Glossary of related terms

A glossary of common terms relating to air pollution is show below. You can use the letters below to quickly jump to a specific term.

  • 1,3-Butadiene (chemical formula C4H6)

    This is an organic compound emitted into the atmosphere mainly from fuel combustion e.g. petrol and diesel vehicles. It is not found in the fuels themselves, but is formed when they are burned. 1,3-butadiene is also an important chemical in certain industrial processes, particularly the manufacture of synthetic rubber. Apart from locations near to these processed, the main source of 1,3-butadiene in the atmosphere is motor vehicle emissions. 1,3-butadiene is known to cause cancer in humans.

  • Acid Deposition

    Total atmospheric deposition of acidity is determined using both wet and dry deposition measurements. Wet deposition is the portion dissolved in cloud droplets and is deposited during rain or other forms of precipitation. Dry deposition is the portion deposited on dry surfaces during periods of no precipitation as particles or in a gaseous form. Although the term "acid rain" is widely recognized, the dry deposition portion ranges from 20 to 60 percent of total deposition.

  • Acid Rain

    When atmospheric pollutants such as sulphur dioxide and nitrogen oxides mix with water vapour in the air, they are converted to sulphuric and nitric acids. These acids make the rain acidic, hence the term 'acid rain'. Acid ran is defined as any rainfall that has an acidity level beyond what is expected in non-polluted rainfall. Acidity is measured using a pH scale, with the number 7 being neutral. Consequently, a substance with a pH value of less than 7 is acidic, while one of a value greater than 7 is basic. Generally, the pH of 5.6 has been used as the baseline in identifying acid rain. Thus any precipitation that has a pH value of less than 5.6 is considered to be acid precipitation.

  • Air Pollution Bandings

    The Air Pollution Information Service uses four bands to describe levels of pollution. The bands are low, moderate, high and very high. Healthy people do not normally notice any effects from air pollution, except occasionally when air pollution is "very high".

  • Air Pollution Bulletins

    Air Pollution Bulletins are issued daily for each zone of the UK. The bulletins show current air quality, and a forecast for the next 24 hours, classified into four bands and also into an index.

  • Air Pollution Index

    A numerical index for air pollution from 1 to 10 related to the air quality bands of "low", "moderate", "high" or "very high".

  • Air Quality Directive

    The European Union's Directive 2008/50/EC of 21st May 2008, on Ambient Air Quality and Cleaner Air for Europe is often referred to as 'the Air Quality Directive'.

  • Air Quality Management Area (AQMA)

    If a local authority finds any places where the objectives are not likely to be achieved, it must declare an Air Quality Management Area there. This area could be just one or two streets, or it could be much bigger. Then the local authority will put together a plan to improve the air quality - a Local Air Quality Action Plan

  • Air Quality Standards

    Standards are the concentrations of pollutants in the atmosphere which can broadly be taken to achieve a certain level of environmental quality. The standards are based on assessment of the effects of each pollutant on human health including the effects on sensitive sub-groups.

  • Air Quality Strategy

    The United Kingdom's own National Air Quality Strategy, containing policies for assessment and management of air quality in the UK. This was first published in 1997, as a requirement of The Environment Act 1995. The Air Quality Strategy for England, Scotland, Wales and Northern Ireland describes the plans drawn up by the Government and the devolved administrations to improve and protect ambient air quality in the UK in the medium-term. The Strategy sets objectives for the main air pollutants to protect health. Performance against these objectives will be monitored where people are regularly present and might be exposed to air pollution.

  • Air Quality Strategy Objective

    The Air Quality Strategy sets objectives for the maximum concentrations of eight pollutants. These are at least as stringent as the limit values of the Air Quality Directive.

  • Ambient Air

    Outdoor air.

  • Annual Mean

    The average of the concentrations measured for each pollutant for one year. Usually this for a calendar year, 1st January – 31st December.

