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    Outdoor air pollution is a major environmental health problem affecting. By reducing air pollution levels, we reduce the burden of stroke, heart disease, lung cancer, and chronic and acute respiratory diseases, including asthma. The lower the levels of air pollution, the better the cardiovascular and respiratory health of the population.
    The World Health Organization Air quality guidelines" provide an assessment of health effects of air pollution and thresholds for health-harmful pollution levels.
    In 2014, 92% of the world population lived in places where the air quality guidelines were not met. Outdoor air pollution in both cities and rural areas was estimated to cause 3 million premature deaths worldwide in 2012.
    Policies and investments supporting cleaner transport, energy-efficient housing, power generation, industry and better municipal waste management would reduce key sources of urban outdoor air pollution.
    Reducing outdoor emissions from household coal and biomass energy systems, agricultural waste incineration, forest fires and certain agro-forestry activities (e.g. charcoal production) would reduce key rural and peri-urban air pollution sources.
Reducing outdoor air pollution reduces emissions of CO2 and short-lived climate pollutants like black carbon particles and methane, thus contributing to the near- and long-term mitigation of climate change.
    Indoor smoke is a serious health risk for some 3 billion people who cook and heat their homes with biomass fuels and coal.
Some lung cancer deaths could have been averted by improving ambient air quality, or by reducing tobacco smoking.
    A 2013 assessment by WHO’s International Agency for Research on Cancer (IARC) found that outdoor air pollution is carcinogenic to humans, with the particulate matter component of air pollution most closely associated with increased cancer incidence, especially lung cancer. An association has been observed between outdoor air pollution and increase in cancer of the urinary tract/bladder.
Exposure to small particulate matter causes cardiovascular and respiratory disease, and cancers.
     Most sources of outdoor air pollution are beyond the control of individuals and demand action by cities and national and international policymakers in transport, energy waste management, buildings and agriculture.
    There are many examples of successful policies in transport, urban planning, power generation and industry that reduce air pollution:
Clean technologies that reduce industrial smokestack emissions; improved management of urban and agricultural waste, including capture of methane gas emitted from waste sites as an alternative to incineration (for use as biogas).
Shifting to clean modes of power generation; prioritizing rapid urban transit, walking and cycling networks and rail interurban freight and passenger travel, and  shifting to cleaner heavy duty diesel vehicles and low-emissions vehicles and fuels, including fuels with reduced sulfur content.
Urban planning:
Improving the energy efficiency of buildings and making cities more compact, and energy efficient.
Power generation:
Increased use of low-emissions fuels and renewable combustion-free power sources (like solar, wind or hydropower); co-generation of heat and power; and distributed energy generation ( mini-grids and rooftop solar power generation).
Municipal and Agricultural waste management:
Strategies for waste reduction, waste separation, recycling and reuse or waste reprocessing and improved methods of biological waste management such as anaerobic waste digestion to produce biogas and low cost alternatives to the open incineration of solid waste. Where incineration is unavoidable, then combustion technologies with strict emission controls are critical.
The "WHO Air quality guidelines" indicate that by reducing particulate matter (PM10) pollution from 70 to 20 micrograms per cubic metre (μg/m), we can cut air pollution-related deaths by around 15%. The Guidelines apply worldwide and are based on expert evaluation of current scientific evidence for:
particulate matter (PM)
ozone (O3)
nitrogen dioxide (NO2) and
sulfur dioxide (SO2), in all WHO regions.
Particulate matter
Definition and principal sources
Particulate Matter affects more people than any other pollutant. The major components of PM are sulfate, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water. It consists of a complex mixture of solid and liquid particles of organic and inorganic substances suspended in the air. Air quality measurements are typically reported in terms of daily or annual mean concentrations of PM10 particles per cubic meter of air volume (m3). The most health-damaging particles are those with a diameter of 10 microns or less, (≤ PM10), which can penetrate and lodge deep inside the lungs. Chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer.
