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The glossary below provides definitions to commonly used terms relating to air pollution which you may find useful.


  • 1,3-Butadiene (C4H6)
    1,3-butadiene, like benzene, is organic compound emitted into the atmosphere principally from fuel combustion of petrol and diesel vehicles. Unlike benzene, however, it is not a constituent of the fuel but is produced by the combustion of olefins. 1,3-butadiene is also an important chemical in certain industrial processes, particularly the manufacture of synthetic rubber. It is handled in bulk at a small number of industrial locations. Other than in the vicinity of such locations, the dominant source of 1,3-butadiene in the atmosphere is the motor vehicle. 1,3-Butadiene is a known, potent, human carcinogen.


  • 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 Pollution Information Service
    The Air Pollution Information Service provides - free of charge - detailed, easy-to-understand information on air pollution. This information is particularly important to people with medical conditions which air pollution may make worse. The latest information is available by freephone, on Ceefax and Teletext, and via the Internet. The Service gives regionally based summaries and detailed information on current pollution levels, as well as forecasts for the next 24 hours.
  • 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 Objectives
    Objectives are policy targets generally expressed as a maximum ambient concentration to be achieved, either without exception or with a permitted number of exceedences, within a specified timescale.
  • 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 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.
  • Ambient Air
    The air occurring at a particular time and place outside of structures. Often used interchangeably with "outdoor air".
  • Annual Mean
    The average of the concentrations measured for each pollutant for one year. Usually this for a calendar year, but some species are reported for the period April to March, known as a pollution year. This period avoids splitting a winter season between two years, which is useful for pollutants that have higher concentrations during the winter months.
  • 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.


  • BAM (Beta Attenuation Mass Monitor)
    The BAM (Beta Attenuation Mass Monitor) measures particulate concentrations automatically. The mass density is measured using the technique of Beta attenuation. A small Beta source is coupled to a sensitive detector which counts the Beta particles. As the mass of particles increases the Beta count is reduced. The relationship between the decrease in count and the particulate mass is computed according to a known equation (the Beer-Lambert law).
  • Benzene (C6H6)
    Benzene is an aromatic organic compound which is a minor constituent of petrol (about 2% by volume). The main sources of benzene in the atmosphere in Europe are the distribution and combustion of petrol. Combustion by petrol vehicles is the largest component (70% of total emissions) whilst the refining, distribution and evaporation of petrol from vehicles accounts for approximately a further 10% of total emissions. Benzene is emitted in vehicle exhaust not only as unburnt fuel but also as a product of the decomposition of other aromatic compounds. Benzene is a known human carcinogen.
  • Black Smoke
    Black Smoke consists of fine particulates. 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 engined vehicles. Black smoke is measured by its blackening effect on filters. It has been measured for many years in the UK. Now interest is moving to the mass of small particles regardless of this blackening effect.
  • BTEX
    BTEX includes Benzene, Toluene, Ethylbenzene and Xylene. These are the hydrocarbon pollutants collected on diffusion tubes.


  • 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.
    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, the relationship between the amount of pollution produced and the number of vehicle miles travelled. By using the emission factor of a pollutant and specific data regarding quantities of materials used by a given source, it is possible to compute emissions for the source. This approach is used in preparing an emissions inventory.
  • 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.
  • 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 exceedence is a concentration greater than the standard value. For air quality bands an exceedence is a concentration greater than, or equal to, the upper band threshold.


  • 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 has, to date, been 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 (50C) 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 Earths 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.


  • 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.


  • Maximum hourly average
    The highest hourly reading of air pollution obtained during the time period under study.
  • Microgrammes per cubic metre (g/m3)
    A measure of concentration in terms of mass per unit volume. A concentration of 1 g/m3 means that one cubic metre of air contains one microgramme (millionth of a gramme) of pollutant.


  • 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.
  • Non-volatile PM10 & Non-volatile PM2.5
    Non volatile PM component is the fraction of particulate matter that does not readily evaporate at "normal" temperatures and pressures. This fraction includes carbon particles from combustion, agricultural and quarry dust, re-suspended road dust, etc.


  • 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)
    Ozone (O3) is not emitted directly into the atmosphere, but is a secondary pollutant produced by reaction between nitrogen dioxide (NO2), hydrocarbons and sunlight. Whereas nitrogen dioxide (NO2) acts as a source of ozone, nitrogen oxide (NO) destroys ozone acting as a local sink. For this reason, ozone levels are not as high in urban areas (where high levels of NO are emitted from vehicles) as in rural areas. Ozone levels are usually highest in rural areas, particularly in hot, still, sunny weather conditions giving rise to "summer smog".


  • PAHs
    Polycyclic Aromatic Hydrocarbons (PAHs) belong to a large group of organic compounds ; several individual PAHs have been shown to be carcinogenic. EPAQS have recommended a standard for PAHs of 0.25 ng/m3 using benzo[a]pyrene as a marker compound.
  • Particulate matter (PM10)
    Particulate matter consists of very small liquid and solid particles floating in the air. Of greatest concern to public health are the particles small enough to be inhaled into the deepest parts of the lung. These particles are less than 10 microns in diameter - about 1/7th the thickness of the a human hair - and are known as PM10 .Concern about the potential health impacts of PM10 has increased very rapidly over recent years. Increasingly, attention has been turning towards monitoring of the smaller particle fraction (PM2.5) capable of penetrating deepest into the lungs, or even to total particle numbers.
  • 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.
  • 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.


  • Sulphur Dioxide (SO2)
    Sulphur dioxide is a corrosive acid gas which combines with water vapour in the atmosphere to produce acid rain. Both wet and dry deposition have been implicated in the damage and destruction of vegetation and in the degradation of soils, building materials and watercourses. SO2 in ambient air is also associated with asthma and chronic bronchitis.


  • TEOM
    The tapered element oscillating microbalance (TEOM) is used to continuously measure particulate concentrations. It measures the mass collected on an exchangeable filter cartridge by monitoring the corresponding frequency changes of a tapered element. The sample flow passes through the filter, where particulate matter collects, and then continues through the hollow tapered element on its way to an electronic flow control system and vacuum pump.
    Toxic organic micro pollutants dioxins (TOMPs) are produced by the incomplete combustion of fuels. They comprise a complex range of chemicals some of which, although they are emitted in very small quantities, are highly toxic or carcinogenic. Compounds in this category include: PAHs (Polycyclic Aromatic Hydrocarbons), PCBs (PolyChlorinated Biphenyls), Dioxins and Furans.
  • Trajectory Model
    The trajectory model is used to predict episodes of photochemically generated pollutants in the summer, where long-range transport is an important factor in producing high UK concentrations. It uses the output of numerical weather prediction models as its input, and predicts how air masses have been transported for the preceding 96 hours. These pathways are known as "back trajectories". The model uses a simplified chemical scheme to predict the formation of ozone as the air travels to the UK. Concentrations of the secondary particle contribution to PM10 are also predicted by this model


  • Volatile organic compounds (VOCs)
    Carbon-containing compounds that evaporate into the air (with a few exceptions). VOCs contribute to the formation of smog and/or may themselves be toxic. VOCs often have an odor, and some examples include gasoline, alcohol, and the solvents used in paints
  • Volatile PM10 & Volatile PM2.5
    Volatile PM component is the fraction of particulate matter that can exist as both aerosol particles and gas as "normal" temperatures and pressures. This includes many volatile organic compounds, ammonium nitrate, etc. Volatile PM is an important component to collect, as it forms a significant proportion of PM in the spring and autumn, but because it readily evaporates, collection and measurement can often be difficult.


  • 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.