  • Automatic Monitoring

    Monitoring is usually termed "automatic" or "continuous" if it produces real-time measurements of pollutant concentrations. Automatic fixed point monitoring methods exist for a variety of different pollutants and these can provide high resolution data averaged over very short time periods.

  • Average Exposure Indicator (AEI)

    The statistic on which the Air Quality Directive's national exposure reduction target is based, for PM2.5 between 2010 and 2020. The AEI for the UK is calculated as follows: the arithmetic mean PM2.5 concentration at appropriate UK urban background sites is calculated for three consecutive calendar years, and the mean of these values taken as the AEI.

  • Benzene (C6H6)

    A chemical compound that is harmful to human health. As an air pollutant, benzene can be emitted from domestic and industrial combustion processes, and road vehicles.

  • Benzo[a]pyrene

    One of a group of compounds called polycyclic aromatic hydrocarbons (PAHs) that can be air pollutants. The main sources of B[a]P in the UK are domestic coal and wood burning, fires, and industrial processes such as coke production.

  • Beta Attenuation Monitor (BAM)

    A type of instrument used for monitoring concentrations of particulate matter.

  • Black Carbon

    Black Carbon is dark, soot-like airborne particulate material. It consists mostly of carbon and is formed from incomplete combustion of any carbon-containing material. It is measured by the optical absorption of specific wavelengths by particulates collected on a filter. Black carbon is one of the constituents of particulate matter; it is also known to contribute to climate change. Black carbon is similar to the 'black smoke' measurement described below.

  • Black Smoke

    Black Smoke consists of fine particles of carbon-containing material. These particles can be hazardous to health especially in combination with other pollutants which can adhere to the particulate surfaces. Black smoke is emitted mainly from fuel combustion. Following the large reductions in domestic coal use the main source is diesel vehicles. Black smoke was measured for many years by its blackening effect on filters, but this has largely been superseded by measurements of black carbon (see above), using automated techniques.

  • Carbon Monoxide (CO)

    A colorless, odorless gas resulting from the incomplete combustion of hydrocarbon fuels. CO interferes with the blood's ability to carry oxygen to the body's tissues and results in adverse health effects.

  • Climate Change

    An increase in the temperature of the Earth's troposphere. The Earth's climate has changed in the past as a result of natural influences, but the term is most often used to refer to the 'anthropogenic' (i.e. caused by humans) warming, occurring as a result of increased emissions of greenhouse gases.


    Committee on the Medical Effects of Air Pollutants COMEAP is an Advisory Committee of independent experts that provides advice to Government Departments and Agencies on all matters concerning the potential toxicity and effects upon health of air pollutants.

  • Data Capture

    Gives the percentage of all the possible measurements for a given period that were validly measured.

  • Days with Exceedences

    The number of days in which at least one period has a concentration greater than, or equal to, the relevant air quality standard (the averaging period will be that defined by that standard).

    Since the national air quality standards cover different time periods (15 min average, 24 hour running mean etc.) this gives a useful way of comparing data for different pollutants.

  • Diffusion Tube Samplers

    Passive diffusion tube samplers collect nitrogen dioxide and other pollutants by molecular diffusion along an inert tube to an efficient chemical absorbent. After exposure for a known time, the absorbent material is chemically analysed and the concentration calculated.

  • Dispersion model

    A dispersion model is a means of calculating air pollution concentrations given information about the pollutant emissions and the nature of the atmosphere. In the action of operating a factory, driving a car, or heating a house, a variety of pollutants are released into the atmosphere. The amount of pollutant which is released can be determined from a knowledge of the process or actual measurements. Air quality objectives are set in terms of concentration values, not emission rates. In order to assess whether an emission is likely to result in an exceedence of a prescribed objective it is therefore necessary to know the ground level concentrations which may arise at distances from the source. This is the purpose of a dispersion model.

