THE WORLD BANK South Asia Urban Air Quality Management Briefing Note No. 13 The Science of Health Impacts of Particulate Matter South Asian cities record some of the highest levels of outdoor particulate pollution worldwide. Scientific research over the last two decades has demonstrated that particulate matter is the major pollutant of concern from the health perspective. Current research is focusing on questions relating to particulate matter characteristics such as size, number, and composition, and the mechanisms by which it causes health impacts. This briefing note presents the current understanding of the answers to those questions. T here is strong evidence linking urban air pollution has been used for measuring mass. Particulate matter to acute and chronic illnesses and premature death less than 10 microns in diameter is referred to as PM , 10 [1], and these adverse health impacts in turn carry and PM less than 2.5 microns is referred to as PM . 2.5 high economic costs to society [2]. The evidence The term fine PM has also come to be associated with correlating air pollution and health impacts is especially PM , ultrafine with PM less than 0.1 microns in diameter 2.5 strong for particulate matter (PM) [1]. Many questions (PM ), and coarse with PM in the size range between 0.1 still remain, however, about how PM affects human health 2.5 and 10 microns. Of late, the focus of PM measurement and what makes it particularly harmful. This note, the in ambient air of South Asian cities has shifted from last in a series of three notes addressing the health effects measuring TSP to mass-based measurement of PM 1 10 . of outdoor air pollution, attempts to answer these questions based on the current understanding of the science of the Origin and Transport of Particulate Matter health impacts of PM. Airborne PM has numerous sources, ranging from naturally occurring dust, sea salt, and pollen, to products What is Particulate Matter? of combustion such as forest fires, domestic cooking and Airborne particulate matter is not a single pollutant, but heating, garbage burning, power generation, and mobile rather a mixture of many subclasses of pollutants--in sources (such as vehicles and ships). Combustion solidandliquidforms,witheachsubclasscontainingmany processes normally contribute much more to the fine and different chemical species. Particulate matter may be ultrafine PM fraction, whereas noncombustion processes classified as primary or secondary. Primary particles contribute more to larger size PM fractions [4]. For are emitted directly by emission sources, whereas example, a study of the chemical composition of PM in secondary particles are formed through the atmospheric Mexico City in 1997 showed that: (a) PM particle mass 2.5 reaction of gases, such as the reaction between ammonia consisted of about 50 percent carbonaceous aerosols likely and oxides of nitrogen or sulfur, that leads to the formation from combustion sources, followed by approximately 30 of PM [3]. percent secondary aerosols, and 15 percent geological matter; whereas (b) PM particle mass consisted of PM is typically classified according to size. The particle 10 about 50 percent geological matter, followed by size can vary from approximately 0.005 microns (0.005 x carbonaceous aerosols (about 30 percent) and secondary 10-6 meters) to 100 microns in diameter (that is, from the aerosols (less than 20 percent) [5]. Unfortunately, such size of a few atoms to about the thickness of a human detailed studies are very limited in South Asia. hair). All ambient PM irrespective of size is referred to as suspended particulate matter (SPM), or total Fine PM tends to be distributed uniformly over large areas, suspended particles (TSP) when a gravimetric procedure thereby making it difficult to trace it to individual sources. 1In India, as per the National Ambient Air Quality Monitoring standards the term respirable suspended particulate matter (RSPM or RPM) is used interchangeably with PM . 10 1 The recent finding of the Asian brown haze, attributed should also increase with decreasing particle size and primarily to combustion sources in Asia, confirms the long increasing surface area. residencetimeandregionaldistributionoffineandultrafine The above discussion suggests that the particle size and particles [6]. Larger particles, however, normally have particle number may be more relevant indicators than shorter atmospheric lifetimes (minutes to hours) and do particle mass: the smaller the particle, the greater the not travel long distances (less than tens of kilometers). fraction of particles deposited in airways and lungs, and As a result they tend to be less evenly distributed, and are the greater the surface area available for interaction with usually found closer to their sources [3]. biological systems. Most of the research, however, on the role of particle size and number has been conducted Why is PM so Damaging to Health? in laboratories using animals or mathematical models. In Epidemiological studies from many different parts of the practice ambient monitoring of particle numbers is not world, conducted by different groups of researchers using common; rather, most of the monitoring the world over different data sets and analysis techniques, have generated still focuses on particle mass. results that confirm the magnitude of PM health impacts (see Briefing Note No. 11 [1] for more detail). While a Composition statistical association has been found between adverse The main components of urban PM are metals, organic health effects and PM , recent studies using PM data 10 2.5 compounds, materials of biologic origin, secondary have shown an even stronger association between health particulate matter (sometimes as ions), and the particle outcomes and particles in this size range. Evidence that core which often forms the bulk of urban PM and is smaller particles are more harmful is further supported frequently composed of pure or elemental carbon. Several by medical and toxicological research, which is studies, mostly on animals, have demonstrated the effect increasingly focused on understanding the role of particle of metallic PM on lung damage. Organic compounds are size (in the fine and