On an industrial site, Volatile Organic Compounds, VOCs, are among the ambient air pollutants, both indoor and outdoor. The term VOC encompasses a multitude of chemical substances.
What do they have in common? They are composed of carbon and hydrogen, and under ambient conditions, they are either in a gaseous phase or in a liquid phase that will easily evaporate. VOCs impact air quality with consequences for human health, the environment, and the economy. They are therefore subject to an official definition at the European level, which is then transposed nationally. Understanding the physicochemical properties of VOCs helps in taking appropriate preventive measures. Here is a quick overview that will familiarize you with the physical and chemical characteristics of VOCs that impact air quality.
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Regulatory definition of VOCs
Article 2, §16 and §17 of Council Directive 1999/13/EC of March 11, 1999 on ” the limitation of emissions of volatile organic compounds due to theuse of organic solvents in certain activities and installations ” gives the following two definitions:
Organic compound: “any compound which, excluding methane, contains carbon and hydrogen, which can be substituted by other atoms such as halogens (e.g., fluorine, chlorine, bromine, iodine), oxygen, sulfur, nitrogen, or phosphorus, with the exception of carbon oxides (e.g., CO2) and carbonates (e.g.: CO32-) and bicarbonates(e.g. HCO3–) . ”
Volatile organic compound (VOC) : any organic compound having a vapour pressure of 0.01 kPa or more at a temperature of 293.15 K(20°C) or having a corresponding volatility under the particular conditions of use.
The directive has been transposed into French law in the Environmental Code, article R224-48. It defines a VOC as “any organic compound whose initial boiling point, measured at a standard pressure of 101.3 kPa, is less than or equal to 250°C.”
Volatile? Yes, but to varying degrees! Physical Classification of VOCs
To understand the extent of VOC emissions degrading ambient air quality, we seek to determine their concentration. To do this, we rely on a physical characteristic: volatility.
Volatility is the ability of a substance to evaporate at ambient temperature and pressure.
VOCs are volatile, but to varying degrees. Since the volatility of a VOC depends on its vapor pressure, the effective saturation concentration ???????? (in µg.m-3) can be used as a classification criterion. Furthermore, volatility decreases as the molecular weight of the VOC increases. VOCs can also be classified according to the number of carbon atoms in their structure.
| volatility to non-volatility | no. of carbon atoms | effective saturation concentration ???????? |
| VOC Very volatile to volatile | nb C ≤ 11 | ???????? > 106 μg .m-3 |
| VOC-IVolatility Intermediate | 12 ≤ nb C ≤ 18 | 103 μg.m-3 <???????? ≤106 μg.m-3 |
| COSVSemi Volatil | 18 < nb C ≤ 32 | 10-1 μg.m-3 <???????? ≤103 μg.m-3 |
| CONV Non-volatile at room temperature (particle) | nb C > 32 | ???????? < 10-1 μg.m-3 |
Note that non-volatile organic compounds at room temperature (CONV) will evaporate under specific conditions of use, linked to industrial processes, or in the event of an accident (fire, explosion).
Another classification of VOCs takes boiling temperature as a criterion.
| Volatility | Boiling temperature |
| Highly volatile | < (50 – 100 °C) |
| Volatile | (50 – 100 °C) to (240 – 260 °C) |
| Semi-volatile | (240 – 260 °C) to (380 – 400 °C) |
VOCs and odors
Some VOCs are odorless (butane, propane). Other VOCs may have more or less characteristic odors. Sulfur compounds, amines, oxygenated compounds (ketones, aldehydes), and certain aromatic compounds are particularly odorous.
Classification of VOCs by chemical structure
VOCs constitute a vast class of chemical compounds, exhibiting a wide diversity of structures and properties. It is their common impact as air, water, and soil pollutants that groups them into this class. However, their structures, particularly the presence of atomic groups other than C and H, influence their chemical properties, and thus on the specific nature of their toxicity for humans and nature; and consequently, on the resulting economic effects.
VOC structural criteria
VOCs are distinguished according to several non-exclusive structural criteria, each contributing to the nature and degree of their polluting properties:
- cyclic(chain of carbon atoms closing in a circle) versus non-cyclic(non-closed carbon chain, C atoms link together in a linear fashion);
- aromatic(a specific hexagonal carbon chain, called a benzene ring, composed of 6 carbon atoms each linked to a hydrogen atom) versus non-aromatic(a chain without this special configuration);
- monocyclic(a single cyclic carbon chain) versus polycyclic(several identical rings linked by one or two carbon atoms in common)
- homocyclic(a ring formed solely of C) versus heterocyclic(a ring containing carbon and other carbon-substituting atoms);
- saturated (presence only of single carbon bonds) versus unsaturated (double or triple carbon bonds). Unsaturation confers increased reactivity to VOCs, impacting their toxicity. Aromatic structures are all unsaturated.