  • EMEP (Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air pollutants in Europe)

    The EMEP programme consists on three main elements: (1) collection of emission data, (2) measurements of air and precipitation quality and (3) modelling of atmospheric transport and deposition of air pollution. EMEP regularly reports on emissions, concentrations and/or depositions of air pollutants, the quantity and significance of transboundary fluxes and related exceedences to critical loads and threshold levels. The EMEP programme is carried out in collaboration with a broad network of scientists and national experts that contribute to the systematic collection, analysis and reporting of emission data, measurement data and integrated assessment results.

  • Emission Factor

    The relationship between the amount of pollution produced and the amount of raw material processed or burned. For mobile sources (such as cars), the emission factor is the amount of pollution produced and the distance travelled by the vehicle.

  • Emission Inventories

    Emission inventories are estimates of the amount and the type of pollutants that are emitted to the air each year from all sources. There are many sources of air pollution, including traffic, household heating, agriculture and industrial processes.


    The Expert Panel on Air Quality Standards was set up in 1991 to provide independent advice on air quality issues, in particular the levels of pollution at which no or minimal health effects are likely to occur. Members of the Panel are primarily drawn from those eminent in the fields of health research, practice and teaching. The Panel's recommendations were adopted as the benchmark standards in the National Air Quality Strategy.

  • Episode (Air Pollution Episode)

    An 'air pollution episode' means a period of time (usually a day or several days) when air pollution is high (air quality is poor).

  • EU Directives

    The European Union has been legislating to control emissions of air pollutants and to establish air quality objectives for the last two decades. After many years of a somewhat piece-meal approach, ambient air quality legislation is now being consolidated. Directive 96/62/EC on ambient air quality assessment and management, the so called Air Quality Framework Directive, sets a strategic framework for tackling air quality consistently by setting European-wide limit values for twelve air pollutants in a series of daughter directives. These will supersede and extend existing European legislation.

  • Exceedence

    A period of time where the concentration of a pollutant is greater than, or equal to, the appropriate air quality criteria. For air quality standards an exceedance is a concentration greater than the standard value. For air quality bands an exceedance is a concentration greater than, or equal to, the upper band threshold.

  • FDMS

    This stands for 'Filter Dynamic Measurement System' and refers to a type of instrument for monitoring concentrations of particulate matter. The FDMS is a modified form of TEOM.

  • FidasTM

    A type of instrument which uses an optical technique for monitoring concentrations of particulate matter.

  • Fourth Daughter Directive

    The European Union's Directive 2004/107/EC, which covers the four metallic elements cadmium, arsenic, nickel and mercury together with polycyclic aromatic hydrocarbons (PAH). (Its name comes from its origin as one of four so-called Daughter Directives set up under an overarching 'framework Directive'.)

  • Global Warming

    An increase in the temperature of the Earth's troposphere. Global warming has occurred in the past as a result of natural influences, but the term is most often used to refer to the warming predicted by computer models to occur as a result of increased emissions of greenhouse gases.

  • Gravimetric Equivalent PM10 Data

    Monitoring of PM10 levels in the UK was, until around 2009, largely based upon the use of TEOM analysers. A principal concern with the TEOM instrument is that the filter is held at an elevated temperature (50°C) in order to minimise errors associated with the evaporation and condensation of water vapour. This can lead to the loss of the more volatile species (some hydrocarbons, nitrates etc.) and has led to the identification of differences between TEOM and gravimetric measurements at co-located sites. Currently a factor of 1.3 is applied to all TEOM measured concentrations to estimate the gravimetric equivalent. Further studies have been commissioned by Defra, the Scottish Executive, the Welsh Assembly Government and the Department for the Environment in Northern Ireland to investigate these effects, and to provide a more robust relationship between the TEOM and the European transfer gravimetric reference method.

  • Greenhouse Gases

    Atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, ozone, and water vapor that slow the passage of re-radiated heat through the Earth's atmosphere.

  • Hydrocarbons

    Compounds containing various combinations of hydrogen and carbon atoms. They may be emitted into the air by natural sources (e.g., trees) and as a result of fossil and vegetative fuel combustion, fuel volatilization, and solvent use. Hydrocarbons are a major contributor to smog.

