Deadly Particles

Concern over particles

That airborne particles kill has been known for a while. But the threat is rising with every scientific discovery

There is nothing new about particulate pollution. Yet, scientists are talking about airborne particles today like never before. As science on air pollution developed in response to environmental and public health crises in the West, these tiny killers came under intense scrutiny. Today, scientists are perplexed by the discovery that even at very low concentrations these particles kill. They have now shifted their focus to the tiniest and the most lethal of them all and their potential to claim lives. This has dramatically altered the approach to its risk management worldwide.

Scientists had known all along that particles are harmful but scientific evidence on the enormity of their health effects is yet to be understood completely. The history of particles goes back many hundred years. Records tell us that during the rule of Edward I (1272-1307 ad) burning of coal was banned in London to control air pollution. His successor Edward II actually ordered persons guilty on this count to be tortured. As the West industrialised and motorised fast, it went through a series of severe air pollution episodes in the early 20th century. Smoke particles and sulphur dioxide were the most talked about pollutants in those days. The first recorded episode was in Meuse valley of Belgium where air pollutants got trapped at ground level for a week in December 1930 killing 60 people. Across the Atlantic, in October 1948, half the population of 14,000 in Donora, Pennsylvania, fell sick, 10 per cent fell severely ill and 20 died due to severe air pollution. But it took the notorious London Smog of 1952 which killed 4,000 people to trigger extensive research on air pollution and its effects on health which ultimately revealed deadly facts about smoke particles.

When the uk government, alarmed by the death count of the 1952 smog episode in London, enforced more stringent air pollution control methods, they actually succeeded in bringing the level of pollution down. Paradoxically, this success in lowering pollution only proved that the objective of reducing risk from the killer particles was still very distant.

Immediately after air pollution levels fell in London following strict enforcement of air pollution measures, it first became more and more difficult to detect the effects on health of day-to-day variation in concentration of smoke. Robert L Maynard of the department of health, uk, who has recently edited a study entitled Particulate Matter: properties and effects upon health , says that this led scientists to believe that it is possible to set a threshold limit for pollutants below which health effects are unlikely to occur. Therefore, setting safe limits for air pollutants was the most obvious thing to do. This led to establishment of guidelines for air pollutants including particulates in uk and these later provided the basis for the World Health Organisation's Air Quality Guidelines for Europe in 1987 and air quality regulations worldwide.

But things changed dramatically when scientists began to observe incomprehensible but serious health effects even at extremely low concentrations of particles. Says Maynard, "A trickle of epidemiological studies that began in the late 1980s and turned into a flood gate 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."

As science on particles developed, scientists were faced with more questions than they had solutions for. It was clear to them that they needed to understand much more about the 'idiosyncrasies' of ultrafine particles and how they chemically and biologically affect human health systems. Only further research and knowledge will enable scientists to identify the most dangerous of all particles and concentrate their efforts towards controlling them.

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.