- unbranched versus branched(the main carbon chain has one or more branches formed either from groups of C and H atoms alone, or incorporating other atoms which then give them a characteristic reactivity [functional group]).
VOC classification
There are two main categories of VOCs: aromatic VOCs and aliphatic (= non-aromatic) VOCs.
Aromatic VOCs have a benzene ring (= benzene ring) as their basic skeleton. They are divided into several subcategories:
- Monocyclic Aromatic Hydrocarbons, including BTEX (short for Benzene, Toluene, Ethylbenzene, Xylene), all of which are toxic and ecotoxic.
- Polycyclic Aromatic Hydrocarbons (PAHs) with carcinogenic properties. They are synthesized during the formation of fossil fuels (petroleum, coal) or during the incomplete combustion of organic matter (fuel heating, forest fires, etc.).
- aromatic heterocyclic compounds, or aromatic heterocycles, in which one or more C atoms of the benzene ring are substituted by other atoms (or groups of atoms) such as those listed in the above definition.
Aliphatic VOCs (= non-aromatic VOCs) include molecules with :
- saturated: alkanes; example: hexane C6H14, which is used in glues, adhesives, degreasing liquids, and is present in gasoline vapors. It can enter the body through respiratory and percutaneous routes. Its effects by inhalation range from dizziness to loss of consciousness. Skin contact causes dermatitis.
- insaturée aux propriétés plus polluantes :
- alkenes (double carbon bond); example: ethylene C2H4, released by most fruits and vegetables as a ripening agent, emitted by exhaust pipes, propane forklifts, and plastic bags under the action of light. By inhalation, it can cause dizziness, headache, loss of consciousness, and contributes to the greenhouse effect;
- alkynes (triple carbon bond); example: ethyne or acetyleneC2H2; extremely flammable and explosive, it is used as a fuel for welding or in certain analysis equipment.
Alkanes, alkenes and alkynes also incorporate :
- non-cycliques, les aliphatiques acycliques, qui sont constituées de chaînes :
- either linear. Example: n-hexane.
- or branched.
- cycliques (cycles non-aromatiques), les alicycliques (= aliphatiques cycliques) : cycloalcanes, cycloalcènes, cycloalcynes.
- Molecules can comprise several cycles
- either linked by 2 common carbon atoms; example: polycyclic cycloalkanes
- or linked by 1 common carbon atom: spirans.
- The ring can include atoms other than carbon (heterocycle)
- A ring can bear a branch. For example, methylcyclohexane (C6H11CH3), used as a base for organic synthesis, a solvent for ethers and cellulose, and aviation fuel. It can irritate the respiratory tract, central nervous system, skin, and eyes.
- Molecules can comprise several cycles
If the aliphatic or aromatic structure has a branch identified as a functional group (a group of atoms that gives it distinctive chemical properties), such as certain alkane derivatives or some PAHs, for example, the VOC will specifically impact air quality. VOCs are then classified into a particular family:
- Halogenated VOCs, e.g. chloromethane CH₃Cl
- Sulfur VOCs, e.g., β-mercaptoethanol C2H6OS,
- COV oxygénés dont des :
- Alcohol VOCs, e.g., ethylene glycol C2H6O2; used as antifreeze, solvent, brake fluids, dyes… Its inhalation causes cough and headache, its ingestion causes abdominal pain, nausea
- Ketone VOCs, e.g., acetone C3H6O, a solvent used in the paint, lacquer, rubber, and plastics industries… Highly volatile, it can be inhaled in large quantities when its concentration in the air is high. It can enter the bloodstream via the lungs and spread throughout the body. Symptoms range from nasal irritation to central nervous system depression.
- VOC aldehydes, e.g. CH2Oformaldehyde, emitted to a greater or lesser extent in all industrial sectors and recognized as a carcinogen.
- Ether VOCs, e.g., ethylene glycol n-butyl ether (EGBE) C6H14O2,
- Ester VOCs, e.g., methyl acetate C3H6O2.
- Nitrated VOCs, e.g., nitroethane C2H5NO2, irritating to the respiratory tract, it can alter blood, and cause convulsions.
- Amino VOCs, e.g. aniline, which adsorb easily to work clothes, walls, machines and worktops.
Several thousand substances meet the definition of VOCs. They affect air quality in all industrial sectors. Understanding their nature helps to comprehend how they react with gases and dust present in the air of an industrial site. Appropriate preventive measures will thus be taken based on their chemical composition, particularly regarding source capture, filtration, and treatment.
