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The process of taking in. For a person or an animal, absorption is the process of a substance getting into the body through the eyes, skin, stomach, intestines, or lungs. (Source: ATSDR Glossary of Terms  )


Acid rain

Falling rain (or snow) which has become acidic (pH less than 5.6) as a result of its combination with gaseous pollutants, such as sulphur dioxide (SO2) and nitrogen oxides (NOx).

Acid rain may cause acidification of surface waters, soils and ecosystems. (Source: GreenFacts)


Additive effect

An additive effect is the overall consequence which is the result of two chemicals acting together and which is the simple sum of the effects of the chemicals acting independently. (Source: GreenFacts)

Adverse health effect

A change in body function or cell structure that might lead to disease or health problems. (Source: ATSDR Glossary of Terms  )

Aerodynamic diameter

Airborne particles have irregular shapes, and their aerodynamic behaviour is expressed in terms of the diameter of an idealised spherical particle known as aerodynamic diameter.

Particles are sampled and described on the basis of their aerodynamic diameter, which is usually simply referred to as particle size.

Particles having the same aerodynamic diameter may have different dimensions and shapes. (Source: based on the GreenFacts Digest on Air Pollution)



Aerodynamics is the study of how air and other gases flow, including the forces that act on an object as it moves in air.

The more aerodynamic (streamlined) a moving object is, the weaker the air resistance that slows it down. (Source: GreenFacts)

Air pollution hot spot

A location where emissions from specific sources may expose individuals and population groups to elevated risks of adverse health effects - including but not limited to cancer - and contribute to the cumulative health risks of emissions from other sources in the area. (Source: California Air Resources Board Glossary of Air Pollution Terms  )


Allergies are inappropriate or exaggerated reactions of the immune system to substances that, in the majority of people, cause no symptoms.

Symptoms of the allergic diseases may be caused by exposure of the skin to a chemical, of the respiratory system to particles of dust or pollen (or other substances), or of the stomach and intestines to a particular food. (Source: ACAAI Allergy-Immunology Glossary  )



Refers to surrounding environmental conditions such as temperature or noise.

In the case of air, ambient air often refers to outdoor air as opposed to indoor air. (Source: GreenFacts)

Antagonistic effect

A biologic response to exposure to multiple substances that is less than would be expected if the known effects of the individual substances were added together. (Source: ATSDR Glossary of Terms  )


A usually chronic inflammatory disorder of the airways characterized by intermittent episodes of wheezing, coughing, and difficulty in breathing, sometimes caused by an allergy to inhaled substances. (Source: American Lung Association Appendix 4: Glossary  )


The mass of air surrounding the Earth.

The atmosphere consists of nitrogen (78%), oxygen (21%), and traces of other gases such as argon, helium, carbon dioxide, and ozone.

The atmosphere plays an important role in the protection of life on Earth; it absorbs ultraviolet solar radiation and reduces temperature extremes between day and night. (Source: GreenFacts)



An inherited tendency to develop an allergic condition, such as some forms of hay fever, asthma, or eczema. (Source: GreenFacts)


Deviation of results or inferences from the truth, or processes leading to such systematic deviation. Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth. (Source: CDC Glossary of Epidemiologic Terms  )


Inflammation of the bronchi, the main air passages that lead to the lungs.

Bronchitis causes a persistent cough and phlegm production. It is especially common in smokers and in areas with atmospheric pollution. (Source: GreenFacts based on Urologychannel HC Glossary )


Canadian Public Health Association

"The Canadian Public Health Association (CPHA) is a national, independent, not-for-profit, voluntary association representing public health in Canada with links to the international public health community. CPHA's members believe in universal and equitable access to the basic conditions which are necessary to achieve health for all Canadians."

Carbon monoxide (CO)

An odorless, colorless, and highly poisonous gas.



Having to do with the heart and lungs. (Source: NCI dictionary  )

Chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease, or COPD, refers to a group of diseases that cause airflow blockage and breathing-related problems.

It includes emphysema, chronic bronchitis, and in some cases asthma.

COPD is a leading cause of death, illness, and disability in the United States. (Source: CDC Facts About COPD  )

Coarse particles

Particulate matter present in air is divided into different categories depending on the size of the particles (aerodynamic diameter).

