HEALTH TOPICS A TO Z

Air pollution, both indoors and outdoors, is a major environmental health problem affecting everyone in developed and developing countries alike. The new WHO Air quality guidelines (AQGs) are designed to offer global guidance on reducing the health impacts of air pollution. The guidelines first produced in 1987¹ and updated in 1997² had a European scope. The new guidelines apply worldwide and are based on expert evaluation of current scientific evidence. They recommend revised limits for the concentration of selected air pollutants: particulate matter (PM), ozone (O₃), nitrogen dioxide (NO₂) and sulfur dioxide (SO₂), applicable across all WHO regions.

Several recent key findings merit special mention:

• Currently there are serious risks to health from exposure to PM and O₃ in many cities of developed and developing countries. It is possible to derive a quantitative relationship between the pollution levels and specific health outcomes (increased mortality or morbidity). This allows invaluable insights into the health improvements that could be expected if air pollution is reduced.
• Even relatively low concentrations of air pollutants have been related to a range of adverse health effects.
• Poor indoor air quality may pose a risk to the health of over half of the world’s population. In homes where biomass fuels and coal are used for cooking and heating, PM levels may be 10–50 times higher than the guideline values.
• Significant reduction of exposure to air pollution can be achieved through lowering the concentrations of several of the most common air pollutants emitted during the combustion of fossil fuels. Such measures will also benefit programmes for the reduction of greenhouse gases.

In addition to guideline values, the AQGs give interim targets related to outdoor air pollution, for each air pollutant, aimed at promoting a gradual shift from high to lower concentrations. If these targets were to be achieved, significant reductions in risks for acute and chronic health effects from air pollution can be expected. Progress towards the guideline values, however, should be the ultimate objective.

Particulate matter

Guideline values

This is the first time a guideline value has been defined for particulate matter (PM).The aim is to achieve the lowest concentrations possible. As no threshold for PM has been identified below which no damage to health is observed, the recommended value should represent an acceptable and achievable objective to minimize health effects in the context of local constraints, capabilities and public health priorities.

Definition
PM affects more people than any other pollutant. It consists of a complex mixture of solid and liquid particles of organic and inorganic substances suspended in the air. The particles are identified according to their aerodynamic diameter, as either PM₁₀ (particles with an aerodynamic diameter smaller than 10 µm) or PM_2.5 (aerodynamic diameter smaller than 2.5 µm). The latter are more dangerous since, when inhaled, they may reach the peripheral regions of the bronchioles, and interfere with gas exchange inside the lungs.

Principal sources
The major components of PM are sulfate, nitrates, ammonia, sodium chloride, carbon, mineral dust and water. Particles may be classified as primary or secondary, depending on how they are formed. Primary particles are emitted into the atmosphere through man-made (anthropogenic) and natural processes including combustion of fuels in vehicle engines or in households; industrial activities; erosion of road surfaces by road traffic and abrasion of brakes and tyres; and work in caves and mines. Secondary particles are also emitted largely from anthropogenic sources, but they are formed in the air, usually by chemical reactions between gaseous pollutants. Particles produced by outdoor sources (industry and traffic) penetrate easily into indoor spaces and add to the burden of PM emitted indoors.

Health effects
The effects of PM on health occur at levels of exposure currently being experienced by most urban and rural populations in both developed and developing countries. Chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer. In developing countries, exposure to pollutants from indoor combustion of solid fuels on open fires or traditional stoves increases the risk of acute lower respiratory infections and associated mortality among young children; indoor air pollution from solid fuel use is also a major risk factor for chronic obstructive pulmonary disease and lung cancer among adults. The mortality in cities with high levels of pollution exceeds that observed in relatively cleaner cities by 15–20%. Even in the EU, average life expectancy is 8.6 months lower due to exposure to PM_2.5 produced by human activities.

Ozone (O₃)

Guideline values

The previously recommended limit, which was fixed at 120 μg/m³ 8-hour mean, has been reduced to 100 μg/m³ based on recent conclusive associations between daily mortality and ozone levels occurring at ozone concentrations below 120 µg/m³.

Definition and principal sources
Ozone at ground level – not to be confused with the ozone layer in the upper atmosphere – is one of the major constituents of photochemical smog. It is formed by the reaction with sunlight (photochemical reaction) of pollutants such as nitrogen oxides (NO_x) from vehicle and industry emissions and volatile organic compounds (VOCs) emitted by vehicles, solvents and industry. The highest levels of ozone pollution occur during periods of sunny weather.

