**2. Methodology**

In the present review, researches on PAHs are briefly summarised based on literatures. In this methodology, the physical and chemical properties and the most important components of PAHs are defined. For this purpose, the effects of PAHs, which have the environmental hazards, on feed and animal production are investigated. As a result, the potential risks of PAHs on human and animal health are summarised.

#### **3. Polycyclic aromatic hydrocarbons (PAHs)**

#### **3.1 The physical and chemical properties of PAHs**

Incomplete combustion does not produce a single type of PAH but results in the generation of a complex array of combustion products [5]. On the other hand, it is possible to produce PAHs as pure compounds for research purposes. PAHs,

which are produced in the form of pure compounds, are either colourless or of light yellow, white or green colour, have a light, pleasant odour and are found in solid state. Except for research purposes, these pure compounds have no further area of use [4].

PAHs, which have carcinogenic and mutagenic effects, are classified according to the number of benzene rings found in their structure. Those containing less than four benzene rings are classified as light PAHs, whilst those with more than five benzene rings are classified as heavy PAHs. Besides their classification, PAHs are also named after the number of benzene rings they possess, such that compounds with two benzene rings are referred to as naphthalene and those with three benzene rings are named as anthracene and phenanthrene. Compounds with a greater number of benzene rings have specific names [6, 9]. Light PAHs include naphthalene, acenaphthene, acenaphthalene, fluorene, anthracene and phenanthrene, whilst heavy PAHs include pyrene, fluoranthene, benzo[a]pyrene, chrysene, benzo[b] fluoranthene, indeno[1,2,3-cd]pyrene, benzo[k]fluoranthene, dibenzo[a,h] anthracene and benzo[g,h,i]perylene [4, 10]. The carcinogenicity and mutagenicity of heavy PAHs such as dibenzo[a,h]anthracene and benzo[a]pyrene are stronger than those of light PAHs [11].

The physical and chemical properties of polycyclic aromatic hydrocarbons vary with the molecular weight of the compounds [2]. Increased molecular weight is associated with reduced water solubility. PAHs with a high molecular weight are capable of enduring without evaporating, given their low solubility and volatility. Lowly volatile PAHs become even less volatile with an increase in the number of benzene rings in their structure. An increase in the molecular weight of PAHs is associated with higher boiling and melting points and lower vapour pressure. The majority of PAHs have a boiling point above 300°C and a melting point below 250°C [4, 12].

While more than a hundred PAHs have been detected in nature, among these, only 16 (**Table 1**) have been described as primary pollutants, in view of their greater toxicity and carcinogenicity [13]. This description of the primary pollutants was made by the US National Priority List (NPL) and is based on the extensive information available on these compounds, the more serious side effects, the greater residual risks, and the higher levels of detectability in hazardous dumpsite analysis [4].