Coarse particles are the relatively large airborne particles mainly produced by the mechanical break-up of even larger solid particles.

Examples of coarse particles include dust, pollen, spores, fly ash, and plant and insect parts.

Coarse particles have an aerodynamic diameter ranging from 2.5 to 10µm (PM10-2.5), which distinguishes them from the smaller airborne particulate matter referred to as fine (PM2.5) and ultrafine particles (PM0.1).

Cohort study

A type of epidemiological study which observes a large number of individuals in a population over a period of time.

It compares individuals who are exposed for instance to a certain chemical to others without the exposure or with a different level of exposure. These two groups are called cohorts and are followed over time to determine the differences in the health outcomes between the exposure subjects. (Source: GreenFacts)


The amount of a chemical or substance present in a particular quantity of soil, water, air, food, blood, hair, urine, breath, or any other media. (Source: GreenFacts)


Controlled study

An experiment or clinical trial in which two groups are used for comparison purpose.


Dose-response relationship

The relationship between the amount of exposure [dose] to a substance and the resulting changes in body function or health (response). (Source: ATSDR Glossary of Terms  )



A biological endpoint is a direct marker of disease progression - e.g. disease symptoms or death - used to describe a health effect (or a probability of that health effect) resulting from exposure to a chemical. (Source: GreenFacts)


Environment DG

"The Environment DG is one of 36 Directorates-General (DGs) and specialised services which make up the European Commission. Its main role is to initiate and define new environmental legislation and to ensure that measures, which have been agreed, are actually put into practice in the Member States."

The mission statement of the Environment DG is: "to promote Sustainable Development, preserving the rights of future generations to a viable environment; to work towards a high level of environmental and health protection and improvement of the quality of life; to promote environmental efficiency; to encourage the equitable use, as well as the sound and effective management, of common environmental resources" (Source: EC DG ENVI website )


Fairness of rights, distribution, and access. Depending on context, this can refer to resources, services, or power. (Source: Millennium Ecosystem Assessment  Glossary )

European Commission

"The European Commission (EC) embodies and upholds the general interest of the [European] Union and is the driving force in the Union's institutional system. Its four main roles are to propose legislation to Parliament and the Council, to administer and implement Community policies, to enforce Community law (jointly with the Court of Justice) and to negotiate international agreements, mainly those relating to trade and cooperation."

The Commission's staff is organised into 36 Directorates-General (DGs) and specialised services, such as the Environment DG and the Research DG. (Source: EC website  )

European Environmental Agency

The European Environmental Agency (EEA) is one of the 15 specialised agencies of the European Union, which handle specific technical, scientific or management tasks. Operational since 1994, the EEA is based in Copenhagen.

"Its mission is to collect, prepare and disseminate timely, targeted, relevant and reliable information on the state and trends of the environment at European level. The founding regulation of EEA stipulates that it is open to countries that do not belong to the European Union but share its concern for the environment. Iceland, Liechtenstein and Norway have been members from the start, and 12 out of 13 candidate countries have joined in 2002 (...)." (Source: EEA website )


Contact of the cells of an organism with a substance, micro-organism or radiation. In the case of humans, this may involve contact with a substance or agent by swallowing, breathing, or through the skin or eyes. Exposure may be short-term [acute exposure], of intermediate duration, or long-term [chronic exposure].

Exposure can be divided into external and internal.

External exposure refers to the whole dose to which an organism is exposed.

Internal exposure refers only to that fraction of the initial chemical dose that is absorbed and distributed throughout the body via systemic circulation. (Source: GreenFacts)


Fine particles

Particulate matter present in air is divided into different categories depending on the size of the particles (aerodynamic diameter).

Fine particles are airborne particles which are smaller than coarse particles. They have an aerodynamic diameter of 2.5 µm or less (PM2.5). The fine particles which are smaller than 0.1 µm are referred to as ultrafine particles (PM0.1).

  • Fine particles are largely formed from gases.
  • Ultrafine particles are formed by nucleation, which is the initial stage in which gas becomes a particle. These particles can grow up to a size of 1µm either through condensation, when additional gas condensates on the particles, or through coagulation, when two or more particles combine to form a larger particle.

