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Some common air pollutants

top of page Sulphur Dioxide

Sulphur dioxide (SO2) is generated mainly from the combustion of fossil fuels containing sulphur. Eighty per cent of worldwide emissions come from burning coal and lignite, 20% from oil. Coal typically contains about 2% sulphur and heavy fuel oil about 3% by weight. Sulphur dioxide in water vapour and clouds produces acid rain.

SO2 emissions in the EMEP area
Source: EMEP

Earlier this century in London high concentrations of SO2 and smoke (from the burning of coal in homes and for industrial purposes) usually occurred at the same time. Effects on health were demonstrated in a series of epidemiological studies. These found that daily death rates, and a worsening of the condition of people with chronic bronchitis, were related to raised levels of smoke and SO2. It was not possible to determine whether the smoke or the SO2 was having the greater effect and the possibility that the effects were due to acid being transported on the surface of the smoke particles was raised; re-analysis of the data has supported the view that this is of major importance in producing effects on health.

Inhalation of SO2 produces narrowing of the airways (bronchoconstriction), which people suffering from asthma are more sensitive to than other individuals. Small effects on the way in which the lungs of asthmatic sufferers function may be expected as a result of exposure for just a few minutes to SO2 concentrations as low as 550 µg/m3. Complaints of effects on the airways and irritation of the eyes and nose from those living close to factory chimneys is often attributed to sulphur dioxide. National and international controls have helped reduce emissions and their impact, which however can vary significantly between countries.

top of page Particulate Matter
Particle size
Particles are classified into the following categories:
  • nucleation mode (< 0.2 µm diameter) - particles recently emitted from a process or freshly formed within the atmosphere; present in large numbers in urban air;
  • accumulation mode (> 0.2 µm - 2 µm diameter) - particles which have grown from nucleation mode by coagulation or condensation of vapours; stable with an atmospheric lifetime of 7-30 days, and;
  • coarse mode (> 2 µm diameter) - mainly formed by weathering processes, including soil dust, sea spray and industrial dusts; their large size means the atmospheric lifetime is short.

Particles < 10 µm are associated with health effects and particles > 2 µm diameter are associated with soiling.

A wide size range of particles occurs in the air and the way they act is determined by size. Larger particles will not stay in the air for very long and will be deposited close to their source; they are unlikely to be inhaled. Smaller particles can be carried long distances and inhaled.

Particles can cause irritation to the eyes, nose and throat. Some of the larger particles (> 10 µm diameter) reaching the nose or throat will be filtered out by the body's natural defense system. However, very tiny particles that reach deep into the lung may be absorbed into the blood stream or cause lung or other health problems. Such particles are those less than 10 µm in diameter - hence the term PM10. Studies in the United States have shown that death rates from respiratory and cardiovascular disease increase with increased concentrations of PM10, and EU monitoring shows significant levels in many countries.

Click on this picture to see it full size

Source: EEA

Both the size and composition of the particles have potential health effects. In the winter smogs of earlier this century, it was soot and SO2 that were identified as a health hazard, responsible for bronchitis and emphysema. These acidic particles can cause lung inflammation and changes in blood clotting. No direct link has been proved between particulate pollution and asthma.

top of page Ozone

There are two sorts of ozone: ozone in the stratosphere (15-50 km above the earth's surface) forms what is known as the "ozone layer" and is essential in limiting the amount of ultraviolet irradiation reaching the earth's surface (see section on global- effects, p. a19). However, ozone in the troposphere - the level that contains the air we breathe - is a pollutant and it can damage health and vegetation.

At ground level ozone is a secondary pollutant(2) formed by the action of sunlight on primary pollutants; these are nitrogen oxides from vehicle emissions and industry and volatile organic compounds from vehicles, solvents and industry. Nitrogen oxides react in sunlight (photochemical reaction) to form ozone, a major constituent of photochemical smog.

High concentrations of ground level ozone are often a particular problem in hot sunny climates such as in southern Europe, with cities like Athens having a particular problem. In northern Europe concentrations are usually higher in rural areas as ozone production occurs in polluted air as it drifts away from cities. Ozone is thus also a transboundary pollution problem and action to deal with it needs to be agreed nationally and internationally and implemented locally.

While ozone can damage crops and vegetation, its activity is such that concentrations indoors are rapidly reduced by reaction with plastics and fabrics; almost alone amongst the major air pollutants, exposure to ozone is nearly always an outdoor problem.

Asthma is a disorder of the airways in which inflammation obstructs airflow, causing breathlessness and wheezing. It is becoming increasingly common throughout the developed world in both urban and rural areas. It is an inflammatory disease which causes the bronchi to overreact to a stimulus. To counteract the irritant, the airways become inflamed and swollen and produce excess mucus causing narrowing of the airways in vulnerable individuals. In the UK asthma is thought to affect between 4 - 6% of children and 4% of adults, when in Finland, over 2.5% of the population suffers from asthma. A common feature through all Western countries is a strong increase in the number of asthma cases. See also WHO pamphlet on Asthma.

