Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Airborne Particulate Matter Airborne particulate matter, which includes dust, haust, and secondary organic compounds formed dirt, soot, smoke, and liquid droplets emitted into by photochemistry). These species may be the most the air, is small enough to be suspended in the abundant fine particles after sulfates. Additionally, atmosphere. Airborne particulates may be a com- atmospheric reactions of nitrogen oxides produce plex mixture of organic and inorganic substances. nitric acid vapor (HNO3) that may accumulate as They can be characterized by their physical at- nitrate particles in both fine and coarse forms. The tributes, which influence their transport and most common combination of coarse particles con- deposition, and their chemical composition, sists of oxides of silicon, aluminum, calcium, and which influences their effect on health. iron. The physical attributes of airborne particulates include mass concentration and size distribution. Terms and Sampling Techniques Ambient levels of mass concentration are mea- sured in micrograms per cubic meter (µg/m3); size Several terms are used to describe particulates. attributes are usually measured in aerodynamic Generally, these terms are associated with the diameter. Particulate matter (PM) exceeding 2.5 sampling method: microns (µm) in aerodynamic diameter is gener- Total suspended particulates (TSP) includes par- ally defined as coarse particles, while particles ticles of various sizes. Some proportion of TSP smaller than 2.5 microns (PM2.5) are called fine par- consists of particles too large to enter the human ticles. The acid component of particulate matter, respiratory tract; therefore, TSP is not a good in- and most of its mutagenic activity, are generally dicator of health-related exposure. TSP is mea- contained in fine particles, although some coarse sured by a high-volume gravimetric sampler that acid droplets are also present in fog. Samples taken collects suspended particles on a glass-fiber fil- in the United States showed that about 30% of par- ter. The upper limit for TSP is 45 microns in diam- ticulate matter was in the fine fraction (Stern et al. eter in the United States and up to 160 microns in 1984). Europe. Particles interact with various substances in the TSP sampling and TSP-based standards were air to form organic or inorganic chemical com- used in the United States until 1987. Several coun- pounds. The most common combinations of fine tries in Central and Eastern Europe, Latin particles are those with sulfates. In the United America, and Asia still monitor and set standards States, sulfate ions account for about 40% of fine based on measurements of TSP. As monitoring particulates and may also be present in concen- methods and data analysis have become more trations exceeding 10 µg/m3 (USEPA 1982b). The sophisticated, the focus of attention has gradu- smaller particles contain the secondarily formed ally shifted to fine particulates. Recent evidence aerosols, combustion particles, and recondensed shows that fine particulates, which can reach the organic and metal vapors. The carbonaceous thoracic regions of the respiratory tract, or lower, component of fine particles—products of incom- are responsible for most of the excess mortality plete combustion—contains both elemental car- and morbidity associated with high levels of ex- bon (graphite and soot) and nonvolatile organic posure to particulates. Most sophisticated stud- carbon (hydrocarbons emitted in combustion ex- ies suggest that fine particulates are the sole factor 201 202 PROJECT GUIDELINES: POLLUTANTS accounting for this health damage, while exposure sult of combustion processes, including the burn- to coarse particulates has little or no independent ing of fossil fuels for steam generation, heating and effect. household cooking, agricultural field burning, die- The particles most likely to cause adverse health sel-fueled engine combustion, and various indus- effects are the fine particulates PM10 and PM2.5 — trial processes. Emissions from these particles smaller than 10 microns and 2.5 microns anthropogenic sources tend to be in fine fractions. in aerodynamic diameter, respectively. They are However, some industrial and other processes that sampled using (a) a high-volume sampler with a produce large amounts of dust, such as cement size-selective inlet using a quartz filter or (b) a di- manufacturing, mining, stone crushing, and flour chotomous sampler that operates at a slower flow milling, tend to generate particles larger than 1 rate, separating on a Teflon filter particles smaller micron and mostly larger than 2.5 microns. In than 2.5 microns and sizes between 2.5 microns and cold and temperate parts of