Urban pollution and air pollutants

The air and pollutants

Air is the first of the elements necessary for life. Every day, we breathe about 14 kg of air, or 11 liters.

Humans introduce substances into the atmosphere with harmful consequences for health and the environment. These substances are emitted by stationary and mobile sources: boilers, industrial, domestic and agricultural activities, road transport of people and goods, etc.

Pollutants are dispersed by winds, dissolved by rains, or blocked when the atmosphere is stable.

The normal chemical composition of air is: nitrogen 78%, oxygen 21%, argon 0,9 and other gases 0,1%


The air we breathe can contain hundreds of pollutants in gaseous, liquid or solid form. The following pollutants are considered as indicators of pollution and are therefore subject to regulation.

The origin of the main pollutants

Sulphur dioxide (SO2)

This gas essentially comes from the combination of sulfur, contained in fossil fuels (coal, fuel oil, poor quality diesel, etc.) with the oxygen in the air during their combustion. Industries and heating installations are the main emitters.

Nitrogen oxide (NO, NO2)

They result from the reaction of nitrogen and oxygen in the air which takes place at high temperature in engines and combustion plants. Vehicles emit most of this pollution; then come the heating systems.

suspended particles (PM10 and PM2,5)

This is dust with a diameter of less than 10 µm or 2,5 µm and which remains suspended in the air. They result from combustion, vehicle wear and tear on the roadway and erosion. This dust can also carry other pollutants such as heavy metals and hydrocarbons. The main emitters are diesel vehicles, incinerators, cement plants and some industries.

PM2,5 are particularly dangerous because they pass faster through the body while PM10 is already more visible but above all more easily stopped by the mucous membranes.

More: the fine particles

Carbon monoxide (CO)

It results from the incomplete combustion of fuels and fuels. In ambient air, it is mainly found near road traffic lanes.
Especially from gasoline vehicles: recent cold engine, small engine (eg gardening) and uncatalyzed old vehicles still passing technical inspection.

Volatile organic compounds (VOCs)

They are multiple, they are mainly hydrocarbons whose origin is either natural or linked to human activity: road transport, industrial or domestic use of solvents, evaporation from oil storage and reservoirs. automobiles, and combustion.

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Polycyclic aromatic hydrocarbons (PAHs)

These are compounds whose molecule is cyclical, highly toxic and peristants.
They consist of carbon atoms and hydrogen molecules whose structure comprises at least two fused aromatic rings. They are part of POPs (see below)

“Pyrolytic” PAHs are generated by incomplete combustion processes of organic matter at high temperature. The mechanisms brought into play during their formation involve the production of free radicals by pyrolysis at high temperature (≥ 500 ° C) of fossil material (petroleum, fuel oil, organic matter, etc.) under oxygen-deficient conditions. PAHs of pyrolytic origin come from the combustion of automobile fuel, domestic combustion (coal, wood), industrial production (steelworks), energy production (power stations running on petroleum or coal, etc.) or more incinerators.

Persistent organic pollutants (POPs)

Persistent organic pollutants (POPs) are a family of pollutant but rather a classification that encompasses several families.
Thus they are molecules defined by the following properties:
- Toxicity: they have one or more proven harmful impacts on human health and the environment.
- Persistence in the environment: these are molecules which resist natural biological degradation.
- Bioaccumulation: molecules accumulate in living tissues and concentrations therefore increase along the food chain.
- Long-distance transport: by virtue of their persistence and bioaccumulation properties, these molecules tend to travel very long distances and deposit far from places of emission, typically hot environments (with strong human activity) towards environments cold (especially the Arctic).

Example of POPs: dioxins, furans, PCBs, Chlordecone ...

Metals (Pb, As, Ni, Hg, Cd ...)

This term encompasses all the metals present in the atmosphere. The main ones with a toxic character are: lead (Pb), cadmium (Cd), arsenic (As), nickel (Ni), mercury (Hg). In the air, they are mainly found in particulate form. Most of them come from road traffic, the steel industries and waste incinerators.

Ozone (O3)

This gas is the product of the photochemical reaction of certain pollutants, in particular nitrogen oxides (NOX) and volatile organic compounds (VOCs), under the effect of solar radiation. This pollutant has the particularity of not being emitted directly by a source; it is a secondary pollutant. It is mainly found in summer, on the outskirts of towns.

The effects of pollution

They are numerous and must be studied on a case by case basis! Of all the environments with which man is in contact, air is the only one from which he cannot escape: it is indeed necessary to breathe in order to live.