Please note that ultrafine particles (PM0.1) are part of the fine fraction (PM2.5). (Source: GreenFacts)

Fly ash

Fine solid particles of ash, dust, and soot that are carried into the air when fuel is burnt. (Source: GreenFacts)

Fossil fuel(s)

A general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. (Source: US EPA Glossary of Climate Change terms  )


Inflammation is the reaction of living tissues to infection, irritation or other injury. (Source: GreenFacts)



Not harmful to health. (Source: GreenFacts)


Not organic. Inorganic compounds are generally structured by ionic bonds and do not contain carbon chemically bound to hydrogen (hydrocarbons) or any of their derivatives. Examples of inorganic compounds include sodium chloride (NaCl) and calcium carbonate (CaCO3) and pure elements (e.g. elemental mercury, elemental lead). (Source: GreenFacts)


Life expectancy

The number of years that a person could expect to live on average, based on the mortality rates of the population in a given year.

Life expectancy can change over the lifecycle. For example, at birth a person may be expected to live for 75 years, but if they survive to 75 they may be expected to live for another 10 years. (Source: New Zealand Ministry of Economic Development Population & Sustainable Development, Glossary  )

Lower respiratory symptoms

Symptoms that relate to the lower respiratory tract, i.e. the lungs, trachea, and bronchi.

They include cough, phlegm, chest pain on deep inhalation, and wheeze.

Lower respiratory symptoms suggest the presence of asthma, an allergic reaction, or an infection. (Source: GreenFacts)


Lung function

The term "lung function" refers to how well a person is breathing. (Source: GreenFacts)


Mass (weight) Units

The Metric System of Measurements uses the mass units: gram (g), kilogram (kg) and tonne (t).

1000 g = 1 kg
1000 kg = 1 tonne

Adding prefixes of the International System of Units (SI) allows to express weight as multiples or fractions of 1 gram:

1 gigatonne (Gt) =1 000 000 000 000 000 g
1 megatonne (Mt) =1 000 000 000 000 g
1 tonne (t) =1 000 000 g
1 kilogram (kg) =1 000 g
1 gram (g) =1 g
1 milligram (mg) =0.001 g
1 microgram (µg) =0.000 001 g
1 nanogram (ng) =0.000 000 001 g
1 picogram (pg) =0.000 000 000 001g

Imperial and US weight units can also be expressed as metric units:

Metric units
1 US ton (ton) =0.907 tonne
1 UK ton (ton) =1.016 tonne
1 lb (pound) =453.59 g
1 oz (ounce) =28.35g

Further information on the International System of Units (SI) is provided by the International Bureau of Weights and Measures (BIPM) 


A disease or the incidence of a disease within a population. (Source: GreenFacts)

Nitrogen oxides (NOx)

NOx is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts [such as nitric oxide (NO) and nitrogen dioxide (NO2)].

Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas.

Nitrogen oxides form when fuel is burned [...]. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

In the atmosphere, nitrogen oxides can contribute to formation of photochemical ozone (smog) and have health consequences. They also lead to acid rain and contribute to global warming. (Source: US EPA NOx: What is it? Where does it come from?   )



In the context of air pollution, nucleation is the first step of the process by which gases are converted to small liquid droplets (ultrafine particles).

This occurs either when certain gases condensate or when different gases react with each other.

The ultrafine particles formed are called "nuclei" and can grow in size when more gases condensate on them or when several droplets merge. (Source: GreenFacts)

Organic arsenic compounds

Arsenic compounds containing carbon. They are mainly found in sea-living organisms, although some of these compounds have also been found in species living on land. (Source: GreenFacts)


Ozone is a form of oxygen having the molecular form of O3. It is a bluish, unstable gas with a pungent odour, found in two parts of the atmosphere: the stratosphere and the troposphere.

The ozone layer: The stratosphere contains a layer in which the concentration of ozone is greatest, the so called ozone layer. The layer extends from about 12 to 40 km. It shields the Earth from ultraviolet radiation's harmful health effects on humans and the environment. This layer is being depleted by human emissions of chlorine- and bromine-containing compounds.

Ground-level ozone: At ground level (in the troposphere), ozone is considered an air pollutant that can seriously affect the human respiratory system. It is a chemical oxidant and a major component of photochemical smog. (Source: GreenFacts)


Panel study

A research method in which data are collected from the same individuals at different points in time. (Source: Online Learning center Glossary )

Particulate matter

Sum of all microscopic solid and liquid particles, of human and natural origin, that remain suspended in a medium such as air for some time. These particles vary greatly in size, composition, and origin, and may be harmful.

