Polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons ( specialized claims;. Standard language polycyclic aromatic hydrocarbons, PAHs short or English PAH = polycyclic aromatic hydrocarbons ) are a group of substances of organic compounds consisting of at least two fused aromatic ring systems, which always lie in a plane. The simplest PAH is naphthalene, in which two benzene rings are fused via a common bond, one also speaks of fused ring systems. Fluorene is also a PAH, since the two rings are joined by an additional methylene unit to each other rigidly. No PAH is biphenyl, here the two benzene rings are not fused.

These ring-shaped hydrocarbons can also (often methyl groups) carry substituents. In an enlarged label, derivatives containing hetero atoms (primarily oxygen and nitrogen) in the form of aldehyde, keto, carboxy and nitro groups, as well as heteroaromatics to the PAHs are counted. This provides a great wealth of variants within the PAH; , several hundred compounds have been known.

Properties

PAHs are predominantly neutral, nonpolar solids. Many exhibit fluorescence. PAHs are very low in water; with increasing number of condensed rings take volatility and solubility ( in organic solvents ) from.

Many PAHs have been shown to be carcinogenic (cancer-causing ) because they epoxidized in the metabolism in the body ( oxidized to epoxides ) and may respond to these epoxides in a nucleophilic ring-opening reaction with DNA. This is not to be confused with the insertion of planar hydrophobic molecules between hydrogen-bonded base pairs of DNA ( intercalation ).

Because of the different toxicological and physico- chemical properties of a division into low molecular weight PAHs (2-3 rings) and higher molecular weight PAHs (4-6 rings) is useful.

PAHs are also aromatic when the number of π - electrons are not the Hückel rule for aromaticity [4n 2] π -electron equivalent.

Compounds

Naphthalene, a colorless solid, the easiest PAH, which consists of two fused benzene molecules. Other important PAHs are anthracene and benzopyrene. In addition, you count acenaphthylene, acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene, benzanthracene, coronene, ovals, tetracene, pentacene, and chrysene to this group of substances. In recent years it has been possible so-called " Superacene " to synthesize and characterize. These compounds consist of a variety of fused benzene units are very stable, have an extremely high melting point and are almost a precursor of the graphite dar.

Properties of various PAHs can be found in the list:

Occurrence

PAHs are a natural component of coal and petroleum. The in coke obtained from coal tar contains high levels of PAHs. Therefore, its use in road construction and eg as roofing banned since 1970. Treated with coal tar products, such as asphalt teergebundener from the period before 1970, tar paper or Teerimprägnierungen ( for telegraph poles or railway sleepers ), therefore, contain a lot of PAHs. Performs the distillation of crude oil gently by, arise only very small amounts of PAHs.

In gasoline and diesel fuel or heating oil can be found traces of PAHs. Also PAHs are present in tobacco smoke and smoked, grilled and roasted meats. PAHs can also accumulate in house dust on busy streets.

PAHs are an important component of interstellar matter and be detected by the methods of infrared astronomy in many areas of our Milky Way and other galaxies. The observed PAHs are excited mainly in the far UV, but emit in the infrared. That's why you can find PAHs in regions with strong UV radiation, such as in the star-forming regions of massive stars. A very successful tool for the detection of such PAHs was IRAC on board Spitzer. The 8 micron channel is dominated by PAH bands. Even through the "IRS" spectrograph of Spitzer, it was possible much already watched "Unidentified Infrared Bands " ( UIBs ) to identify as PAH emission band by ISO in the interstellar medium.

Formation

PAHs occur in the pyrolysis (incomplete combustion ) of organic material (eg coal, fuel oil, fuel, wood, tobacco ) and are therefore prove worldwide. The majority of the PAHs today comes from anthropogenic processes, but they can also be of natural origin (forest fires ). An important process in regard to the problem of contaminated sites is the production of coke and gas from coal. Waste products of coke ovens and former gas works (tar) are often the starting point of serious ground water contamination. PAHs are also formed by condensation reactions of humic acids. In nature are observed the production of biologically active PAC by microorganisms, fungi, plants and animals.