The effects of air pollution depend on the amount of pollutant with which the organism is in contact; we speak of "dose". This dose varies depending on 3 factors:

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- The concentration of pollutants in the atmosphere,
- The duration of the exhibition,
- The intensity of physical activity,

The disorders occur mainly in susceptible individuals that are:
- Children,
- Old people,
- Asthmatics,
- The respiratory insufficiency,
- Cardiacs,
- Chronic bronchitis,
- Smokers,
- Pregnant women,
- Professionals in contact with chemicals (garage owners, building trades, industry agents, etc.).

Health Impacts

Depending on the nature of the pollutants, the health consequences are different, even if the various harmful constituents often act synergistically.

The effects on human health of some pollutants

Sulphur dioxide (SO2)

It is an irritating gas. It causes an alteration of pulmonary function in children and an exacerbation of acute respiratory symptoms in adults (cough, respiratory discomfort, etc.).
People with asthma are particularly susceptible.

Nitrogen oxide (NO, NO2)

It is an irritant gas that penetrates the finer branches of the respiratory tract, causing bronchial hyperresponsiveness in asthma patients and increased sensitivity of the bronchi to infections in children.

suspended particles (PM10)

Larger particles are retained by the upper respiratory tract. They are therefore less harmful to health than the finer PM2,5 particles (<10 µm in diameter) which penetrate deeper into the body.
They then irritate the lower respiratory tract and alter respiratory and even, ultimately, cardiovascular function.

Some, by nature, also have mutagenic and carcinogenic properties.

Carbon monoxide (CO)

Deadly gas. It binds instead of oxygen to the hemoglobin in the blood leading to a lack of oxygenation of the nervous system, heart and blood vessels. The central nervous system and sensory organs are the first to be affected, causing headache, dizziness, asthenia or sensory disturbances. In case of very high and prolonged exposure, it can be fatal or leave irreversible neuropsychic sequelae.

Volatile Organic Compounds (VOCs) including benzene

These molecules have very different effects depending on their family. Simple olfactory discomfort (odors), some cause irritation (aldehydes), or even a decrease in respiratory capacity. Others, like benzene, cause mutagenic and carcinogenic effects.

Metals (Pb, As, Ni, Hg, Cd ...)

These different elements accumulate in the body, which entails a risk of long-term toxicity involving possible carcinogenic properties.

Ozone (O3)

This gas, very oxidizing, easily penetrates to the finest respiratory tracts. It causes coughing and lung damage, especially in children and asthmatics, as well as eye irritation.

Environmental impact

In the long term the effects on the environment can take place with concentrations lower than those harmful to humans.

The most visible consequences are often blackening of buildings and monuments, including the cost of restoration is often very heavy.
Nitrogen oxides and sulfur dioxide strongly contribute to the phenomenon of acid rain which destroys the natural environment as well as construction materials.

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The most oxidizing pollutants (ozone) reduce the photosynthetic activity of plants, which is visible in the appearance of spots (necrosis) on the surface of the leaves of the most sensitive plants. This causes growth slowdowns in plants. Reductions in agricultural yield have even been observed.

The influence of weather on pollution

Pollutants are dispersed by winds, dissolved by rains, or blocked when the atmosphere is stable.

Thus, the high pressure periods characterized by calm weather, with a weak wind, sometimes accompanied by a temperature inversion in winter, contribute to a rapid increase in the concentration of pollutants at ground level.
Under normal conditions, the air temperature decreases with altitude. The warm air containing the pollutants tends to rise naturally. The pollutants disperse vertically.

In a temperature inversion situation, the ground cooled significantly overnight (eg, winter on a clear day). The temperature at an altitude of a few hundred meters is then higher than that measured at ground level. The pollutants are thus trapped under a "cover" of hot air, called the inversion layer.

The SFX index

The ATMO index was designed, at the initiative of the Ministry of Regional Planning and the Environment, in order to qualify the air quality of a homogeneous urban unit.

This index is representative of the background urban air pollution of an agglomeration, felt by the majority of its inhabitants. It is calculated over a day (from 0 h to 24 h). In order to provide information as quickly as possible, a partial index is calculated at the end of the day with the values ​​measured up to 16 p.m.

It does not make it possible to highlight particular or localized pollution phenomena, of proximity for example. It is a synthetic figure of the state of the air, associated with a qualifier:

very good 1
very good 2
3 bon
4 bon
Means 5
poor 6
poor 7
8 bad
9 bad
10 bad

Four pollutants are used to construct the ATMO index: sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3) and suspended particles (PM10).

These chemical species are considered as indicators of air pollution.

For each of these pollutants, a sub-index is determined with reference to a correlation table where a value is assigned to each concentration range. The final index is the sub-index which is the largest.

Example of measures:
Subindex SO2 = 1
Subindex PM10 = 2
Subindex O3 = 5
Subindex NO2 = 2
SFX index = 5

Learn more

- The dead of pollution in France
- urban and alternative transport Study on pollution

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