Particulate matter may be in the form of fly ash, soot, dust, fog, fumes etc. (Source: GreenFacts)


Persistent organic pollutants

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. This group of priority pollutants consists of pesticides (such as DDT), industrial chemicals (such as polychlorinated biphenyls, PCBs) and unintentional by-products of industrial processes (such as dioxins and furans).

Persistent Organic Pollutants are transported across international boundaries far from their sources, even to regions where they have never been used or produced. (Source: European Commission Environment DG POPs  )


Thick, sticky, stringy mucus secreted by the mucous membrane of the respiratory tract, as during a cold or other respiratory infection. (Source: Phlegm  )

Photochemical reaction

Refers to any chemical reaction which occurs as a result of light energy from the sun.

For example, ozone is formed through a photochemical reaction involving nitrogen dioxide and reactive organic compounds. (Source: DNREC online   A history of air pollution events )


Polycyclic aromatic hydrocarbons

A group of over 100 different organic compounds composed of several benzene rings. Some of them are persistent and carcinogenic.

PAHs are commonly formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat.

Generally, tobacco smoke is by far the most important source of exposure for humans. (Source: GreenFacts)



A group or number of people living within a specified area or sharing similar characteristics (such as occupation or age). (Source: ATSDR Glossary of Terms  )


Primary & secondary particles

Primary particles are directly released into the atmosphere by wind, combustion processes, or human activities.

Secondary particles are those that form in the atmosphere from other gaseous pollutants, particularly sulfur dioxide, nitrogen oxides, ammonia, and volatile organic compounds. (Source: GreenFacts)

Primary & Secondary pollutant

A primary pollutant is an air pollutant emitted directly from a source.

A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants) react in the atmosphere.

Examples of a secondary pollutant include ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight; NO2, which is formed as NO combines with oxygen in the air; and acid rain, which is formed when sulfur dioxide or nitrogen oxides react with water. (Source: GreenFacts)

Respiratory tract

The organs that are involved in breathing.

These include the nose, throat, larynx, trachea, bronchi, and lungs. Also known as the respiratory system. (Source: St. Jude Children's Research Hospital Glossary   )



Carbon dust that is emitted into air as a result of incomplete fuel combustion. (Source: GreenFacts)



Sulphate is a salt of sulphuric acid. It refers both to the SO42- anion and to any compound that contains this ion.


Sulphur dioxide (SO2)

A corrosive gas produced by the burning of fuels, such as coal and oil, that contain sulphur. It is also produced from sea spray, organic decomposition and volcanic eruptions.

When combined with water in the air, it produces a weak, corrosive sulfuric acid - an ingredient of "acid rain". (Source: GreenFacts)


The likelihood of producing a significantly larger-than-average response to a specified exposure to a substance.



When the combined effect of several forces operating is greater than the sum of the separate effects of the forces. (Source: Millennium Ecosystem Assessment  Glossary )

Threshold (in an ecosystem)

The level of magnitude of a system process at which sudden or rapid change occurs. (Source: Glossary of terms  )


Threshold (in health science)

The lowest dose of a chemical at which a specified measurable effect is observed and below which it is not observed.

(Source: National Safety Council Environmental Glossary   )


Time-series studies

A study in which periodic measurements are obtained prior to, during, and following the introduction of an intervention or treatment in order to reach conclusions about the effect of the intervention. (Source: The Evaluation Center Glossary of Program Evaluation Terms )


The capacity or property of a substance to cause adverse effects. (Source: GreenFacts)



The study of the harmful effects of substances on humans or animals. (Source: ATSDR Glossary of Terms  )

Transition metals

A (loosely defined) group of 38 elements with specific chemical properties.

Examples of transition metals include Iron (Fe), Zinc (Zn), Nickel (Ni), Copper (Cu), Silver (Ag), Manganese (Mn), Cobalt (Co), etc.

The name transition comes from their position in the periodic table (groups 3 to 12).

These elements are very hard with high melting points and high electrical conductivity. (Source: GreenFacts)



An expression of the degree to which a future condition (e.g., of an ecosystem) is unknown.

