What’s wrong with the air in our cities?

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When the use of coal to heat houses was banned in Dublin in 1990, concentrations of fine particulate matter dropped and mortality from respiratory diseases and cardiovascular diseases declined and has remained low. This is probably the closest to an experimental intervention that can be found in the history of air pollution epidemiology, although it was not planned for this purpose, and the result became apparent only when death registers were surveyed several years later.

The first study effectively associating fine particulate matter with mortality was the so-called "Six city study" published in 1993 by researchers from Harvard School of Public Health. Subsequently, many studies have confirmed that fine particulate matter is the single most important health factor in the urban air. The size of particles is crucial in determining the health effects, because only those particles with a diameter less than 10 micrometres (10 μm, 0.01 mm)[1] penetrate deep into the respiratory tract. Larger particles impact or deposit in the upper respiratory tract: nose, throat and upper bronchioles, and are first transferred to mucus which is swept up into the throat and swallowed. The most important parameter for health effects seems to be PM2.5, the concentration of particulate matter smaller than 2.5 μm. These particles are the so-called respirable particles; they able to penetrate to all parts of the lungs.

Sources of fine particulate matter

Sources of particulate matter are both natural and man-made. The natural sources include volcanoes and forest fires. The greatest man-made source is combustion in different forms. Traffic and wood burning are the principal sources in the developed countries which have strict emission controls for industries and energy production, but older power plants and steel industries are still important, especially in the less developed countries.

Atmospheric gaseous precursors can be a significant source of PM2.5. Most of this fraction is made up of soluble ammonium sulphate or ammonium nitrate derived from sulphur dioxide and oxides of nitrogen. These may not be as dangerous as primary particles (smoke) from combustion. Another soluble and possibly innocuous type of particles consists sea salt that is formed along the coasts due to the evaporation of sea water spray.

The most important source of fine particles in the indoor air is the outdoor air. Even if there are no specific indoor sources, particle concentrations indoors can be about 65–90% of those outdoors. The most common indoor source is dust stirred by movement and the physical activity of the inhabitants; this includes mainly different mineral particles.

In many houses, the most important indoor source is the cigarette smoker. If tobacco is smoked indoors, on average the indoor particle concentration triples. Indoor concentrations are also increased by any open type of burning indoors, from candles to gas stoves, as well as kerosene lamps and fireplaces, to say nothing of equipment with internal combustion motors. Never let your car engine run in a garage with the doors closed.

Levels of exposure

Even without specific indoor sources in work or home environment, the personally measured exposure level to fine particulate matter is slightly higher than the outdoor concentration. Since today, people spend most of their time indoors, good filtration in the ventilation system will decrease the exposure to outdoor fine particles. Exposure to traffic-derived fine particulate matter is another issue, because on average, one half of the exposure is acquired during the time spent in traffic, be it on streets or in subway tunnels. Clearly, this exposure is highly variable – it depends where you live and how you commute to work. Indoor sources are also highly variable, and tobacco smoke is still an essential part, even though fortunately many cigarette smokers no longer smoke inside, even in their own homes.

Exposure to fine particulate matter is clearly lower in Northern Europe as compared in Central and Southern Europe. In the north, local sources constitute only about one third of outdoor air concentrations, but indoors the local contribution is about two thirds. If there is no smoking, the contribution of indoor sources is on average one fourth of the total exposure.

Health effects

It is known that exposure to urban fine particulate matter (PM2.5) increases mortality from respiratory diseases and heart diseases. Deaths and illnesses increase one or two days after bad pollution episodes, as shown in many studies in several countries. It is more difficult to study the long-term effects in long-lasting cohort studies, but in a few studies these have also been demonstrated quite clearly. Also in long-term studies, deaths from heart or lung diseases correlate with fine particulate exposure. Lung cancer mortality increases at about the same rate as heart deaths. The increase starts at very low concentrations; in fact it has not been possible to establish a safe limit.

Outdoor fine particulate matter has been estimated to cause hundreds of thousands of premature deaths every year in Europe alone, and high concentrations seem to shorten life by about one year on average. People with coronary heart disease, diabetes or obstructive pulmonary disease seem to be at special risk, but there are signs that fine particle exposure may be also a risk for causing sudden infant death syndrome. The symptoms of asthmatic patients increase, requiring increased drug use and even hospitalization. Urban fine particulate matter is a rather important environmental health problem not only in Europe and the U.S. but throughout the world. The European Union is striving to pass legislation intended to decrease exposures to these particles.

Outdoor particulate matter originates from a wide variety of sources. Population studies have clearly indicated the health impact of particles due to local use of coal, emissions from steel foundries as well as tobacco smoking. There is also evidence about the risks of traffic exhaust emissions, and the general view – although not yet fully proven – is that incomplete burning produces the most harmful particles.

In countries with a cold climate, a sizable portion of the fine particulate matter comes from local wood burning and from sand and dust dispersed by vehicles in the spring after snow has melted. Mineral particles are thought to be less dangerous than smoke-particles, but this assessment is hampered by inadequate knowledge of the mechanisms behind the toxicity.

Alternatives of risk management

Urban fine particulate matter is one of the most important environmental health risks we face every day. If we want to have speedy and significant decrease of exposures in our towns we must be ready to make profound and expensive changes in our economic preferences as well as in everyday life. Up till recently, the control measures have been targetted at the most serious sources, and today point sources are already effectively regulated. One can predict that increasing and more accurate data will lead to new conclusions and more effective abatement.

Some of the measures have already decreased exposures. Emission controls of power plants and industries have lowered the smoke coming from chimneys, especially coarse particle, sulphur dioxide and carbon monoxide emissions, but to some extent also fine particulate emissions. Major reductions can be achieved by regional heating provided by cogenerating plants producing heat and electricity at an overall efficiency of 85–90% while most power plants operate at 30 to 40%. Regional heating has been one of the most efficient ways of reducing both carbon monoxide and fine particles as compared with heating your house with coal, oil or firewood.

Traffic is one of the most difficult parts of the equation. Technical improvements are nullified by increasing numbers of vehicles. A good measure is the amount of oxides of nitrogen and these have hardly decreased at all, and the levels of ozone seem actually to be increasing. This does not mean that we should not strive to reduce traffic-derived pollution – technical developments are still highly important, and all new diesel vehicles should be equipped with particle traps – this is a high-priority issue.

Fine particulate matter is the most important direct environmental health factor in the urban air of the developed countries. Even in developing countries, air quality competes with water quality and food hygiene as a major life-threatening factor. Its impacts on deaths and diseases are clear. Effective measures mean we must accept changes to the basic ways of life, and they are politically unpopular. Until recently, politicians have been able to shelter behind a shield of ignorance and not introduce these essential, though possibly unpopular, changes in air quality legislation.

Notes and references

  1. The accurate term here is the aerodynamic diameter, meaning particles that behave like spheres of 10 μm; because of the different shapes, their actual dimensions may be smaller or larger than 10 μm.

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