoke abatement or fuel combustion control methods in his locality, , but it does not provide a suitable index of the suspended dust that air cleaners in a ventilating system are expected to capture.3'4,6 Gravimet ric or weight data of the type given in Table 2 are preferable. In some cases airborne particle counts may be necessary, as for pollen, bacteria, spores, and insoluble dusts causing illness or lung disease. Dust concentrations by weight cannot be converted readily to concen trations by particle count because of the variability of particle size, shape and specific gravity, and the inherent characteristics of dust counting and weighing procedures. One milligram of dust per cubic meter of air may represent dust counts from 1 million to 100 million particles per cubic foot of air (lightfield microscope technic) according to the size distribution of Air Contaminants 155 the airborne dust sample. Information of this type for a specified applica tion is best obtained by simultaneous sampling for both counting and weighing and noting carefully at the time all factors that might affect the reproducibility of the count-weight ratio. Smoke Abatement and Air Pollution Control Successful abatement of atmospheric pollution caused by smoke requires the combined efforts of the combustion engineer, industrial executive, public health officer, city planning commission and the community at large. Electrification of industry and railroads, increases in the use of domestic oil and gas furnaces, and segregation of industrial districts are gradually providing effective aid in the solution of this problem. In the large cities where nuisance from smoke, fly-ash and cinders is more serious, limited areas obtain some relief by the use of district heating. (See Chap ters 14, 15 and 16 for further discussion on fuel burning technic.) Table 2. Dust Concentration Ranges Location Gbains per 1000 Cu Ft* Rural and suburban districts.............................. Metropolitan district............................. Industrial districts...................... Ordinary factories or workrooms...................... Excessively dusty factories or mines............. Minimum explosive concentrations......... 0.02-0.2 0 04-0 4 0.1 -2 0 0.2 -4 0 4-100 4000-200,000 * 1 grain per 1000 cu ft = 2.3 milligrams per cubic meter, i o* per cubic foot =* 1 gram per liter =* 1000 grams per cubic meter Milligrams peb Cubic Meter 0.05- 0.5 0.1 - 1.0 0.2 - 5.0 0.5 -10 10-1000 10,000-500,000 Many present ordinances limit the number of minutes in any one hour that smoke of a specified density (determined by comparison with a Ringelmann Chart which is described in Chapter 52) may be discharged. There is now considerable interest and activity in the control of air pol lution factors in addition to smoke. Difficulty in the establishment of acceptable criteria for certain corrosive and irritant gases, such as fluorides and the oxides of sulphur and nitrogen discharged with the gases of com bustion, and the frequently complicated technical and economic problems encountered in control, have delayed the drafting and enforcement of egislative measures. Recent reports of an increased incidence of diseases, such as pneumonia and lung cancer, in areas high in certain air contami nants, require further critical investigation before acceptance. The values TMally adopted will undoubtedly be lower than the M;A.C. (Maximum Uowable Concentration) limits for use in industry, because the exposure i^tinuous compared with the 8-hour day, 5 or 6-day week upon which va^ues are based, and because the exposed population contains waividuals with greater variation in age and health status. In foggy weather, or with an inversion of atmospheric conditions, accu sation of gaseous contaminants may cause irritation of eyes, nose, and piratory passages, and possibly cause even more serious physiological ^ects The Meuse Valley fog disaster (Belgium 1930) and the Donora air ^ (j^DDsylvania 1948) are classic examples in the history of gaseous pollution. In both instances it is believed that irritant gases, princi-