Polluted Air

Primary reference(s)

WMO, 1992. International Meteorological Vocabulary. WMO-No. 182. World Meteorological Organization (WMO). Accessed 26 November 2019.

UNEP, no date. Frequently Asked Questions on Air Pollution. International Day for Clean Air for blue skies, United Nations Environment Programme (UNEP). Accessed 11 November 2020.

Additional scientific description

Air pollution is caused by gases and particles emitted to the atmosphere by a variety of human activities, such as the inefficient combustion of fuels, agriculture, and farming. There are also natural sources contributing to air pollution, including particles of soil dust and salt in sea spray (UNEP, no date).

Air pollutants can be emitted directly from a source (i.e., primary pollutants) or can form from chemical reactions in the atmosphere (i.e., secondary pollutants). When concentrations of these substances reach critical levels in the air, they harm humans, animals, plants and ecosystems, and reduce visibility and corrode materials, buildings and cultural heritage sites (UNEP, no date).

The main atmospheric pollutants affecting human health are particulate matter, ground-level ozone (O3) and nitrogen dioxide (NO2) (US EPA, 2020a,b,c). The fine particles that damage human health are known as PM2.5 (particles with a diameter of less than 2.5 micrometres), which can penetrate deep into the lungs and pass into the bloodstream, affecting different organs and bodily functions. These particles can either be emitted directly or formed in the atmosphere from several different emitted pollutants (e.g., ammonia [NH3] and volatile organic compounds [VOCs]) (Air Pollution Information System, 2016; US EPA, 2020d; UNEP, no date).

Ground-level ozone is an important secondary pollutant. It is a potent lung irritant and stunts growth in plants. It also oxidises surfaces with which it comes into contact, degrading the materials from which they are made. Ozone is also a powerful greenhouse gas. Tropospheric ozone is different to ozone in the upper atmosphere (stratosphere), which protects us from ultraviolet light from the sun (UNEP, no date).

Nitrogen oxides (NOx) are a group of air pollutants, comprising nitrogen dioxide (NO2) and nitrogen monoxide (NO). Nitrogen dioxide is the most harmful of these compounds and is generated from human-driven activities. It impacts human health, reduces atmospheric visibility, and can play a significant role in climate change, at high concentrations. It is also a critical precursor to the formation of ground-level ozone (UNEP, no date).

Particulate matter (both the aerosol that is directly emitted to the atmosphere and the secondary aerosol that is formed in the atmosphere) has a wide range of negative impacts, which depend on the chemical composition of the particles (UNEP, no date). Black carbon, which is a carbon particle produced as a result of partial combustion of hydrocarbons that contributes to air pollution, has strong negative impacts on health and contributes to climate warming (No More Planet, 2021). Aerosol has negative impacts on biodiversity in terrestrial ecosystems (especially sulphur- and nitrogen-containing aerosol), and in high concentrations impacts visibility and has a soiling effects on surfaces (UNEP, no date). 

Metrics and numeric limits

While all individuals experience different levels of health impacts from air pollution, across large city or country populations, there is no evidence of a completely safe level of air pollution, especially in the case of particulate matter. However, to help guide countries to achieve cleaner air for health, the World Health Organization (WHO) has set normative guideline values for all major air pollutants, above which, negative impacts on population health are likely (WHO, 2005). For example, the WHO estimates that reducing annual average fine particulate matter (PM2.5) concentrations from levels of 35 μg/m3 (an interim air quality guideline commonly used in many developing country cities), to the WHO guideline level of 10 μg/m3, could reduce air pollution related deaths by around 15%. This does not mean that there are no health effects below those guidelines, but that they represent health-based targets useful for tracking the burden of disease from air pollution, informing national level targets and standards, and monitoring the effectiveness of air quality management efforts designed to improve health (WHO, 2005).

There are currently no set International standards for the other impacts of air pollution.

The impact of air pollution on climate is defined through the global warming potential, such as used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2014).

Examples of drivers, outcomes and risk management

The key message is that air pollution is now the greatest environmental threat to health and ecosystems, but it is preventable. The World Meteorological Organization (WMO) reports that there are solutions and technology for mitigating air pollution. Improving air quality requires all stakeholders on board, from individuals to private companies to governments. Air pollution contributes to heart disease, stroke, lung cancer and other respiratory diseases. It is estimated to be responsible for 7 million premature deaths per year, predominantly in low- and middle-income countries. Air pollution also threatens the economy, food security and the environment (WMO, 2020).

The WMO works toward enhanced availability and quality of observations of several important air pollutants such as black carbon, tropospheric ozone, and atmospheric dust through the Global Atmosphere Watch programme (WMO, 2021a) to support evidence-based policy on the environment (WMO, 2021b). Members are working on the development of forecasting systems that will help reduce acute air pollution episodes (WMO, 2020).

The WMO also leads global research and strengthens scientific knowledge on connections between air quality and climate and is developing the science-based tools to support policy-making on air pollution and climate change mitigation (WMO, 2020).

The WMO Global Atmosphere Watch initiated the Global Air Quality Forecasting and Information System (GAFIS) initiative, which builds a platform for providers and users of air quality forecast and information systems (WMO, 2021c). It will enable access to, and use of, air quality prediction and analysis products at various temporal and spatial scales by a multiplicity of communities and stakeholders interested in air quality (WMO, 2020).