Montfort A. Johnsen, Ph.D. technicAl eDitor Aerosol Particle Size The Basics The human pulmonary system, despite its sophisticated complexity, is relatively ill-equipped to tolerate the effects of very finely divided inhaled particles. Over the last few hundred years, millions of miners have sickened or died from over-exposure to these insidious, virtually invisible particles. Around 300 B.C. the Greeks coined the word asthma to describe lung diseases of miners. Georgius Agricola published a book about 1450 A.D. linking lung diseases with demons that should be removed from mines by ventilation. It remained for Paracelcius, in his book published in 1569, to finally connect these diseases with dust inhalation. In it, he described mountain women in the Carpathian mining regions who had up to seven successive husbands; each man dying of “miner’s disease”. By about 1780, silica dust was finally recognized as the culprit in gold, silver and copper mines. The term silicosis was used to describe the disease. Meanwhile, coal miners often died of “black lung” disease from long-term inhalation of coal dust (carbon, silica and clay) that caused massive fibrosis, emphysema, consumption (tuberculosis) and cancer. Publications as early as 1650 described these effects. During 1853, the “Great Winter Smog” of London caused heavy smoke from coal burning furnaces to invert downward and linger, creating an epidemic that killed over 18,000 people. In 1920, tunnel workers in West Virginia bored through combinations of quartz (silica) and coal. Of 2,500 laborers, a reported 2,264 were severely affected or died from silicosis. Millions of about two-micron silica splinters were found embedded in the deep recesses of their lungs during autopsies. The first mine ventilation regulation was passed in the UK, limiting inhalable particles to 7mg/cubic meter. About 1925, the U.S. passed similar legislation, limiting these particles to 2mg/cubic meter. (This is lower than barely toxic levels of hydrogen cyanide: 100mg/cubic meter). The last of the historic “big three” is the thin fibrous form of asbestos, identified about 1945 as instigating an unusual type of lung cancer: mesothelioma. The huge financial and other impacts continue even today. These and related developments have opened the way for the U.S. National Ambient Air Quality Standards listing under the Clean Air Act Amendments (CAAA), where maximum permitted concentrations of particulate matter (PM), lead, ozone, sulfur dioxide and nitrogen dioxide are designated. The standards are modernized periodically. The four PM categories deal with PM-2.5 (annual), PM-2.5 (24 hour), PM-10 (annual) and the PM-10 (24 hour), where the numeric value expresses the arithmetic mean particle size in microns. Thus, for PM-2.5, half the weight of sampled particles will have a diameter of 0.000098" (0.0025 mm). A PM-2.5 particle has the ability to penetrate down to the terminal bronchioles and alveolar sacs of the lungs—the deepest part of the pulmonary system—where its removal (called clearance) is usually very slow. The retentive half-lives of some of these particles can be measured in years, especially if they become partially embedded in the cellular linings. The CAAA regulatory limit for PM-2.5 (24 hour) is 0.065mg/cubic meter for 98% of the distributed particles averaged over a three year period. The CAAA also regulates the concentration of airborne particles having an arithmetic mean diameter of 10 microns. A PM-10 (24 hour) particle distribution must not exceed 0.150mg/cubic meter for 99% of the particles. For further details, consult the CAAA, Section 112. A sample with a PM-10 mean diameter will normally have about 99% of the total weight of individual particles between 7–13 microns in diameter. Considering that the distribution may contain non-spherical solids, higher density micro-droplets, charged particles and other oddities, it is assumed that a very small portion may penetrate down to the terminal bronchioles, respiratory bronchioles and alveolar locales. There is no sharp cut-off between particles that can West Virginia coal miners in 1946. 22 Spray March 2016
Spray March 2016
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