Uncertainty can result from lack of information or from disagreement about what is known or even knowable. It may have many types of sources, from quantifiable errors in the data to ambiguously defined terminology or uncertain projections of human behavior. Uncertainty can therefore be represented by quantitative measures (e.g., a range of values calculated by various models) or by qualitative statements (e.g., reflecting the judgment of a team of experts). (Source: Millennium Ecosystem Assessment  Glossary )

US Environment Protection Agency

The Environment Protection Agency's of the USA was founded in 1970. It's "mission is to protect human health and to safeguard the natural environment — air, water, and land — upon which life depends."

The EPA enforces federal environmental protection laws. It registers and regulates pesticides, enforces laws covering outdoor air and drinking water quality and regulates the disposal of hazardous and solid wastes.

It has now grown into a big and powerful administration: "18,000 people in Headquarters program offices, 10 regional offices, and 17 labs across the country, EPA employs a highly educated, technically trained staff, more than half of whom are engineers, scientists, and environmental protection specialists. A large number of employees are legal, public affairs, financial, and computer specialists."

U.S. EPA's scientific publications are widely recognized as reference materials. (Source: US EPA website  )

US National Institutes of Health

"The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research. Helping to lead the way toward important medical discoveries that improve people’s health and save lives, NIH scientists investigate ways to prevent disease as well as the causes, treatments, and even cures for common and rare diseases. Composed of 27 Institutes and Centers, the NIH provides leadership and financial support to researchers in every state and throughout the world." (Source: NIH website  )

Ventilation rate

The amount of air inhaled in a specified time period (e.g., per minute, per hour, per day, etc.); also called breathing rate and inhalation rate. (Source: OEHHA   Glossary and List of Acronyms )

Volatile organic compound

Any organic (carbon-containing) compound that evaporates readily to the atmosphere at room temperature.

VOCs contribute significantly to smog production and certain health problems.

VOCs often have odors, examples include gasoline, alcohol, and the solvents used in paints. (Source: GreenFacts)


Vulnerability (in health science)

The likelihood of being unusually severely affected by a substance either as a result of susceptibility to the effects of these substances or as a result of a greater than average [exposure]. (Source: WHO Europe  Answers to follow-up questions from CAFE )

WHO Air quality guidelines (AQG) in Europe

WHO Air Quality Guidelines for Europe aim to protect public health.

In general, the guidelines address single pollutants, whereas in real-life, exposure to mixtures of chemicals occur, with additive, synergistic or antagonistic effects. The guidelines are only tentative, although emissions should be reduced to the lowest achievable level. (Source: WHO Update and revision of the air quality guidelines for europe.  
 Air Quality Guidelines for Europe, Second Edition )

Compound Guideline value Averaging time
The WHO published a global update of these guidelines in 2005:
120 micrograms/ cubic meter
(0.06 ppm)

8 hours
Nitrogen dioxide
200 micrograms/ cubic meter
(0.11 ppm)

1 hour

40 to 50 micrograms/ cubic meter
(0.021 to 0.026 ppm)

Particulate matter
No guideline values were set for particulate matter because there is no evident threshold for effects on morbidity and mortality
World Health Organization

"The World Health Organization  (WHO) is the directing and coordinating authority on international health within the United Nations’ system. WHO experts produce health guidelines and standards, and help countries to address public health issues. WHO also supports and promotes health research. Through WHO, governments can jointly tackle global health problems and improve people’s well-being.

193 countries and two associate members are WHO’s membership. They meet every year at the World Health Assembly in Geneva to set policy for the Organization, approve the Organization’s budget, and every five years, to appoint the Director-General. Their work is supported by the 34-member Executive Board, which is elected by the Health Assembly. Six regional committees focus on health matters of a regional nature."

WHO's scientific publications are widely recognized as a reference source.

The WHO has a number of regional offices which address the specific issues of those regions.

WHO World Regional Offices
  WHO African Region  (46 countries)
  WHO European Region  (53 countries)
  WHO Eastern Mediterranean Region  (21 countries)
  WHO Region of the Americas  (35 countries)
  WHO South-East Asia Region  (11 countries)
  WHO Western Pacific Region  (27 countries)

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