A close look at particles

Simply put, particles are tiny killers thatadversely affect humans at very lowconcentrations and even with minimal increase

Published: Wednesday 15 December 1999

A close look at particles

Small and dangerous: Ultra-fin even at low concentrations, airborne particles are lethal: T he great puzzle was if air pollution levels had visibly and measurably been vastly improved, how could they continue to have such detrimental effects on health? By the late 1970s, studies conducted in the us found that health damage was more strongly associated with particulate and not with sulphur dioxide.

This defied reason. Toxicologists were at a loss to explain how even small doses of particles, as shown by the results of the new studies, could have any harmful effects, let alone death. For a long time many scientists were doubtful of the result and this led to many acrimonious debates on the issue. However, eventually these new studies proved wrong everything that was earlier said or believed about there being no possible harmful health effects below a certain level of concentration.

Even more interesting is that the scientists are looking for an answer not only in the chemical properties but also in the physical property of the particles, which, simply put, means the size of a particle. Says Maynard, "This has shifted the debate in a new direction as it was suggested that very small particles might be playing the role."

Explains Anthony Seaton of the University of Aberdeen, uk , who has researched airborne particles and their health effects, "A clue has come from the consideration of the size distribution of particles produced when fuel is burned and a realisation that the smaller they are, the greater their surface area. Therefore, their potential to carry toxic elements deep into the lung increases. The greater majority of these particles are less than 1 micron (one-fiftieth the diameter of a human hair) in diameter and most fall in the size range of 20-100 microns -- being so small in size they penetrate deep into the lung." This simply means that as the size of the particles get smaller and their number increases, other toxic chemicals in the emissions get more surface area to cling to and, therefore, become more lethal. Maynard states that "studies have shown that carbon black have very different and more toxicological properties when presented as ultrafine." Thus, the scientific focus is getting more microscopic, looking for particles smaller than one-millionth of a metre.

Minimal increase in particulate levels can have adverse health effects: Scientists now believe that even a very small increase in concentration in particles can cause great harm. Says Anthony Seaton, "Despite the considerable reductions in the concentrations of particles in urban air, it has been recognised that increases in deaths and illness still occur in relation to rise in the level of particles in the air of major cities." There is a scientific consensus that very small rises in concentrations even from a baseline level of 30 microgrammes per cubic metre (g/cum) can be associated with measurable increases in death rates and hospital admissions.

Such evidence has disturbed the Western world. Though the West has been successful in keeping the peak levels of particles close to the air quality standards, they do not feel safe anymore. According to Shankar Prasad, community health adviser to the California Air Resources Board ( carb ), the pm 10 (particles with a diameter of 10 microns or one-fifth the width of a human hair) level for most part remains close to the who guidelines of 70 g/cum and rarely reaches 170-200 g/cum. Yet particulate matter is one of the important parameters for issuing smog alerts to warn citizens about the dip in air quality on a daily basis. Richard Wilson of the Department of Physics and Centre for Risk Analysis in the Harvard University, warns that the consistent picture that a us public health authority should consider is that 60,000 people might be dying prematurely due to problems related to air pollution in the us each year.

Smaller particles
Small particles are typically designated according to size. Studies on particle size now show that particles in the ambient air range from 0.01 micron to 100 micron in diameter (one micron is one-millionth of a metre). Particles in the 0.1-2.5 micron range are considered 'fine'. In fact, particles 10 micron in size are not even treated as fine particles any longer. Smaller the particle, greater the risk: The size of the particles now assumes vital importance from the environmental point of view as that would determine how long the particles would remain airborne and also from the point of view of their impact on human health. The size of particles determine how deep they can penetrate our lungs.

Bigger particles settle fast due to gravity. The fine particles remain in the air for a time period ranging from days to weeks and can be carried by wind for thousands of kilometres. On the other hand, coarse particles take minutes or even hours to clear and cover distances of 1 to 10 km.

Scientists are no longer bothered about coarse dust particles, which are largely from natural sources. They are more worried about the tiny chemical particles from fuel combustion which are extremely small in size and go deep into the lungs, staying there longer and causing maximum damage. The smaller they are, the greater their ability to penetrate deeper into the respiratory tract (see box: Tinier they are, deeper they go ).

Deadly cocktail
Particles are dangerous because they carry a very complex mixture of toxic pollutants. They are produced by diverse sources, vary in size and chemical composition, and contain large numbers of both organic and inorganic compounds. Scientists at the Health Effects Institute (hei) in Boston, usa, have found that the nature of mixture varies from place to place and over time. Yet, similar effects of exposure to particulate matter have been reported by epidemiological studies based on day-to-day observation conducted in a variety of locations where the relative amount of pollutants and associated pollutants are different. This has puzzled the hei scientists. They now want to know what attributes of the particulate mixture may be important in causing toxicity, and what factors affect an individual sensitivity?

