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76 CHAPTER 7 1960 Guide Air Contaminants 77 Compiled by W. G. frank od eopytightad. amd by penruKioa. fig. 1------ Sizes and Characteristic of Airborne Particulate Matter liquid form by increasing the pressure, decreasing the tem perature, or applying both processes simultaneously. They are removed from the air by condensation with les difficulty than are the gases. Gases are normally formless fluids which tend to occupy a space or enclosure completely and uniformly at ordinary temperatures and pressures. The following substances quabfy as gases: oxygen, nitrogen, carbon.dioxide, carbon monoxide, hydrogen, ammonia, sulfur dioxide. Gases, likewise, may be solidified or liquefied by the proper control of temperature and pressure. The preceding classification is not suitable for the airborne living organisms, which,range in sire from the submicroscopnc viruses to the largest pollen grains, not considering the small est insect life. Bacteria range from about 05 to 5 microns in sire, fungus spores from l to 10 microns, and pollen from 5 to ISO microns. SIZES OF AIRBORNE PARTICLES Fig. 1 is a graphic tabulation of the properties of airborne solids and liquids arranged according to sire on~ the micron . scale. There are 25,400 microns in 1 inch. Particles larger than 10 microns are unlikely to remain suspended in air currents of moderate strength, but settle out by gravity at speeds dependent upon the shape, size and spe cific gravity of the particle, wind velocity, orientation of the collecting surface, and topography. These larger particles are of major interest to the engineer in the solution of nui sance problems, but it is usually the smaller particles, or those below 10 microns, that remain in the air long enough to be of hygienic as well as economic significance. Industrial dust particles are predominantly of the order of 1 micron in sire. Tremendous numbers are also present in the submicroscopic range below 05 micron, but those below 0.1 micron are not believed at present to be of practical'im portance, possibly due to their exceedingly small mass in com parison with the balance of airborne matter. In fact, particles this small may become the permanent atmospheric impurities that have little, if any, opportunity of settling because of the continual motion imparted to them by air currents and the molecular activity of gases (Brownian Movement). The survey* of atmospheric pollution in 14 American cities conducted from 1931 to 1933 indicated the average sire of out door dust particles to be 0.5 micron, as collected by the Owens jet dust counter and measured under the microscope. Inability of the light field microscope to reveal particles in the 0.1 micron vicinity may have influenced the determination of average particle sire. The lower limit of particle size visible to the naked eye cannot be stated definitely. It depends not only upon the in dividual eye, but also upon the shape and color of the parti cle, intensity and quality of the light, and nature of the back ground or opportunity for contrast. Under ideal conditions -a particle of 10-micron sire might be recognized, while under less favorable conditions it may be impossible to distinguish a particle smaller than 50 microns. The lower limit of visi bility probably ranges from 10 to 50 microns. Dusts, powders, and granular materials are frequently classified by reference to the sire of screens used for separa tion. Particles above 40 microns arc said to be the screen sizes and those below, the subscreen or microscopic sires. Ap proximate or theoretical sires corresponding to the mesh scale of the U. S. Standard Sieve Series are given in Table 1. Microscopic examination of screened dust indicates that Table \ .... Relation of icreen Mesh to Particle Size U. S. Standard mf Noatrncf per* optmg ta 400 325 200 140 too 60 35 37 44 74 105 149 250 500 18 1000 the average diameter of a sample of irregular particles may be substantially larger than the openings of the screen through which it has passed, if the particle shapes deviate consider ably from the spherical form* The smallest dimension of many such particles will correspond to the maximum per missible distance between the wires of commercial screens made to ASTM Standard Specifications. Screening does not give sharp separation into sire 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 Table 2 ..., Dust Concentration Ranges location Grata* per 1000 Mnfigrarai per Co Ft* Cubic Meter Rural aod suburban districts. Industrial districts................... Ordinary factories or work- Excessively dusty factories Minimum explosive concert- 0.02-0.2 0.04-0.4 0.1-2.0 0.2-4.0 4-400 4000-200,000 0.05-0.5 0. t-1.0 0.2-5.0 0.5-10 10-1000 10,000-500,000 * 1 {nia per 1000 ca ft -- IS mfliicnme per eubio meter, ! o* per eubic foot * j gnts per liter ~ 1000 stem* per cubic meter. data published for the cities in this country range from 20 to 200 tons per (square mile) (month). To the air-condition ing engineer this information may indicate the effectiveness 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.*' * * Gravimetric 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 concentrations 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 cvbicy foot of air (light field microscope technic) according to thex size distribution of the airborne dust sample. Information of this type for a specified application is best obtained by simul taneous sampling for both counting and weighing and noting carefully at the time all factors that might affect the repro ducibility 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 engi neer, industrial executive, public health officer, city planning