The biological response to ozone is dependent upon the concentration to which an individual is exposed and the duration of exposure; the dose received is also dependent upon the volume of air inhaled per minute as nearly all the ozone inhaled is absorbed. Thus people taking strenuous physical exercise, e.g. jogging, inhale more ozone and are likely to show a proportionally greater response to its effects. The biological response to ozone is unusual though not unique. If exposure is continued over several days, the response falls away and the person is described as having developed tolerance to ozone. This phenomenon has not been adequately explained and decrease in response should not be taken as an indication that exposure day after day does no more harm than a single day's exposure.

The importance of duration of exposure, and the fact that this may extend over 8 hours on a sunny day, has led to guidelines and air quality standards for ozone usually being defined in terms of an 8-hour average concentration. This is commonly exceeded in Europe.

In terms of producing inflammation of the respiratory tract, ozone is one of the most toxic of the common air pollutants. According to WHO hourly concentrations of 200 µg/m3 can cause eye, nose and throat irritation, chest discomfort, cough and headache; exposure for about six hours to concentrations of 160 µg/m3 have been shown to produce inflammation of the airways and changes in standard indices of lung function.

Individuals show an enormous variation in their response to ozone, with asthma sufferers not being significantly more affected than others; there is however limited evidence to suggest that any ill health effects may be longer lasting in asthmatics.

Ozone has also been shown to increase the response, or to increase the sensitivity, of some people when exposed to the substance to which they are allergic. This has been confirmed in several volunteer studies and in an epidemiological study of hay fever sufferers in London. The fact that peak levels of pollen and of ozone often occur together may make this an important observation.

top of page Nitrogen Dioxide

Nitrogen dioxide (NO2) is produced both as a primary and as a secondary pollutant by combustion processes. In many countries some 50% of NO2 is produced by motor vehicles. Thus concentrations tend to be higher in busy streets than in rural areas.

Trends in NOx emissions from 1980 to 1994

Source: EMEP

Concentrations of NO2 show a distinct daily variation with peak levels typically being recorded during the morning and evening rush hours. In cold, still weather concentrations may increase due to trapping of pollutants in a layer of wet air close to the ground. These episodic increases in concentrations of NO2 do not reflect a change in sources but a change in the pattern of dispersion of the pollutants. The atmospheric conditions which cause an increase in concentrations of NO2 also cause an increase in concentrations of other pollutants, such as particles and carbon monoxide.

Nitrogen dioxide is an irritant gas and exposure to exceedingly high concentrations produces narrowing of the airways in both asthmatic and non-asthmatic individuals. Asthmatics are more sensitive to NO2 than non-asthmatics with exposure to concentrations of about 560 µg/m3 for 30 minutes producing a small change in standard indices of lung function; in non-asthmatics exposure to about 1800 µg/m3 would be necessary to produce a similar response. The exposure-response relationship for NO2 is erratic. Exposure to concentrations of 560 µg/m3 may produce a response whilst exposure to double that may not; the response does seem to reappear and remain as concentrations approach and exceed 1800 µg/m3. The explanation for this is not clear.

Smog in London, December 1991
A dramatic example of the effect that weather can have on the dispersal of pollutants occurred in London in December 1991: from the morning of 12 December until the evening of 15 December, London was blanketed in its worst smog since 1952. During those four days nitrogen dioxide concentrations were well above 190 µg/m3 for most of the time, peaking at 800 µg/m3 in South West London on Friday 13 December. Up to 160 additional deaths are thought to have occurred during this period. Deaths from respiratory disease (including asthma) were 22% higher and from cardiovascular disease 14% higher than expected when compared with the same period the previous year.

Children living in homes using gas cookers have been shown to have an increased risk of respiratory infections compared to those living in homes with electric cookers. This has been attributed to long-term exposure to raised concentrations of NO2 although the evidence is not conclusive. Recent studies have also shown that long-term exposure to the pollutants generated by gas cookers is associated with increased symptoms of asthma amongst women. This effect has also previously been attributed to NO2, but again the evidence is inconclusive. (See pamphlet Indoor air quality.)

Exposure to NO2 at concentrations not very much in excess of those occasionally experienced both indoors and out has also been shown to increase the response of sensitive individuals to allergens. The importance of this effect on public health is uncertain though some, but not all studies, have suggested an increased prevalence of asthma in those living close to busy roads. Further research in this area is required. Emissions of total nitrogen oxides, including NO2 are also declining slowly, but remain above acceptable levels.

top of page Carbon Monoxide

Carbon monoxide (CO) has an affinity for haemoglobin (Hb), the substance that carries oxygen around our bodies in our blood, 200 times greater than that of oxygen. It impairs the oxygen-carrying capacity of blood as carboxyhaemoglobin (COHb) is formed rather than oxyhaemoglobin. When too much oxygen is displaced by CO, it can progressively lead to oxygen starvation. Thus at levels of only a few percent COHb effects may be significant in tissues which are already deprived of oxygen, perhaps as a result of poor blood supply. The escalating symptoms of CO poisoning are headache and vomiting and, in severe cases, collapse and death, although the effects of brief exposure are reversible.

