These are the substances present in the atmosphere as they were delivered. Among these pollutants, the following substances, some of which are of particular importance:

Sulfur dioxide (SO2): emitted by certain industrial processes (especially in the paper industry or refining) and especially by the use of fossil sulfur fuels. It is one of the main contributors of acidic deposition due to its transformation in the atmosphere into sulfuric acid (H2 SO4);
Nitrogen oxides (NOx), in particular nitrogen dioxide (NO2), the emission of which mainly results from the combustion of fossil fuels, in particular by vehicles, which have the effect of contributing to the formation of ozone in the atmosphere;
Polycyclic Aromatic Hydrocarbons (PAHs), emitted by the incomplete combustion of fuel oils or coals and which are generally in the air, bound to particles. Some of them are recognized as very carcinogenic;
Volatile organic compounds (VOCs), which include hydrocarbons (including benzene, toluene and xylenes). They are issued by many sources, including various industrial processes and vehicles. Methane, a greenhouse gas that is rapidly increasing in concentration in the atmosphere, is a volatile organic compound.

They are substances whose presence in the atmosphere results from chemical transformations linked to the interaction of so-called precursor compounds.

Ozone is the main secondary pollutant. Its transformation results from a photochemical process in the presence of certain primary pollutants (carbon monoxide, nitrogen oxides and volatile organic compounds). It is a naturally occurring gas in the atmosphere at low concentrations and high altitude. At lower altitudes, on the other hand, the evolution of its concentration results essentially from human activities. Measurements are taken from 240 micrograms / m³ of air, alternating circulation is set up from 360ug / m³ of air (Decree 2003-1085).
Sulfuric acid and nitric acid which are formed in the atmosphere by the action of moisture from, respectively, sulfur dioxide and nitrogen oxide.


Fine particles. From a health point of view, these particles deserve special attention. Indeed, they may serve as vectors for other substances, such as carcinogenic polycyclic aromatic hydrocarbons (PAHs), which is particularly worrying given the capacity of the finest particles (less than 1um) to develop in the pulmonary alveoli, or even to enter the blood.
Effective solutions to combat this particulate pollution are now well known: the synthesis of the main standards and recommendations identified to date in the tertiary sector is clearly pointing towards filtration levels well above the requirements of the work code.