ultrafine range) and composition in known to lead to mutations and even cause cancer. PM toxicity. Materials of biological origin such as fungal spores and pollen are known to induce a variety of allergic responses. Size and number Sulfate and nitrate ions lead to significant impairment of Size determines how different particles deposit in different the respiratory tract because of their acidic potential. parts of the respiratory tract. Studies have shown that Carbonaceous material in the particle core can by itself particles of different size vary in their respiratory tract lead to lung irritation and damage after chronic exposure. deposition, movement, clearance, and consequent In general fine and ultrafine particles are composed mainly retention time in the human body. Ultrafine particles tend of particles with a carbon core that contains a variety of to behave more like gases and hence travel to the lower metals, organic compounds, and secondary particulates region of the lungs as compared to larger particles which [4]. The surface area of the elemental carbon core is tend to get deposited in the upper or middle region of the considerably increased by its porous nature, greatly respiratory tract. Particles larger than about 10 microns enhancingtheadsorptionprobabilityofairbornesubstances in diameter are deposited almost exclusively in the nose such as organic compounds. and throat, whereas particles smaller than 1 micron are While particle composition is known to play an important able to reach the lower regions of the lungs. The role in inducing adverse health effects, much less is known intermediate size range gets deposited in between these about its role compared to that of particle size and mass. two extremes of the respiratory tract. The strongest evidence of the role of particle composition comes from studies investigating the effects of metallic Ultrafine particles are highly toxic to the lungs, even when particles on health. More recently, studies have started they comprise materials that are not toxic when present focusing on the impact of organic compounds associated in larger particles. The mechanisms for toxicity of ultrafine with fuel combustion, such as those contained in particles particles are still under investigation, but several produced by diesel-powered vehicles. observations may be made. For a given mass, the number of particles and the total surface area increase A recent study by the U.S. Environmental Protection dramatically with decreasing particle size. If toxic Agency(EPA)highlightedthelikelycancerriskfromdiesel components are adsorbed on the surface of ultrafine emissions [7], declaring it as a potential carcinogen. The particles, the level of interaction between the lungs and diesel particles, many of which are smaller than 1 micron the surface of ultrafine particles is likely to increase with in diameter, have a carbonaceous core with a large surface increasing surface area. It is not immediately clear, area to which various organic compounds are adsorbed, however, why the adverse impact of non-toxic particles including carcinogenic polycyclic and nitro-polycyclic 2 hydrocarbons. Diesel particles have also been shown to Specific health effects adsorb allergens from grass pollen, thus potentially The PM damage to lung defenses manifests itself in the increasing allergen deposition in the respiratory tract [8]. form of health effects such as acute respiratory infection (both upper and lower respiratory tract infections), chronic The composition of PM varies with factors such as the obstructive lung disease (especially bronchitis), asthma nature of sources and/or geographical location. For attacks, cardiovascular disease, and lung cancer. Further, example, the particle composition in the northeastern part recent research has increasingly shown that particles can of the United States, which has a high concentration of also affect other parts of the body, including the nervous coal-based thermal power plants, is dominated by system, by physically moving out of the airways and into secondary sulfate particles linked to sulfur dioxide the blood stream [4]. Thus particle deposition in airways emissions from the power plants. As a result, the health can set off a chain of events, potentially affecting parts effects of inhaled particles may also be affected by their of the body other than just the respiratory tract. composition. This suggests that more epidemiological studies based on PM and smaller particles with varying 2.5 As can be expected, the changes in the body are likely to composition are needed to see whether there is a be more severe in cases where the body's defenses are significant impact of particulate composition on health. already weak or previously damaged. Hence, certain population subgroups, such as the elderly, children, and Mechanism of Health Effects individuals with existing respiratory or cardiovascular Dose diseases, are at increased risk from exposure to PM. Whileambientconcentrationsarenormallyusedasaproxy Conclusions and Policy Implications for linking ambient air pollution to health effects, it is the dose that actually matters. Dose is defined as the quantity As discussed above, the science of health impacts of of material that reaches a target. Knowledge of the dose particulate matter is quite complex. After more than two of PM delivered to a target site or sites in the respiratory decades of research, however, answers to some of the tract is important for understanding possible health effects key research questions regarding the science of health associated with human exposure to ambient PM. The effects of PM are beginning to emerge. effect, however, of varying dose and potential duration of M Whilealargebodyofstudieslinksadversehealtheffects the effect are functions of the retention and clearance of to particles less than 10 microns in diameter (PM ), 10 particles from the respiratory tract, which in turn are the latest evidence strongly indicates that the fine (less affected by the characteristics of the inhaled particles than 2.5 microns) and ultrafine (less than 0.1 micron) (size, number, and composition) and the physical and fractions of PM are most harmful because of their