Particulate Matter

Properties and Effects on Health

Dr R L Maynard, Vyvyan Howard

• Publisher: Garland Science
• Year: 2020
• eBook ISBN: 9781000144666
• Topic: Ciencias biológicas
• Subtopic: Ecología

This is a rapidly expanding and highly topical research area. Written by authors and editors who are well known and respected in their fields, this text looks at the health effects caused by particulate aerosols, and discusses recent legislation and future strategies.

1 Introduction

Robert L Maynard

The views given here are those of the author and should not be taken as those of the UK Department of Health.

Air pollution is not a new phenomenon. Pollution of the air by particles is also not a new phenomenon; indeed, of the commonly studied air pollutants smoke, consisting of course of a particulate aerosol, was the first pollutant to be complained of and studied in detail. It is usual to trace the history of complaints about smoke in the UK to the 13th century when the use of coal for domestic heating and the firing of furnaces produced severe air pollution in London (Brimblecombe, 1987). Though there have probably been significant fluctuations in the degree of pollution, the problem of particles has persisted in London for 700 years. The lack of measurements of concentrations of particles in the air make it difficult to say when pollution in London was at its worst. It was certainly bad in the 17th century when the diarist John Evelyn published his Fumifugium or the Inconvenience of the Aer and Smoake of London Dissipated in 1661, and again in the 19th century when Charles Dickens described with characteristic detail, the smoke and fogs of London and other British cities and towns. Cold weather was noticed to be associated with fogs and the exceptionally cold winters of the last decade of the 19th century were associated with dense fogs in London. The combination of temperature inversions and a source, domestic chimneys, of unusual strength, led to the fogs being heavily laden with smoke.

In 1905 Des Voeux, Secretary to the Coal Smoke Abatement Society (later the National Society for Clean Air), coined the word smog to describe this combination of smoke and fog. The word has been used more recently to describe the products of photochemical reactions: ozone, aldehydes and secondary particles, as encountered in cities such as Los Angeles. It should be noted that the London smog that Des Voeux was describing was a reducing rather than an oxidizing chemical mixture and contained high concentrations of smoke particles and droplets of sulphur dioxide. This ‘old fashioned’ smog still exists in those parts of the world where coal is still burned on a large scale.

Despite efforts to reduce the production of smoke in London conditions remained poor and in the early decades of the 20th century, and in 1948 and 1952, severe smogs occurred. The 1952 London episode, The Great Smog’, was for many people literally the last straw, and some 4000 extra deaths occurred as a result (Ministry of Health, 1954). A disaster of such proportions could not be ignored and the Government, adopting a Private Member’s Bill advanced by the MP Gerald Nabarro, passed the Clean Air Act in 1956. This Act, the first of its kind, provided reimbursement to Local Authorities which established smoke-free zones and made grants to citizens to enable them to convert their domestic open fires to grates suitable for the combustion of smokeless fuel. The Act was successful and, as the smokeless zones spread, cities emerged from the winter gloom of centuries. True, the blackened buildings still bore witness to the past conditions, but gradually, clean-up work was undertaken and buildings such as the Houses of Parliament and Westminster Cathedral returned to their original colours. It is difficult, today, to find an example of a truly black public building: the memorial to Sir Walter Scott in Princes’ Street, Edinburgh, remains black and thus a memorial, also, to the days when Edinburgh was dubbed ‘Auld Reekie’.

The London smog of 1952 also triggered research into the effects of air pollutants on health. This had previously been patchy and work had focused on episodes of pollution such as those which occurred in the Meuse Valley in Belgium in 1930, and in Donora, Pennsylvania, USA in 1948 (Firket, 1936; Schrenk et al., 1949). Work at the Medical Research Council’s Air Pollution Research Unit at St Bartholomew’s Hospital, under the Directorship of Professor P. J. Lawther, led the way. He, with colleagues, studied not only the effects on health of episodes of pollution, but also the pathophysiology of the effects of the main pollutants and the effects of day-to-day variations in levels of pollution on the health of patients (Waller, 1991). These patients were drawn from a clinic for those suffering from chronic bronchitis. Just as smog characterized the great cities of the UK, so did chronic bronchitis, ‘The British Disease’ characterize the old age of so many of her citizens. That the prevalence of this disease might be in some way linked with exposure to pollution did not escape Lawther and he argued that the effects of long-term exposure were as deserving of study as the effects of severe air pollution episodes (Lawther, 1961). This call has still not been adequately heeded. It was also in the Air Pollution Unit that Mr Robert Waller first applied the techniques of electron microscopy to the examination of the particles found in London air (Waller et al., 1963). Estimates of the number of particles found in unit volumes of air were produced: these proved to be remarkably accurate. Modern electron-microscopic techniques are now being applied to the problem. These are discussed by Professor Pooley and Dr BéruBé in Chapters 3 and 4.

As levels of air pollution fell in London so it became more and more difficult to detect the effects on health of such day-to-day variations in concentrations of smoke and sulphur dioxide as still occurred. This led to the identification of thresholds of effect: the view being that as long as the smoke concentration remained below 250 µg m⁻³ and the sulphur dioxide concentration below 500 µg m⁻³ (24-h average concentrations) such patients as had been studied would be unlikely to detect effects upon their health as a result of day-to-day fluctuations in concentrations. This work was seminal in establishing guidelines for air quality and played a large part in providing a basis for the World Health Organization’s Air Quality Guidelines for Europe 1987. The figures provided by these studies stood as accepted thresholds until the early 1990s (WHO, 1990). Then things changed.