Use

Few individual PAH compounds are intentionally and are used as final or intermediate product use. Naphthalene is used in the chemical industry as intermediates principally azo dyes, insecticides, stabilizers, pharmaceuticals, cosmetic additives and plasticizers. It was used to a lesser extent than moth control agent. 1-methylnaphthalene used for the production of the phytohormone 1- naphthalene acetic acid. In the textile industry, a mixture of isomers of 1 - and 2-methylnaphthalene used as a solvent. Anthracene is an intermediate in the paint and plastic production. Some perylene derivatives are used as high-quality pigments.

PAHs are a natural part of extender oils based on mineral oil. These are, for example, in rubber products application. Tend to have black ( eg car tires, rubber grips of tools ) rubber products to a higher PAH content than light rubber products. This, however, depends heavily on the type of carbon black, or of its proportion in the rubber compound. In addition, it should be noted that the entrained with the soot PAHs are tightly bound to the soot surface and therefore only extracted with organic solvents (not practical! ) Can be.

Other sources include, for example, Solid hardwood floors, especially mosaic, on edge lamella and rod hardwood floors but also wood-blocks that were glued in the 1950s to 1970s, with tar or bitumen- based adhesives on cement or asphalt screeds, these adhesives are usually contaminated with PAHs.

PAHs as environmental pollutants

Because of their persistence, toxicity, and their ubiquitous distribution PAHs are of great importance as pollutants in the environment. Already in the 1980s, the U.S. federal Environmental Protection Agency ( U.S. EPA ) has received from the several hundred counting individual PAH compounds 16 substances on the list of "Priority Pollutants " (see table at right). These 16 " EPA PAHs " have since been mainly and representative of the whole group of substances analyzed.

PAHs enter mainly from the combustion of fossil fuels with the exhaust gases into the air. With the deposition they are registered in and on the ground, where PAHs are comprehensively demonstrated. Local importance as PAK issuers are contaminated, such as former gas works and coke ovens, or old deposits with PAH-containing waste ( eg ashes, waste oil).

Higher molecular weight PAHs with four or more rings are in the air and in the soil predominantly particle bound before. Low molecular weight PAHs with two and three rings are in the air in front of mainly gaseous, dissolved in the ground in seepage or groundwater.

PAHs in consumer products

The Federal Institute for Risk Assessment (BfR ) in collaboration with the CEC ( Central Experience Exchange Circle ) a PAH document (CEC 01.2-08 ) was developed for the investigation of materials for PAH. This ensures that all of these testing by leading laboratories use a standard test method for determining the PAH content.

The study on PAHs must from April 1, 2008 with the award of the GS mark (tested safety ) in accordance with a decision of the " Committee for Technical Work Equipment and Consumer Products ( AtAV ) " of 20 November 2007 mandatory for products according to the requirements of Clause .4.1 the PAH document CEC be applied 01-08. Products without GS mark can contain significantly more PAHs.

There is a guide for the maximum PAH concentration in consumer products, but no legal limit. The TÜV -Rheinland complained that therefore many manufacturers do not adhere to the recommended values. In a series of studies in March 2009, the TÜV found alarmingly high PAH levels in rubber products such as hammer stems, bicycle horn, beach sandals and watches. Here, the PAK is taken over the long skin contact.

From 1 January 2010, the use of plasticizers for the ( auto) tire production is only admissible if their content does not exceed the following limits:

Effect in humans

The inclusion of pollutants takes place through food and drinking water, by breathing the polluted air through the lungs (where car exhaust and tobacco smoke for the general population are most significant ) as well. Across the skin In children, the contaminant receptacle is particularly high.

PAK degrease the skin, leading to discomfort and can cause corneal damage and the respiratory tract, eyes and digestive tract irritation.

Some PAHs are clearly carcinogenic in humans (eg, lung, larynx, skin cancer and gastrointestinal cancer or bladder cancer). The possibility of fetal damage or impairment of reproductive performance is.

For example, benzo [ a] pyrene is among chimney sweeps for skin cancer blamed.

Biomonitoring in humans

Currently, the determination of 1- hydroxypyrene in the urine is the method of choice for the assessment of exposure to polycyclic aromatic hydrocarbons.

Proof

The reliable identification and quantification is performed by GC -MS after appropriate sample preparation. Results of available rapid test with PAH indicator strips should be protected by the aforementioned methods to prevent misinterpretation and to assess health and economic consequences reliable.