Health effects
Excessive ozone in the air can have a marked effect on human health. It can cause breathing problems, trigger asthma, reduce lung function and cause lung diseases. In Europe it is currently one of the air pollutants of most concern. Several European studies have reported that the daily mortality rises by 0.3% and that for heart diseases by 0.4 %, per 10 µg/m³ increase in ozone exposure.

Nitrogen dioxide (NO₂)

Guideline values

The current WHO guideline value of 40 µg/m³ (annual mean) set to protect the public from the health effects of gaseous NO₂ remains unchanged from the level recommended in the previous AQGs.

Definition
As an air pollutant, NO₂ has several correlated activities:

• At short-term concentrations exceeding 200 μg/m³, it is a toxic gas which causes significant inflammation of the airways.
• NO₂ is the main source of nitrate aerosols, which form an important fraction of PM_2.5 and, in the presence of ultraviolet light, of ozone.

Principal sources
The major sources of anthropogenic emissions of NO₂ are combustion processes (heating, power generation, and engines in vehicles and ships).

Health effects
Epidemiological studies have shown that symptoms of bronchitis in asthmatic children increase in association with long-term exposure to NO₂. Reduced lung function growth is also linked to NO₂ at concentrations currently measured (or observed) in cities of Europe and North America.

Sulfur dioxide (SO₂)

Guideline values

A SO₂ concentration of 500 µg/m³ should not be exceeded over average periods of 10 minutes duration. Studies indicate that a proportion of people with asthma experience changes in pulmonary function and respiratory symptoms after periods of exposure to SO₂ as short as 10 minutes.

The revision of the 24-hour guideline for SO₂ from 125 to 20 μg/m³ is based on the following considerations:

• Health effects are now known to be associated with much lower levels of SO₂ than previously believed.
• A greater degree of protection is needed.
• Although the causality of the effects of low concentrations of SO₂ is still uncertain, reducing SO₂ concentrations is likely to decrease exposure to co-pollutants.

Definition
SO₂ is a colourless gas with a sharp odour. It is produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulfur.

Principal sources
The main anthropogenic source of SO₂ is the burning of sulfur-containing fossil fuels for domestic heating, power generation and motor vehicles. The use of tall chimneys at power stations has caused widespread dispersion of SO₂ affecting populations located far away from the sources. In many developing countries, the usage of coal high in sulfur is increasing.

Health effects
SO₂ can affect the respiratory system and the functions of the lungs, and causes irritation of the eyes. Inflammation of the respiratory tract causes coughing, mucus secretion, aggravation of asthma and chronic bronchitis and makes people more prone to infections of the respiratory tract. Hospital admissions for cardiac disease and mortality increase on days with higher SO₂ levels. When SO₂ combines with water, it forms sulfuric acid; this is the main component of acid rain which is a cause of deforestation.

The implications for public health

Public health recognizes air pollution as an important determinant of health, especially in developing countries. There is significant inequality in the exposure to air pollution and related health risk: air pollution combines with other aspects of the social and physical environment to create a disproportional disease burden in less affluent parts of society. Exposure to air pollutants is largely beyond the control of individuals and requires action by public authorities at the national, regional and even international levels. The health sector can play a central role in leading a multisectoral approach to prevention of exposure to air pollution. It can engage and support other relevant sectors (transport, housing, energy production and industry) in the development and implementation of long-term policies to reduce the risks of air pollution to health.

The way forward

Reducing the impact of air pollution on health requires a review of the national policies and programmes on air quality management to ensure that they address health concerns as a priority, in line with the WHO AQGs. In many cases, regulatory instruments, such as air quality standards or limits, should also be revised. Monitoring of PM₁₀ and/or PM_2.5 needs to be improved to assess population exposure and to assist local authorities in establishing plans for improving air quality. Emission of pollutants into the atmosphere should be addressed by relevant regulatory instruments, such as national emission ceilings, ad hoc limits for specific sources of emission (e.g. enterprises, cars and heating stations) and regulations limiting the use of polluting technologies. WHO will assist the Member States in sharing information on successful approaches, on methods of exposure assessment and monitoring of health impacts of pollution.

¹Air quality guidelines for Europe. Copenhagen, World Health Organization Regional Office for Europe, 1987 (WHO Regional Publications, European Series, No. 23).
²Air quality guidelines for Europe, 2nd ed. Copenhagen, World Health Organization Regional Office for Europe, 2000 (WHO Regional Publications, European Series, No. 91).