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Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons


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Polycyclic aromatic hydrocarbons are enemies of the environment and human and animal health, they have been shown to be toxic to numerous aquatic organisms and also to certain birds, they are able tocontaminate entire cropsagricultural. From the series: “if you know them, you avoid them” and above all, we avoid producing them.

Polycyclic aromatic hydrocarbons: what they are

It is about hydrocarbons characterized by two or more aromatic rings such as that of benzene, fused together which, in this way, are able to form a single structure, usually planar. Like other hydrocarbons, they do not contain heteroatoms in the cycle or substituents.

In most cases they are found in the solid state, those with low molecular weight generally sublimate at room temperature, which happens for example for naphthalenes. In water, polycyclic aromatic hydrocarbons are not soluble at all, at most they are relatively poorly soluble but highly lipophilic and we owe theirs to this last characteristic propensity to bioaccumulate.

We mentioned them molecular structure which instead strongly binds their stability in the atmosphere to ultraviolet solar radiation. THE polycyclic aromatic hydrocarbons larger, less soluble in water and less volatile, they reside in the soil or in oily sediments more than in water or air, even if that modest percentage found in the air constitutes a worrying component when we analyze atmospheric particulate matter. Heavy polycyclic aromatic hydrocarbons can stay in the air because they adhere superficially to the particles suspended in the air.

Polycyclic aromatic hydrocarbons: list

It is long and rather difficult to read for those who do not chew a lot of chemistry, what is important is to know that these Polycyclic aromatic hydrocarbons in nature are found in hard coal and in oil and it is in fact from these two that we extract them. The simplest example of polycyclic aromatic hydrocarbon is naphthalene, those with high molecular weight, such as benzo [and] pyrene and benzo [a] pyrene, are present in high quantities in tars, bitumen, pitch and coals, as well as in related products such as asphalts.

There are also polycyclic aromatic hydrocarbons which derive from carbon black and wood soot. Then there are the lower molecular weight polycyclic aromatic hydrocarbons, such as naphthalene and also fluorene: they are considered ubiquitous pollutants, they are more soluble in water so they are able to reach and contaminate groundwater.

Polycyclic aromatic hydrocarbons: carcinogenic

The alert is high, on polycyclic aromatic hydrocarbons, because some of them have been identified as carcinogenic, mutagenic and teratogenic. There IARC defines them as "probable" or "possible human carcinogens", Benzo (a) pyrene has recently even been reclassified in the group of" human carcinogens ". The most present in the environment are benzo (a) pyrene, benzo (b) fluoranthene, benzo (k) fluoranthene, indene (1,2,3-c, d) pyrene, benzo (a) anthracene, benzo (j) fluoranthene and dibenzo (a, h) anthracene.

In addition to being carcinogenic, they are also very powerful air pollutants and are formed mainly due to anthropic causes, during incomplete combustion of fossil fuels, wood, fats, foliage, incense and organic compounds in general. Therefore, they are produced when urban waste and biomass are burned, we find them in tobacco smoke and also in cooked and slightly "scorched" foods grilled.

IPA polycyclic aromatic hydrocarbons

We will hear more often about PAHs and polycyclic aromatic hydrocarbons, in international contexts, also of PAH. Whatever language you want to call them, these polycyclic hydrocarbons have planar molecules generally made up of a group of sp2 hybridized carbon atoms, linked together in rings that have at least two adjacent carbon atoms in common. Each of the latter has its own pz orbital occupied by an unpaired electron and thus forming a aromatic system in which 4n + 2 delocalized electrons are shared on the entire part of the molecule affected by the phenomenon.

Polycyclic aromatic hydrocarbons: limit values

Whether you understand what has just been said or not, what must be clear even to those who do not even want to hear Chemistry is that there are levels to be respected as regards the amount of polycyclic aromatic hydrocarbons found in the environment.

Years ago the Ministerial Decree of 25/11/1994 had set an annual average daily value of 1 ng / mc with reference to benzo (a) pyrene which is used as a marker for the carcinogenic risk of polycyclic aromatic hydrocarbons in ambient air.

In September 2010, Legislative Decree no.155 of August 13, 2010 came into force and replaced the previous directive on IPAs. Regarding limit values ​​in work environments, the National Institute for Occupational Safety and Health (NIOSH) recommends that the levels in the air of volatile tar products be less than 0.1 mg / mc for a 10-hour working day within 40 hours of work per week.

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Video: 1 polynuclear aromatic hydrocarbons (July 2022).


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