Health effects
There is a close, quantitative relationship between exposure to high concentrations of small particulates and increased mortality or morbidity, daily and over time. Small particulate pollution has health impacts even at very low concentrations – indeed no threshold has been identified below which no damage to health is observed.
The effects of PM on health occur at levels of exposure currently being experienced by many people both in urban and rural areas and in developed and developing countries – although exposures in many fast-developing cities today are often far higher than in developed cities of comparable size.
"WHO Air Quality Guidelines" estimate that reducing annual average particulate matter concentrations common in many developing cities, to the guideline could reduce air pollution-related deaths by around 15%.
    There are serious risks to health from exposure to ozone (O3), nitrogen dioxide (NO2) and sulfur dioxide (SO2). As with particulate matter, concentrations are often highest in the urban areas of low and middle - income countries. Ozone is a major factor in asthma morbidity and mortality, while nitrogen dioxide and sulfur dioxide play a role in asthma, bronchial symptoms, lung inflammation and reduced lung function.
    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 (NOx) from vehicle and industry emissions and volatile organic compounds (VOCs) emitted by vehicles, solvents and industry. As a result, the highest levels of ozone pollution occur during periods of sunny weather.
    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 one of the air pollutants of most concern.
Nitrogen dioxide (NO2)
    As an air pollutant, NO2 has several correlated activities. At short-term concentrations, 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 ozone. The major sources of anthropogenic emissions of NO2 are combustion processes (heating, power generation, and engines in vehicles and ships).
    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 linked to NO2 at concentrations currently measured in cities of Europe and North America.
Sulfur dioxide (SO2)
    SO2 is a colorless gas with a sharp odor. It is produced from burning of fossil fuels (coal and oil) and smelting mineral ores that contain sulfur. The main anthropogenic source of SO2 is the burning of sulfur-containing fossil fuels for domestic heating, power generation and motor vehicles. 
  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. Health effects are now known to be associated with much lower levels of SO2 than previously believed. Reducing SO2 concentrations is likely to decrease exposure to co-pollutants.
    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, a cause of deforestation.
Indoor Air Pollution and Health
    Understanding and controlling common pollutants indoors can help reduce your risk of indoor health concerns. 
    Some effects may show up after a single exposure or repeated exposures to a pollutant. These include irritation of the eyes, nose, and throat, headaches, dizziness, and fatigue. Such immediate effects are usually short-term and treatable.
It is best to eliminate the person's exposure to the source of the pollution, if it can be identified. After exposure to some indoor air pollutants, symptoms of diseases such as asthma may show up or worsened.
    Immediate reactions to indoor air pollutants depends on age and preexisting medical conditions. Sometimes whether a person reacts to a pollutant depends on individual sensitivity. Some people can become sensitized to biological or chemical pollutants after repeated or high level exposures. Some effects may be made worse by an inadequate supply of outdoor air coming indoors or from the heating, cooling or humidity conditions prevalent indoors or air filters left unchanged..
    Health effects may show up years after exposure has occurred or after long or repeated periods of exposure. These effects, which include respiratory diseases, heart disease and cancer, can be severely debilitating or fatal. It is prudent to try to improve the indoor air quality in your home even if symptoms are not noticeable.
While pollutants commonly found in indoor air can cause many harmful effects, there is considerable uncertainty about what concentrations or periods of exposure are necessary to produce specific health problems. People also react very differently to exposure to indoor air pollutants.
    In developing countries, indoor exposure to pollutants from the household 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 cardiovascular disease, chronic obstructive pulmonary disease and lung cancer among adults.
WHO Member States recently adopted a resolution and a road map for an enhanced global response to the adverse health effects of air pollution.
WHO’s "Health in the green economy" series is assessing the health co-benefits of climate mitigation and energy efficient measures that reduce air pollution from housing, transport, and other key economic sectors.
WHO’s work on "Measuring health gains from sustainable development" has proposed air pollution indicators as a marker of progress for development goals related to sustainable development in cities and the energy sector.