Organic Toxics In The Air

Volatile organic compounds (VOCs) are one of the main components of air pollution, it is a group that covers a wide range of chemicals. Its main physical characteristic is that VOCs are molecules whose properties allow them to evaporate under “normal” atmospheric conditions of temperature and pressure; other compounds of this type can have boiling points from less than 0 ° C to about 400 ° C, when this temperature is found in a process or the environment, the chemicals become part of the gaseous mixture in the air. These chemical compounds differ from other air pollutants, initially, since they are molecules that have no charge (they are non-ionics), they are not metals, they have Carbon-Hydrogen bonds and they do not usually form particles because they are individual molecules in the gaseous state. 

Image 1: The resin of conifers contains gaseous molecules as odors.

VOCs are present in both outdoor and indoor air, several hundred of these organic compounds have been found in all environments. Regarding VOC emissions from industrial processes, the EPA reported that the activities with the highest emissions of VOCs to the atmosphere were the production of oil and gas, from these sources are produced Hydrocarbons and other VOCs, Carbon Monoxide and Nitrogen; with a total of more than 3 million tons produced per year, followed by storage and transportation, they generate both air and land pollution. VOCs are too many different molecules, but they can be classified into a few groups to understand their main similarities, which are described below along with some examples of molecules that fit into each category:

Aromatic and Chain Hydrocarbons: They include several chemical families, note that they only have Carbon (C) and Hydrogen (H) atoms. Image 1 shows the chain hydrocarbons and their linear structures, which have defined limits and do not form cycles; propane contains only single bonds (alkane), ethene has one double bond (alkene), and 2-butyne has one triple bond (alkyne).

Image 1: Chemical structures of chain hydrocarbons.

In Image 2 we find the aromatic hydrocarbons, those are the molecules formed by Carbon and Hydrogen that form cycles and have double bonds around the cycle (called aromaticity or double ring). The molecules may present ramifications and a cycle as the nucleus of the molecule, or they can be they can present as several linked cycles. In figure 2 we observe Benzene, that is the basic aromatic hydrocarbon; a cyclic hydrocarbon that contains double ring or conjugated double bonds. Toluene shows how these molecules may have ramifications around their main cyclic nucleus. Finally, Naphthalene is part of the quite dangerous classification called polycyclic aromatic hydrocarbons (PAHs) that have more than one cycle in their structure, this type of compound is derived from oil, gasoline, and combustion reactions.

Image 2: Chemical structures of Aromatic hydrocarbons.

Terpenes: The Terpenes are a large and diverse class of natural products derived from plants. They are synthesized within plant cells according to their metabolism or by the industries for economic purposes. All terpenoids come from the same plant precursors; the primary substrates of photosynthesis, form the key molecules for the synthesis of terpenes (caused by the enzyme isoprene synthase). These compounds are part of the plant's own defenses, metabolism, and growth, among others; many of these compounds are characterized by having olfactory properties such as peppermint and lavender. They have the ability to evaporate easily into the air, many of these have pharmacological effects that could intervene in people's health, some of these are even used as medicines. In Image 3 we observe the chemical structures of some Terpenes. Isoprene is produced by various plants, animals, and even humans, although it is actually toxic in inhaled concentrations; It is the second most abundant endogenous compound in the human breath, the concentration mostly varies between ~ 100-300 ppb in adults. Pinene, as the name suggests, shapes are important components of the pine resin aroma and Limonene is one of the main components of citrus fragrance, peel, and oil.

Image 3: Chemical structures of Terpenes.

Oxygenated VOCS: Refer to organic compounds with at least one oxygen atom. They are relatively soluble compounds in water. They can be classified, according to their chemical functions, into aldehydes (such as formaldehyde, acetaldehyde, and acetone), alcohols (such as propanol and butanol), carboxylic acids (formic acid, acetic acid, for example), organic hydroperoxide, etc. Therefore, an intrinsic reactivity is observed for these compounds, which makes them very toxic for humans; Its production occurs mainly in industry and chemical products. In Image 3, formic acid has been produced on a large scale for industrial use around the world. Ethanol is found in alcoholic beverages and other processes, followed by Acetone, widely used as a solvent.

Image 4: Chemical structures of Oxygenated VOCs.

Others: There are other families of organic compounds that have other atoms in their structure in addition to Carbon, Hydrogen, and Oxigen. Some examples are organosulfates, organonitrates, and organohalides or chemicals such as insecticides, fungicides or herbicides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAH), and peroxyacyl nitrates (PAN). Its properties are diverse and complex due to its chemical structure obtained usually by synthetic methods. In Image 5, we see the Organosulfate nucleus (OR indicates various types of ramifications) used in pesticides; also Acetyl chloride, derived from acetic acid, highly toxic and highly produced on an industrial scale

Image 5: Chemical structures of other types of VOCs.

Volatile organic compounds including hydrocarbons and the other classifications mentioned above are mainly emitted to the atmosphere by humans, followed by natural sources. They are also transformed in chemical reactions, specifically photooxidation leading to the formation of ozone (O3) and secondary organic aerosol (SOA). By altering the organic fraction of the particles in the atmosphere, VOCs modify the Earth's balance by a direct effect of absorption and dispersion of solar radiation or indirect effect by altering the clouds with their chemical properties; they also have a direct effect on human health and the environment. For example, in several cities of Mexico, it has been found ethanol or ethyl alcohol as the greatest air pollutant of this class of compounds, is contained in intoxicating beverages; prevalence probably due to the industrial environment managed in that country. The sources of these are very diverse, as we mentioned earlier, their significant polluting production occurs at an industrial level, for use as raw material, or for the production of these molecules. Other sources include: 

• A dishwasher that produces chlorinated hydrocarbons and Ethanol.
• Driving produces aromatics chlorinated hydrocarbons (for example, Tetrachloroethylene).
• Dry cleaning household and consumer products produce a large variety of volatile organic compounds including fragrances. 
• Paint produces Alkanes, Glycols, Glycol Ethers, Texan.
• Personal care products are the source of Siloxanes, fragrances, Ethylene Glycol Butylether, and more.
• The shampoo contains showering chlorinated hydrocarbons.
• Social activities produce Ethanol, smoke, terpenes, and dodecane.
• Smoked tobacco produces Aliphatic hydrocarbons and aldehydes.
• Cera contains Benzene, Styrene, 3-vinyl pyridine, 2- and 3-picolines, etc.

Due to their chemical nature, the VOCs are one of the most toxic and dangerous group of chemicals that compose air pollution; this group contains a large number of different molecules, and their sources are very diverse. Scientific studies have shown great ability to alter health causing death (when inhaling large amounts), permanent damage, neurotoxicity, cancer, teratogenicity, damage to the heart and cardiovascular system, among others. For this reason, it is important not to dispose of these types of chemical products in conventional garbage, and not to spread them in the air or waters as waste.

References

• Miao Wang. Study of Volatile Organic Compounds (VOC) in the cloudy atmosphere : air/droplet partitioning of VOC. Earth Sciences. Université Clermont Auvergne, 2019. English. ⟨NNT : 2019CLFAC080⟩.

• Wolkoff, Peder. (1995). Volatile Organic Compounds Sources, Measurements, Emissions, and the Impact on Indoor Air Quality. Indoor Air. 5. 5 - 73. 10.1111/j.1600-0668.1995.tb00017.x. 

• Montero-Montoya, Regina et al. “Volatile Organic Compounds in Air: Sources, Distribution, Exposure and Associated Illnesses in Children.” Annals of global health vol. 84,2 225-238. 27 Jul. 2018, doi:10.29024/aogh.910.