  • Limit value

    The Air Quality Directive sets 'limit values' for ambient concentrations of pollutants. Limit values are legally binding and must not be exceeded. All Member States of the EU must make the limit values part of their own air quality legislation.

  • Local Air Quality Action Plan

    Where a local authority has set up an AQMA, it must produce an action plan setting out the measures it intends to take in pursuit of the air quality objectives in the designated area. The plan should be in place, wherever possible, within 12-18 months of designation and should include a timetable for implementation.

  • Local Air Quality Management (LAQM)

    LAQM requires local authorities periodically to review and assess the current and future quality of air in their areas. A local authority must designate an air quality management area (AQMA) if any of the objectives set out in regulation is not likely to be met in its area in the relevant period.

  • Long-Term Objectives

    As well as limit values and target values, the Air Quality Directive sets 'long-term objectives' for ozone concentration. These are similar to limit values but are not legally mandatory. Member States must take all necessary measures not entailing disproportionate costs to meet the target values and long-term objectives.

  • Maximum hourly average

    The highest hourly reading of air pollution obtained during the time period under study.

  • Member States

    Countries that are part of the European Union.

  • Microgramme per cubic metre (µg/m3)

    Unit often used to express concentration of a pollutant in air. 1 µg = 1 millionth of a gramme or 1 x 10-6 g.

  • Micrometre (µm)

    Unit of length often used for the size of particulate pollutants. 1 µm = 1 millionth of a metre (1 x 10-6 m) or one thousandth of a millimetre.

  • Milligramme per cubic metre (µg/m3)

    Unit often used to express concentration of carbon monoxide in air. 1 mg = 1 thousandth of a gramme or 1 x 10-3 g.

  • National Atmospheric Emissions Inventory (NAEI).

    The NAEI compiles estimates of emissions to the atmosphere from UK sources such as cars, trucks, power stations and industrial plant. These emissions are estimated to help to find ways of reducing the impact of human activities on the environment and our health. The NAEI is funded by Defra, the Scottish Executive, the Welsh Assembly Government and the Department for the Environment in Northern Ireland.

  • National Statistics

    Emissions and concentration statistics shown in the air quality database are National Statistics. National Statistics are produced to high professional standards set out in the National Statistics Code of Practice. They undergo regular quality assurance reviews to ensure that they meet customer needs. They are produced free from any political interference.

  • Oxides of Nitrogen (NOx)

    Combustion processes emit a mixture of oxides of nitrogen, primarily nitric oxide (NO) and nitrogen dioxide (NO2), collectively termed NOx. Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant which may exacerbate asthma and possibly increase susceptibility to infections. In the presence of sunlight, it reacts with hydrocarbons to produce photochemical pollutants such as ozone. Nitrogen dioxide emissions can also be further oxidised in air to acid gases, which contribute to the production of acid rain.

  • Ozone (O3)

    A pollutant gas which is not emitted directly from any source in significant quantities, but is produced by reactions between other pollutants in the presence of sunlight. (This is what is known as a 'secondary pollutant'.) Ozone concentrations are greatest in the summer. O3 can travel long distances and reach high concentrations far away from the original pollutant sources. Ozone is an irritant to the airways of the lungs, throat and eyes: it can also harm vegetation.

  • Particulate Matter (PM)

    Small airborne particles. PM may contain many different materials such as soot, wind-blown dust or secondary components, which are formed within the atmosphere as a result of chemical reactions. Some PM is natural and some is man-made. Particulate matter can be harmful to human health when inhaled, and research shows a range of health effects associated with PM. In general, the smaller the particle the deeper it can be inhaled into the lung.

  • Percentile

    A value that is the rank at a particular point in a collection of data. For instance, a 98th percentile of values for a year is the value that 98% of all the data in the year fall below, or equal.