A trickle of new epidemiological studies that began in the late 1980s and turned into a flood in the 1990s, provided evidence that day-to-day variations in the already low concentrations of particles and other pollutants were still associated with effects on health. A number of reviews of this literature have been published (Department of Health, 1995; Lambert et al., 1998; Wilson and Spengler, 1996). These effects included increases in daily numbers of deaths and hospital admissions and less severe effects such as visits to general practitioners, symptoms and the consumption of anti-asthma remedies. To say that these studies have provoked controversy would be to seriously understate the case. Vigorous and sometimes acrimonious debate followed both in the correspondence columns of learned journals and in a less restrained form on the floors of international symposia. Critics argued that the results were artefactual and contrary to both common sense and established doctrine. Why were the arguments so ill-tempered? Three reasons can be suggested:

● the results of the new studies were so significantly different from those of the older studies that it seemed to some that if the new results were right then the old results had to be wrong. The results of the older studies had been reflected upon for years and were generally considered to be sound, though doubts had been expressed;

● toxicologists wondered how such small doses of pollutants as implied by the results of the new studies, could produce any effects, let alone death, and felt that the epidemiological studies must be flawed;

● those who represented, or who were associated with, industries and manufacturers that produced pollutants or devices such as motor vehicles that emit pollutants, saw that demands for cleaner operations and products would follow and realized that these would be both difficult to meet and inevitably expensive.

It is fair to say that the results of the new epidemiological studies are still not accepted by all as indicative of causal associations between low concentrations of pollutants and effects on health. Of the several pollutants studied, most emphasis has been placed on the results regarding particles. Several well argued and detailed papers have been published disputing the interpretation of these studies and have argued that the reported associations are produced, perhaps, by inadequate accounting for confounding factors and should not be taken as indicating a causal relationship (Gamble, 1998; Gamble and Lewis, 1996; Lipfert and Wyzga, 1995). The case cannot be argued again here. It might just be said that a number of detailed examinations of the evidence have been undertaken by independent experts: their conclusions have been similar and have suggested that reported associations are important and should be regarded as causal. This view has been accepted by the UK Department of Health. This acceptance of the evidence has moved the debate forward to a search for an explanation of the effects. This has not proved easy to find and, though several hypotheses have been produced, none has been accepted (Oberdörster et al., 1995; Seaton et al., 1995).

The sticking point for many toxicologists can be summed up simply: effects are said to occur when the daily dose of particles increases by as little as 250 µg; airborne particles do not contain any extraordinarily toxic substances and, therefore, such a dose could not have much effect, certainly could not kill anybody, and therefore the original evidence is flawed. This has been put deliberately starkly.

One of the most interesting ideas put forward to circumvent this objection has been that it is not the chemistry of the particulate aerosol but, rather, its physical properties that are important. This has shifted the debate into a new direction. It was suggested that very small particles, of nanometre dimensions, might be playing a role. This has been supported by studies that showed that materials such a titanium dioxide and carbon black had very different, and more marked, toxicological properties when presented as ultrafine (<100 nm diameter) particles than as particles of about 250 nm diameter (Amdur et al., 1988; Gilmour et al., 1997; Li et al., 1996; Oberdörster et al., 1992). Such a theory is appealing as it can make sense of the similar results produced by epidemiological studies undertaken in areas between which there are significant differences in the chemical composition of particles. For example, studies in European cities will largely reflect the effects of a primary aerosol produced by motor vehicles; studies in rural areas, on the other hand, might largely reflect the effects of a secondary aerosol produced by photochemical reactions. The aerosols differ significantly in chemical composition but each contains many ultrafine particles. How such ultrafine particles act remains unknown.

A point that has been ignored until recently is the capacity of ultrafine particles to be retained in the lungs. It was often assumed that such particles would simply waft in and out of the respiratory system in the breath. Nothing could be further from the truth. Recent modelling studies have shown that some 60% of inhaled particles of 20 nm diameter are deposited in the gas exchange zone of the lung (ICRP, 1994). Interestingly, it is true that few even smaller particles (<10 nm diameter) are retained in the lung: this is because they do not reach the lung but are essentially all deposited in the nose and pharynx.

A recurring question regarding the possible effects of ambient concentrations of particles has been, if man (and other animals) have always been exposed to particles, why have we (and they) not evolved a defence against them? We should ask in return whether particles of nanometre dimensions have always been common. Aerosols generated by physical means certainly do not contain many such small particles – it takes a great deal of energy to reduce, say, a rock to ultrafine particles. Fires, on the other hand, generate prodigious numbers of ultrafine particles: a coal gas flame burning for 15 s can raise the number count of particles in a chamber from 109 000 to 860 000 per cm³ (Gadle and Magill, 1956). These small particles, ‘nuclei’, tend to be short lived and rapidly aggregate to produce particles of 0.5–2.5 µm diameter. It is interesting to note that particles of <0.5 µm diameter are minimally deposited in the respiratory system: the physics of diffusion and sedimentation conspiring against their deposition.

Particles deposited in the gas exchange zone of the lung are removed by macrophages. Our knowledge of how macrophages recognize particles has increased dramatically during the past few years (Kobzik, 1995). Despite this, we lack information on how macrophages recognize, ingest and handle ultrafine particles. We do know that macrophages can be overloaded with particles and that they then lose their capacity to migrate and leave the gas exchange zone. Recent studies have suggested that overloading with very small particles may occur at lower percentage volume loadings than with larger particles (Donaldson et al., 1998). This point is taken up by Professor Donaldson in Chapter 8. Donaldson also takes up the interesting property of ultrafine particles of generating free radicals. One of the more remarkable recent discoveries in the area of particle toxic...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Abbreviations
• Contributors
• Foreword
• 1. Introduction
• Physicochemical properties of particulates
• Biological and health effects
• Monitoring and abatement
• Future policy considerations
• Glossary
• Index