Chemical coating makes the particles dangerous: John Spengler of the Harvard School of Public Health points out that the fine particles from burning of coal, petrol, diesel and wood are a complex mixture of sulphate, nitrate, ammonium, hydrogen ions, elemental organic compounds, metals, poly nuclear aromatics, lead, cadmium, vanadium, copper, zinc, nickel, and so on. It is this cocktail of chemicals which makes these tiny killers so lethal (see box: A matter of size ). The harmful chemicals present in the emissions coat the surface of these tiny particles. When these particles penetrate the respiratory tract, these toxic substances trigger serious health problems.

Diesel particles
Scientists have paid a lot of attention to particles emitted by diesel engines. In recent years, the composition of diesel particles has been the focus of increased scrutiny as diesel vehicles are known to emit extremely small particulate matter. A recent study in the uk has found that 90 per cent of the particles emitted by diesel vehicles one micron in diameter or smaller and are thus highly respirable. It has been further proved that diesel exhaust has a larger quantity of fine and ultra-fine toxic particles in comparison to particulate matter in which there is a total absence of diesel exhaust.

Characteristics of diesel particles: Moreover, particles from the exhaust of diesel engines (simply called diesel exhaust) have a carbon core which is unique to them, as are some of the organic compounds that these particles absorb like the polycylic aromatic hydrocarbons (pahs). Besides particulate matter, diesel exhaust also contains toxic gases. Both these components -- particles and gases -- in the diesel exhaust contain hundreds of lethal chemical compounds, including organic compounds. Further, these components of a diesel exhaust have toxicological and carcinogenic properties that pose a serious health hazard for humans.

Sulphate particles
The atmosphere plays its own role in aiding the formation of deadly particles -- especially sulphate particles. Sulphate particles are also emitted due to combustion of fuels with high sulphur content like fossil fuels. Diesel is a major source of sulphate particles. Most fossil fuels contain sulphur, which forms sulphur dioxide (so2) when burnt. Once it is out in the atmosphere so2 is further oxidised and turns into sulphate particles. These can be carried to considerable distances through the air. Sulphate particles are now treated as more dangerous than pm10 and pm2.5 particles.

Components of sulphate particles: While some so2 oxidises to form sulphate particles, some reacts with hydroxyl radical (oh) to form sulphur trioxide and then, again, it gets transformed into fine droplets of sulphuric acid in the presence of moisture. Sulphuric acid droplets can again react with other gases to form sulphate particles. The fine aerosols of sulphuric acid form droplets that are in the nanometre range of size. These exceedingly small droplets then grow a little bigger through thickening by coalescing with other sulphuric acid droplets or with other suspended particles in the air. But their size remains below one micron. Since it takes time to oxidise, sulphate particles can appear in locations far away from the source of the precursor gas, thus making even less polluted areas in the urban periphery vulnerable to its deadly effect.

Health risks posed by sulphate particles: Latest information available from the who implicates sulphates as being a major causative factor in increasing hospital admissions and even mortality. Specifically, an increase of 10 g/cum sulphates can cause an increase in hospital admissions and daily mortality to be as high as 50 to 60 per cent (see graph: The health effects ). Researchers found that summertime hospital admissions in Ontario for children are associated with increases in ambient levels of sulphates and ozone. No data on sulphate levels in India is available, but given the scale of diesel consumption it could be high.

Constraints on monitoring
While scientists are increasingly discovering health effects of tiny particulate matter, air quality regulators are yet to catch up and come up with risk management plans. Maynard is convinced that "the rate of development of thinking in the area of particle toxicology and epidemiology has posed problems and regulations are behind the leading edge of science." "This has been especially the case regarding the optimal metric (measurement) that should be chosen to reflect the concentration of particles in the air. In a number of countries, including the uk, attention was focused on pm 10, but pressure to move to pm 2.5 and even pm 0.1 is mounting. Also, just measuring the mass of particulate matter is not enough; we need to monitor the number of ultra fine particles," he explains.

Measuring the tiniest of them all: So, what should be the size of particles that are monitored to estimate the risk from particulate pollution? This is a matter of great debate across the world.

The West has so far responded by shifting focus from the monitoring of total suspended particulate matter (tspm) which includes dust, to particles of size less than 10 micron and 2.5 micron. Till about 1987, tspm was the only available measurement of particles in the us. But after 1987, the United States Environmental Protection Agency (epa) started monitoring only pm 10.