physiological characteristics of the exposed population. ability to penetrate deeply into the lungs. Lung defenses M Small particles with toxic materials adsorbed onto surface are especially damaging. Ultrafine particles, Theadulthumanlung,withasurfaceareaof40-120square however, have been found to have adverse health meters, comes in contact with between 10,000 and 20,000 effects even when they comprise substances that are liters of ambient air daily. The lung has evolved a multi- not harmful when present in larger particles. layered defense mechanism to counter inhaled particles, M For a given mass, the surface area and particle number and it responds to particulate pollution with the same increase dramatically as the particle size decreases. defense mechanism as it does to pollens and spores [9]. As a result, a smaller mass of PM which happens to 10 consist primarily of ultrafine particles can be more The first layer of defense is a barrier of cells and fluids damaging than a greater mass of PM but with a that the foreign matter must penetrate before it enters 10 significant fraction falling in the coarse range. the tissues of the body. Fluid secretion, such as mucus lining the airways, is an important part of the first layer M Certain plausible mechanisms by which PM affects human health have been identified, strengthening that essentially traps and removes the larger particles. evidence from epidemiological studies. Coughing,activatedbythepresenceofparticles,alsohelps in the removal process. If the particles infiltrate the first Policy decisions for reducing the health risks of PM in layer of defense, "scavenger" cells come into play. These many countries are being made based on the current cells ingest the particles and attempt to destroy them. If understanding of the science of PM health impacts. A the burden of foreign matter overwhelms this line of review of the scientific evidence shows that the production defense, as can occur in response to inhaled particles, the of fine and ultrafine PM is concentrated in various lung defenses may be weaker from subsequent attacks. combustion processes, ranging from household cooking 3 and garbage burning to vehicle operation and industrial 4. Health Effects Institute (HEI). 2002. "HEI Perspectives: processes. These findings provide a sufficient basis for Understanding the Health Effects of Components of the an urgent need for identifying and supporting measures Particulate Matter Mix: Progress and Next Steps," April. to reduce combustion-related emissions across these Available at . sources. Before moving to expensive control measures in a specific sector or group of sources, however, it is 5. J.C. Chow, J. G. Watson, S.A. Edgerton, and E. Vega. 2002. important to gain a better understanding of their "Chemical Composition of PM and PM in Mexico City 2.5 10 contribution to health impacts and the effect of proposed during Winter 1997," Science of the Total Environment, control measures, especially if these measures also have No.287,pp.177-201. a welfare and equity impact. 6. United Nations Environment Programme. 2002. The Asian The monitoring and regulation of PM is on its way to Brown Cloud: Climate and Other Environmental Impacts, 2.5 becoming standard across the United States and other UNEP/DEWA/RS.02-3.Availableat. discussion presented in this note is whether standards 7. US EPA. 2002. Health Assessment Document for for, and monitoring of, PM levels in South Asia should be Diesel Exhaust, EPA/600/8-90/057F, May. Available at revisited in favor of regulating and monitoring PM . 2.5 . the ambient concentrations and sources of fine particles 8. UKDepartmentofEnvironment,Food&RuralAffairs.2001. (and hence the need for well-designed networks for Expert Panel on Air Quality Standards, Airborne Particles: measuring these particles), it is also important to account What is the appropriate measurement on which to base a for the complexity of measurement techniques (for more standard? A Discussion Document--Review of information, see references [10] and [11]). These Possible Mechanisms of Toxicity. Available at techniquesareexpensiveandmoretechnicallychallenging . Therefore, careful consideration should be given to the 9. UKDepartmentofEnvironment,Food&RuralAffairs.2001. pros and cons of monitoring PM , the monitoring of 2.5 Expert Panel on Air Quality Standards, Airborne Particles: which is relatively new even in industrial countries, and What is the appropriate measurement on which to base a PM for which there are many well-established and 10 standard? A Discussion Document--Size Distribution proven monitoring methods with years of experience to and Chemical Nature of Airborne Particles. Available at learn from. . References 10. UKDepartmentofEnvironment,Food&RuralAffairs.2001. 1. South Asia Urban Air Quality Management Briefing Note Expert Panel on Air Quality Standards, Airborne Particles: No. 11. 2003. "Health Impacts of Outdoor Air Pollution," What is the appropriate measurement on which to base a February. Available at . Measurements of Airborne Particles. Available at 2. South Asia Urban Air Quality Management Briefing Note . Reduction in Air Pollution," February. Available at 11. Chalmers University of Technology. 2002. "Lecture: . Measurement Methods II--How to detect, collect 3. US EPA. 2002. Third external review draft of air quality and analyse Aerosol particles." Available at criteria for particulate matter, Volume 1, April. Available < h t t p : / / f y. c h a l m e r s . s e / ~ m o l n a r / l e c t u r e s / at . Measurement%20Methods%20II.htm>. This briefing note was prepared in March 2003 as part of the South Asia program on urban air quality management, funded in part by the joint UNDP/World Bank Energy Sector Management Assistance Programme (ESMAP). The program's objective is to support the regiowide process of developing and adopting cost-effective and viable policies and efficient enforcement mechanisms to reverse the deteriorating trend in urban air quality. A full set of briefs and other materials are available at . For further information, contact Sameer Akbar (sakbar@worldbank.org) or Masami Kojima (mkojima@worldbank.org) about the program, and Sameer Akbar about this note. 4