  • PM10

    Particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 10 µm aerodynamic diameter, as defined in ISO 7708:1995, Clause 6. This size fraction is important in the context of human health, as these particles are small enough to be inhaled into the airways of the lung – described as the 'thoracic convention' in the above ISO standard. PM10 is often described as 'particles of less than 10 micrometres in diameter' though this is not strictly correct.

  • PM2.5

    Particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 2.5 µm aerodynamic diameter, as defined in ISO 7708:1995, Clause 7.1. This size fraction is important in the context of human health, as these particles are small enough to be inhaled very deep into the lung – described as the 'high risk respirable convention' in the above ISO standard. PM2.5 is often described as 'particles of less than 2.5 micrometres in diameter' though this is not strictly correct.

  • Polycyclic Aromatic Hydrocarbons (PAH)

    PAHs are a large group of chemical compounds that are toxic and carcinogenic. Once formed, they can remain in the environment for a long time, and can be passed up the food chain. The main sources are domestic coal and wood burning, outdoor fires, and some industrial processes. The pollutant benzo[a]pyrene is a PAH, and because it is one of the more toxic PAH compounds it is measured as a 'marker' for this group of pollutants.

  • POPS

    Persistent Organic Pollutants (POPs) are chemical substances that persist in the environment, bioaccumulate through the food web, and pose a risk of causing adverse effects to human health and the environment. These include dioxins and furans (see TOMPS)

  • ppb

    Parts per billion. The concentration of a pollutant in air in terms of volume ratio. A concentration of 1 ppb means that for every billion (109) units of air, there is one unit of pollutant present.

  • ppm

    Parts per million. The concentration of a pollutant in air in terms of volume ratio. A concentration of 1 ppm means that for every million (106) units of air, there is one unit of pollutant present.

  • Running mean

    This is a mean - or series of means - calculated for overlapping time periods, and is used in the calculation of several of the national air quality standards.

    For instance, an 8 hour running mean is calculated every hour, and averages the values for eight hours. The period of averaging is stepped forward by one hour for each value, so running mean values are given for the periods 00:00 - 07:59, 01:00 - 08:59 etc. By contrast a non-overlapping mean is calculated for consecutive time periods, giving values for the periods 00:00 - 07:59, 08:00 - 15:59 and so on. There are, therefore, 24 possible 8-hour means in a day (calculated from hourly data) and 3 non-overlapping means.

  • Secondary pollutant

    A pollutant which is formed by chemical reactions from other pollutants in the atmosphere. Ozone, for example, is a secondary pollutant.

  • Sulphur Dioxide (SO2)

    An acid gas formed when fuels containing sulphur impurities are burned. SO2 irritates the airways of the lung and is also harmful to the environment, contributing to acid precipitation.

  • Target Value

    As well as limit values, the Air Quality Directive and Fourth Daughter Directive set target values for some pollutants. These are similar to limit values but are not legally mandatory. Member States must take all necessary measures not entailing disproportionate costs to meet the target values.

  • TEOM

    This stands for 'Tapered Element Oscillating Microbalance'. This is a type of instrument used to monitor concentrations of particulate matter.

  • TOMPs

    This stands for 'Toxic Organic Micropollutants'. These are compounds that are present in the environment at very low concentrations, but are highly toxic and persistent. They include dioxins and dibenzofurans.

  • Volatile organic compounds (VOCs)

    Carbon-containing compounds that evaporate into the air. Examples include the solvents used in paints. VOCs are involved in the formation of other pollutants, such as ozone.

  • Zones and Agglomerations

    The UK has been divided into zones and agglomerations for the purposes of air pollution monitoring, in accordance with EC Directive 96/62/EC. There are 16 zones. They match:

    1. The boundaries of England's Government Offices for the Regions
    2. The boundaries agreed by the Scottish Executive, National Assembly for Wales, and Department of the Environment in Northern Ireland.

    There are 28 agglomerations in the UK. An agglomeration is defined as any urban area with a population greater than 250,000.