the world, domestic 10 microns. No generally accepted conversion coal burning has been a major contributor to the method exists between TSP and PM10, which may particulate content of urban air. Traffic-related constitute between 40% and 70% of TSP emissions may make a substantial contribution to (USEPA1982b). the concentration of suspended particulates in ar- In 1987, the USEPA switched its air quality stan- eas close to traffic. Some agroindustrial processes dards from TSP to PM10. PM10 standards have also and road traffic represent additional anthropo- been adopted in, for example, Brazil, Japan, and genic sources of mostly coarse particulate emis- the Philippines. In light of the emerging evidence sions. on the health impacts of fine particulates, the The largest stationary sources of particulate USEPA has proposed that U.S. ambient standards emissions include fossil-fuel-based thermal for airborne particulates be defined in fine particu- power plants, metallurgical processes, and ce- lates. ment manufacturing. The physical and chemical Black smoke (BS) is a particulate measure that composition of particulate emissions is deter- typically contains at least 50% respirable particu- mined by the nature of pollution sources. Most lates smaller than 4.5 mm in aerodynamic diam- particles emitted by anthropogenic sources are eter, sampled by the British smokeshade (BS) less than 2.5 microns in diameter and include a method. The reflectance of light is measured by larger variety of toxic elements than particles emit- the darkness of the stain caused by particulates ted by natural sources. Fossil fuel combustion gen- on a white filter paper. The result of BS sampling erates metal and sulfur particulate emissions, depends on the density of the stain and the opti- depending on the chemical composition of the cal properties of the particulates. Because the fuel used. The USEPA (1982b) estimates that more method is based on reflectance from elemental than 90% of fine particulates emitted from sta- carbon, its use is recommended in areas where tionary combustion sources are combined with coal smoke from domestic fires is the dominant sulfur dioxide (SO2). Sulfates, however, do not component of ambient particulates (WHO and necessarily form the largest fraction of fine UNEP 1992). After reviewing the available data, particulates. In locations such as Bangkok, Ostro (1994) concluded that BS is roughly equiva- Chongqing (China), and São Paulo (Brazil), or- lent to PM10. However, there is no precise equiva- ganic carbon compounds account for a larger lence of the black smoke measurements with fraction of fine particulates, reflecting the role of other methods. The BS measure is most widely emissions from diesel and two-stroke vehicles or used in Great Britain and elsewhere in Europe. of smoke from burning coal and charcoal. Al- though sulfates represent a significant share (30– Sources of Particulates 40%) of fine particulates in these cases, care is required before making general assertions about Some particulates come from natural sources such the relationship between sulfates and fine par- as evaporated sea spray, windborne pollen, dust, ticulates, since the sources and species charac- and volcanic or other geothermal eruptions. Par- teristics of fine particulates may vary significantly ticulates from natural sources tend to be coarse. across locations. Combustion devices may emit Almost all fine particulates are generated as a re- particulates comprised of products of incomplete Airborne Particulate Matter 203 combustion (PICs, which may include toxic organ- Population-based cross-sectional and longitudi- ics) and toxic metals, which are present in the fuel nal studies (see, for example, Lipfert 1984; Dockery and in some cases may also be carcinogenic. Par- et al. 1993) have found an association between ticulates emitted by thermal power generation may long-term exposure and mortality. Using 14-to-16- contain lead, mercury, and other heavy metals. The year studies in six U.S. cities, and controlling for melting, pouring, and torch-cutting procedures of individual risk factors, including age, sex, smok- metallurgy emit metal particulates containing lead, ing, body-mass index, and occupational exposure, cadmium, and nickel. Particles emitted by the ce- Dockery et al. (1993) found a significant connec- ment industry are largely stone or clay-based par- tion between particulate air pollution and excess ticulates that may contain toxic metals such as mortality at average annual PM10 concentrations lead. as low as 18 µg/m3, well below the current U.S. ambient standard of 50 µg/m3. Studies on the ef- Impacts of Exposure fect of particulates on human health summarized by Ostro (1994) suggest an increase in human mor-