In 1997 the epa issued standards for pm 2.5. Scientists now believe that particles get more volatile and toxic as they get smaller than 2.5 microns. Some scientists now hold that the decision to measure pm 10, or for that matter pm 2.5, to represent the small particulate pollution levels in cities is arbitrary and may still fail to estimate the real risk from even tinier particles. According to Lidia Morawska, associate professor with the department of physical sciences at the Queensland Institute of Technology in Brisbane, Australia, the cut-off point at pm 2.5, set by the epa, was determined by the technology that was available to measure such small particles. The decision was arbitrary as appropriate instruments to monitor the ultrafine particles were not available. So there is no clear scientific reason to base standards on pm 2.5. According to Morawska, there is a need to take into account the ultrafine particles smaller than 2.5 microns. Here the number of particles goes very high and can be determined only with the help of very sophisticated gadgets, which are still in the making. The size now in focus is 0.03 microgramme. Clearly, technical limitations have constrained monitoring of pm 1 and smaller particles.

Particles smaller than 1 micron: Mary Amder, professor of toxicology at the Harvard School of Public Health in the us informs that a study by scientist K Dreher and others, has collected samples of ambient particles in the size range: smaller than 1.7 micron, between 1.7 and 3.7 micron, and between 3.7 and 20 micron in Washington, dc. The study found that particulates smaller than 1.7 micron had the highest percentage of soluble metal content, the highest sulphate content and were the most acidic of the three size fractions. In animals exposed to these particles, greatest lung damage was caused by the particles smaller than 1.7 micron.

The epa now recognises the need to monitor pm 1. Says Dale Evarts from the department of air pollution planning and standards at the epa, "There is no routine monitoring but epa is sponsoring studies in 10 cities that include measuring of these particles (pm 1s)."

Health is first priority: R L Maynard says that arguments over the health effects of particulate matter has been quite "ill-tempered". One reason is that industry representatives saw that demands for cleaner operations and products would follow this new science on particulate and realised that these would be both difficult to meet and very expensive. And he was proved right when epa proposed revision of national standard for pm 2.5 from 65 g/cum to 50 g/cum as 24-hour average in November 1998. Since this would require reductions in pollution from automobiles, power plants and industrial facilities, the proposed standards drew considerable opposition from the industry and the standards have been challenged in us courts. Though the decision on the proposed standard is still pending, epa has estimated that implementation of the proposed standard could cost $8.6 billion per year, but health benefits of implementing the new standards would outweigh this cost by a factor of 2-11.

However, the American Lung Association ( ala ) still feels that the new standard is not adequate to protect public health. In their report, entitled Gambling with Public Health II , the ala has said that the "proposed pm 2.5 ambient standards of the epa would still potentially expose 89 million people to serious health hazard". Instead they want the standard of 18 g/cum averaged over 24 hours, and 10 g/cum annually. The important message is that usa is prepared to take up such high cost abatement programme where even at the worst case scenario the pm 10 levels peak to only 170-200 g/cum, and that, too, rarely, compared to more than 800 g/cum in Delhi.

A matter of size
The sources of particles render them unique

Sizeof particles
Down to Earth

Source: Resuspended dust, soil dust, street dust. Coal and oil fly ash. Metal oxides of Silicon, Aluminium, Magnesium, Iron. Mold spores and pollen.
Particles less than 10 microns in size

Source: Wind blown dust and grinding operations. Fog droplets, pollen, bacteria.
Source: Diesel exhaust. Combustion of Coal, Oil, Gasoline. High temperature processes, smelters, steel mills etc. Coal-fired thermal power plants.
Source: Diesel exhaust. Combustion of Coal, Oil, Gasoline, Wood. Atmospheric transformation products of NOx, SO2 and organic compounds.

Strict control
The air quality standards for PM10 set by international agencies

  ALA (proposed) 10 g/cum   Annual average
  ALA (proposed) 18 g/cum   24-hour average
  California 30 g/cum   Annual Geometric Mean
  US EPA 50 g/cum   Average annual ambient standard
  California 50 g/cum   24-hour average
  United Kingdom 50 g/cum   24-hour average
  Indian Standard 60 g/cum   Annual average
  WHO 70 g/cum   European Ambient Air Quality Guideline
  Indian Standard 100 g/cum   24-hour average
  US EPA 150 g/cum   24-hour average

Source: Mary Amdur et al 1996, in Richard Wilson and John Spengler (Eds) Particles in Our Air — Health Effects and Concentrations, Harvard University Press, USA
ALA: American Lung Association
US EPA: United States Environmental Protection Agency
WHO: World Health Organisation

Note: All figures in microgrammes per cubic metre (g/cum)

Subscribe to Daily Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.