A close look at particles

PARTICULATE MATTER: PROPERTIES AND EFFECTS UPON HEALTH· Edited by: R L Maynard and C V Howard·BIOS Scientific Publishers Ltd, UK·186pp· Price £67.50

Published: Tuesday 29 February 2000

growing air pollution is posing a serious threat to human, plant and animal life. Suspended particulate matter emitted from mobile and stationary combustion sources, mining and a whole lot of other activities have biological and health effects. There is no doubt that some particles are indeed hazardous. However, the degree of hazard associated with specific types of particles and the precise mechanisms by which exposure to particulate pollution leads to ill health are yet to be fully understood and is at present the subject of intense research.

This book has a number of contributors drawing attention to large scale epidemiological studies of recent origin, which suggest that there may be a relationship between pm 10 (particulate matter with an aerodynamic diameter of 10 microns or m) exposure and cardiovascular disease. One micron is one-millionth of a metre or one-fiftieth the diameter of a human hair. Ken Donaldson, Vicki Stone and William MacNee in chapter 8 of this book give evidence of increase in coagulation of blood upon exposure to pm 10 particles, which may lead to inflammation in the lung. This local inflammation may in turn lead to heart attacks and stroke in susceptible individuals.

There does not appear to have been any systematic analysis of respective pm 10 emissions from different industries and mobile combustion sources with respect to the basic physical chemistry or the types of ill effects that have been discussed in this book. This needs exhaustive investigations. In Chapter 8 Donaldson, Stone and MacNee point out that traffic pollution is a major source of urban pm 10.

There is evidence that pm 10 contains an ultrafine component derived from combustion sources such as diesel exhaust. The diameter of most of the diesel particles is approximately 0.2 m, with 90 per cent by mass less than 1 m (Health Effect Institute, Massachusetts, us , 1995). Kelly A BruB, Timothy P Jones, Ben J Williamson and Roy J Richards point out in chapter 4 that the particles in diesel exhaust being tinier in size and larger in number, offer a large surface area to absorb organic compounds such as carcinogenic polyaromatic hydrocarbons ( pah s) and nitroarenes. They state that since these particles are respirable, the International Agency for Research on Cancer has classified diesel exhaust particulates as probable human carcinogen.

Currently much research is being directed at ways and means of reducing particulate emissions from diesel vehicles. Particle traps with secondary combustion are being developed to reduce the mass of particulate matter emitted with diesel exhaust. This will in turn drive the size distribution of particles in exhaust towards smaller sizes, which might or might not be beneficial.

For the general reader one of the more demanding presentations in the book is chapter 5 on "The structural and physical chemistry of nanoparticles." It is a crucial contribution by D A Jefferson and E E M Tilley and points the way forward for much of the research that needs to be performed expeditiously. It transpires that materials regarded as chemically relatively inert can become highly reactive and electrically charged when transformed into small particles. Not only does the process of fine particle formation lead to an increase in chemical reactivity but also, on occasions, it leads to the formation of completely new substances that do not exist in the material from which the particle originated. The study of the chemistry of such novel configurations is in its infancy.
Some recent discoveries The effect of mixtures of particles of differing chemical composition entering the blood stream in large numbers on a daily basis, is only just beginning to be investigated. Obviously, the surface chemistry of nanoparticles and their effect on organic molecules in an aqueous environment is going to be difficult but very important to study and will need an inter-disciplinary approach. In addition to biologists and pathologists who would want to know their possible effects in vivo aqueous systems, nanoparticles will also attract scientists with interest in atmospheric chemistry, meteorology, vehicle engine design and so on.

Again, analyses of the surface chemistry of nanoparticles are to be encouraged in association with toxicological studies to help resolve the dilemma faced by regulators of vehicle emission vis--vis use of particle traps. The trend towards the use of shorter chain-length fuels, such as liquefied petroleum gas ( lpg ) and compressed natural gas ( cng ) is already receiving support from governments the world over and may prove to be a better route to particulate emission control.

Until recently, it was believed that toxic effect of particles could be eliminated by decreasing the mass of particles. Recent epidemiological studies cited in this book have proved this theory wrong. It may be that numbers of very small particles present in the air have more potential for toxic effects than the total mass of particles present.

Studies in particle toxicology have undergone significant changes over the last few years, and this book provides an up-to-date and comprehensive account of the current knowledge in this area. It reviews physical and chemical properties of particulate matter, explains the adverse health effects that particles can cause, discusses how particles can be monitored and investigates instruments suitable for particle size distribution analysis and number-based concentration determination.

However, there are techniques in microscopy not covered in this book. These include 'Super sfem ' microscopy, scanning tunnel microscopy, atomic force microscopy and scanning proton micros-copy, besides others. Levels of pm 10 in the atmosphere can be considered as being composed of three main categories primary, secondary and course particles. The primary particles are emitted directly by combustion processes and are generally less than 2.5 m and often 1 m in aerodynamic diameter. Secondary particles are those which are formed from chemical reactions and include sulphates and nitrates formed from the reactions of oxides of sulphur ( so x ) and nitrogen ( no x ). They are also generally less than 2.5 m, but their size could vary depending on humidity.

The third category is the coarse particles formed from a variety of non-combustion sources. These include natural fires and human-influenced sources such as construction and mining/quarrying activities, resuspended road dust, tyre debris and so on. The particles generated by these sources mostly arise from mechanical attraction and are thus relatively large, that is, they generally have a diameter greater than 2.5 m. Biological particles like pollens and fungal spores tend to be relatively large, but can be found in the pm 10 category.

This categorisation of the sources of atmosphere particles is convenient from the point of view of air quality management. Very broadly, in a given urban area, the primary particles will arise from combustion sources in and around the urban area. However, the secondary particle will have a significant proportion of long-range transported material since the particle size and deposition velocities of sulphate and nitrate particles are small and their atmospheric life times are long. The life times of the coarse particles are short and their atmospheric transport distances are also short, they predominantly arise from within that area. In designing a rational control policy for atmospheric particles the contributions of different source categories to atmospheric particulate levels need to be quantified according to Martin Williams who has discussed air quality, strategies with respect to particles in chapter 11. The chapter highlights the challenges posed to policy makers by the rapidly developing science of atmospheric particulate matter.

While on one hand there is emerging epidemiological evidence that the finer fractions of pm 10 are the most important, evidence on pm 2.5 (particles with a diameter of 2.5 m) is growing. However, even in the event that consensus is reached that the fine (less than 2.5 m) or the ultrafine particles should be regulated, it would be reckless to move away from the regulation of pm 10 until it is conclusively proven that the toxic or harmful components of pm 10 don't reside in the coarse particles. This indeed is the rationale behind the move by the us Environmental Protection Agency for having retained air quality standards for pm 10 while promulgating a new air quality standard for pm 2.5.

The cumulative expertise contained between the covers of this book is enormous. The book provides an important review of the latest developments in the active research area of particulate matter which individuals interested in particulate pollution control need to know.

The author is professor emeritus with the Delhi College of Engineering

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