Cigarette smokers seem to adapt to high levels of COHb: often as much as 10% of total haemoglobin is combined with CO as COHb in the blood of heavy cigarette smokers. Indeed, cigarette smokers are on average net contributors to ambient CO levels.

In healthy, non-smoking, individuals effects of exposure to CO appear at a COHb concentration of about 5%. In those with poor circulation to the heart it has been shown that levels of COHb as low as 2.5% may produce an effect upon the electrical activity of the heart.

Carbon monoxide is a problem both as an indoor and as an outdoor air pollutant. In terms of accidental deaths, indoor exposure presents the major problem. Despite the undoubted and well understood toxic effects of CO there was until recently little evidence to suggest that outdoor exposure posed a risk to health. However, recent studies have shown that the increase in emergency admissions to hospital for heart attacks, which had been reported as being related to changes in levels of particles, was also significantly related to changing levels of CO. This suggests that exposure to low concentrations of CO may be dangerous and that rather more deaths than previously thought may occur from outdoor exposure to CO.

top of page Lead

Lead is a heavy metal which exists naturally in the earth's crust. It is found throughout the environment in water and air. Natural concentrations of lead are low but exploitation of lead as a useful metal has increased the level of exposure to this contaminant. In those countries which still mainly use leaded petrol, most of the lead in the air comes from petrol-fuelled vehicles. While the dangers of exposure to high levels of lead are well known, there is very serious concern that low levels of lead may affect the mental development of children (see WHO pamphlet on Lead and Health).

top of page Carcinogens as Outdoor Air Pollutants
Association and causality
Source: Handbook on air pollution and health
It is often fairly easy to show that measure of ill-health, e.g. the number of admissions to hospital per day, is associated with a possible cause such as the day-to-day variation in levels of air pollutants. To show real cause and effect, i.e. that a causal relationship exists, a number of guidelines or tests have been developed. These include looking at the consistency of the results of a number of different studies, the way in which the results of different studies fir together (coherence), whether there is a "dose-response relationship" such as that as the proposed causal factor increases so does the effect and whether the sequence of events makes sense, i.e. the cause always precedes the effect.

Proof of causality is often impossible but by the application of these and other criteria an expert judgment as to whether an association is likely to be causal can often be reached.

A considerable number of substances, carcinogenic or supposedly carcinogenic, occur as outdoor and indoor air pollutants. Of these benzene, 1,3-butadiene and the polycyclic aromatic hydrocarbon compounds (PAHs) are perhaps the best known. Some of them are genotoxic carcinogens and as such any level of exposure (no matter how small) may, in theory, be associated with an increased risk of cancer. The term "cancer" is used here to embrace all malignant tumours and related disorders such as leukemia. Attempts to quantify the risk posed by exposure to ambient levels of carcinogens have been made though the assumptions underlying these calculations have been challenged.

For air pollutants which are considered to be genotoxic carcinogens, WHO experts have used mathematical models to calculate so called "unit risk". These calculations are based on information from animal studies or from epidemiological studies. This unit risk is generally expressed as the additional cancer risk in a given population that inhales this compound at a concentration of 1 µg/m3 during its entire lifetime. For benzene, for instance, the unit risk factor is 4.10-6. This means that in a population of one million persons, 4 additional cases of cancer can be expected following a continuous inhalation of 1 µg/m3 during its lifetime or one additional cancer in a population of 250 000 people.

summary of urban benzene measurements in Europe on cities. Annual averages if not otherwise noted. (µg/m3)
Country Street sites
UK, 1995 5.8
The Netherlands, 1995/96 3.5-8.7
Germany, 1994 10-12
Belgium, 1995 15
Denmark, 1996 15.8
Sweden, 1996 6.7-10.3
Source : "EEA, Air Pollution in Europe, 1997"

It should be realized, however, that these theoretical risk estimates contain large safety factors, but also a number of uncertainties.

To derive standards, countries have to choose their acceptable level of risk for each hazard, balancing risks and benefits, and establishing the degree of urgency of public health problems among sub-populations inadvertently exposed to carcinogens. In the United Kingdom, the standard for benzene is 5 µg/m3. This level is exceeded in many instances (see table above) in many cities, but one should keep in mind when comparing measured concentrations in the air to a guideline value that:

  • models used to derive the guideline values are likely to be conservative; and
  • exposure is rarely continuous.

