WHO updates fact sheet on Ambient (outdoor) Air Pollution and Health (2 May 2018)

The World Health Organization (WHO) has updated its fact sheet on Ambient (Outdoor) Air Pollution.

Background Information:

Particulate Matter (PM)

Particulate Matter (PM) is a common proxy indicator for air pollution. It 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.

While particles with a diameter of 10 microns or less, (≤ PM10) can penetrate and lodge deep inside the lungs, the even more health-damaging particles are those with a diameter of 2.5 microns or less, (≤ PM2.5). PM2.5 can penetrate the lung barrier and enter the blood system.


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.

Chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer.


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.

Nitrogen dioxide (NO2)

NO2 is the main source of nitrate aerosols, which form an important fraction of PM2.5 and, in the presence of ultraviolet light, of ozone.

Sulfur dioxide (SO2)

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.

When SO2 combines with water, it forms sulfuric acid; this is the main component of acid rain which is a cause of deforestation.


Key Messages:

Air pollution is a major environmental risk to health. By reducing air pollution levels, countries can reduce the burden of disease from stroke, heart disease, lung cancer, and both chronic and acute respiratory diseases, including asthma.

The lower the levels of air pollution, the better the cardiovascular and respiratory health of the population will be, both long- and short-term.

In 2016, 91% of the world population was living in places where the WHO air quality guidelines levels were not met.

Ambient (outdoor) air pollution in both cities and rural areas was estimated to cause 4.2 million premature deaths worldwide in 2016.


WHO estimates that in 2016, outdoor air pollution was responsible for

  • 17% of all adult deaths from ischaemic heart disease,
  • 14% from stroke, 25% from chronic obstructive pulmonary disease,
  • 16% from lung cancer, and
  • 26% from all deaths from acute lower respiratory infections.

Some 91% of those premature deaths occurred in low- and middle-income countries, and the greatest number in the WHO

  • South-East Asia and
  • Western Pacific regions.


Policies and investments supporting cleaner transport, energy-efficient homes, power generation, industry and better municipal waste management would reduce key sources of outdoor air pollution.

In addition to outdoor air pollution, indoor smoke is a serious health risk for some 3 billion people who cook and heat their homes with biomass, kerosene fuels and coal.

WHO Air Quality Guideline Values and Health Effects:

Guideline values

Particulate Matter (PM)

  • Fine Particulate Matter (PM2.5)
    10 μg/m3 annual mean
    25 μg/m3 24-hour mean
  • Coarse Particulate Matter (PM10)
    20 μg/m3 annual mean
    50 μg/m3 24-hour mean

Ozone (O3)

  • 100 μg/m3 8-hour mean

Nitrogen dioxide (NO2)

  • 40 μg/m3 annual mean
  • 200 μg/m3 1-hour mean

Sulfur dioxide (SO2)

  • 20 μg/m3 24-hour mean
  • 500 μg/m3 10-minute mean

Health Effects

Particulate Matter (PM)

Both morbidity and mortality increase with a rise in PM

Ozone (O3)

It can

  • cause breathing problems,
  • trigger asthma,
  • reduce lung function and
  • cause lung diseases.

Nitrogen dioxide (NO2)

Symptoms of bronchitis in asthmatic children increase in association with long-term exposure to NO2

Sulfur dioxide (SO2)

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.


Useful Links:

Link to the updated fact sheet:


Link to WHO infographics on Air Pollution:


Link to WHO Air Quality Guidelines, 2005 Update:


Link to WHO Global Health Observatory data on Ambient Air Pollution:


Link to WHO International Agency for Research on Cancer (IARC)’s 2013 Assessment: Air Pollution and Cancer (English) [PDF]: 

Click to access pr221_E.pdf

Link to Review of Evidence on Health Aspects of Air Pollution (REVIHAAP) Report (2013): 



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