Nevertheless, efforts should be made to reduce the level of carcinogens in the air.

top of page Indoor Air Pollutants

As well as nitrogen dioxide, carbon monoxide and lead there are a number of other pollutants that affect air quality in enclosed or confined spaces. More detailed information is available in the pamphlet indoor air quality as well as in other pamphlets such as Radon, Asbestos and health or Asthma and respiratory allergies. Clearly, indoor air pollution affects people both in their homes and at the workplace. In the latter case, workers cannot choose the air they breathe and there is a need for legislative or regulatory action. It is the same with public spaces where local authorities can enforce regulations. (see WHO pamphlet on Indoor air quality)

top of page Radon

In specific geographical areas, and in buildings which have not been designed to be radon-proof, concentrations of radon gas may reach levels that cause lung cancer. The increased risk of cancer from radon exposure is strongly enhanced for smokers, and several thousands of extra cancers in Europe can be attributable to radon in regions of high emission.(see WHO pamphlet on Radon)

top of page Asbestos

As for radon, smoking substantially increases the risk of cancer in asbestos-exposed populations (see WHO pamphlet on Asbestos and health).

top of page Volatile organic compounds (VOCs)

VOCs originate mainly from solvents and chemicals used at home or in offices. The main indoor sources are: perfumes, hair-spray, furniture polish, glues, paints, stains and varnishes, wood preservatives, pesticides, air fresheners, dry cleaning, moth repellents and many other products. The main health effects are eye, nose and throat irritation. In more severe cases there may be headaches, loss of coordination and nausea. In the long-term, some of them are suspected to damage the liver, the kidneys and the central nervous system as well as being potential carcinogens. Simple measures, such as proper use of these products according to manufacturers' instructions, and ventilating the rooms when using such products, will keep the contamination at levels which can be considered as safe for the health of most people. Acute contamination will be prevented by the usual measures of home safety regarding the use of chemicals.

top of pageTobacco smoke

To know more about WHO policy on tobacco Smoking generates a wide range of harmful chemicals including nicotine, tar, formaldehyde, oxides of nitrogen and carbon monoxide. It is now well known that smoking causes cancer. It is also well known that passive smoking causes a wide range of problems to the passive smoker ranging from being disturbed by the smell, and eye, nose and throat irritation in children, to cancer, increased risk of bronchitis and pneumonia, severe asthma crises, and decreases in lung function. However, legislation varies considerably between European countries. Other factors such as the cost of tobacco have an obvious influence on the number of smokers in countries. Local authorities should ban smoking in all areas where they can: schools, parking lots, cinemas, all municipal buildings and restaurants, with spaces reserved for non-smokers when necessary.

Consumption of Tobacco throughout the European Community
Member state % adults smokers %male %female Per capita consumption smoking legislation
Austria 30 40 21 1725 minimal legislative control
Belgium 25 31 19 1515 Complicated, vague provision
Denmark 42 45 39 1574 very limited restrictions
Finland 24 29 19 1613 widespread ban
France 34 40 27 2025 The most comprehensive in the EU
Germany 29 37 22 1939 No national regulations
Greece 37 46 28 3297 relatively limited
Ireland 30 28 32 2151 Fairly widespread public ban
Italy 32 38 26 1849 Mainly public places and transport
Luxembourg 27 32 26   relatively widespread
Netherlands 33 37 30 2465 complicated provision
Portugal 28 46 12 1969 similar to Luxembourg
Spain 36 48 25 2312 Extensive ban in public places
Sweden 23 23 23 1227 widespread ban
United kingdom 28 29 28 1982 voluntary code of practice

top of page Formaldehyde

Formaldehyde comes mainly from particle board, carpets and insulation foams. Beside eye, nose and throat irritation, it may in some allergic people cause wheezing, coughing, skin rash and other severe allergic reactions. High concentrations may trigger attacks in people with asthma. The measures that local authorities can take concentrate mainly on avoiding the use of pressed wood products which contain urea - formaldehyde resins.

top of page Biological pollutants

There is a wide range of biological pollutants which includes pollen from plants, mites, pet hair, fungi, parasites and some bacteria. Most of them are allergens and can cause asthma, hay fever and other allergic diseases. (See relevant pamphlets).

top of page Pesticides

As indoor air pollutants, pesticides are a minor risk to health when used according to manufacturers' instructions and when stored according to safety rules. (For outdoor effects of pesticides, see WHO pamphlet Pesticides and health). Global effects of air pollutants

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Exposure pathways and monitoring


Gobal effects of air pollutants


(2) Primary pollutants are those emitted directly into the air, e.g. sulphur dioxide and volatile organic compounds. Secondary pollutants are those formed by a combination, or reaction (e.g. photochemical), with other primary pollutants, e.g. ground level ozone.following


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