# Methods for Determining Polycyclic Aromatic Hydrocarbons (PAHs) in Milk: A Review
## 2 Polycyclic Aromatic Hydrocarbons  
Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants commonly found in nature (Table 1). They are formed through the incomplete combustion of organic matter via two primary pathways: natural processes (such as volcanic eruptions, forest fires, and biological activity, categorized as pyrogenic or petrogenic) and anthropogenic/human activities (including energy production, agriculture, and industrial processing). In the literature, they are also referred to as polycyclic organic matter (POM), polynuclear aromatic hydrocarbons (PNAHs), polynuclear aromatics (PNAs), or polynuclear hydrocarbons [27]. Over 200 compounds have been identified, of which more than a dozen have been categorized by the EU and the US as requiring special attention due to their toxicity [27; 28]. These compounds rarely occur individually in nature; instead, they are typically present as complex mixtures [29]. Certain PAHs, such as benzo[a]anthracene, chrysene, benzo[b]fluoranthene, and benzo[a]pyrene, serve as marker compounds in tests designed to assess contamination levels in samples [30].  
Structurally, PAHs consist of two or more linked aromatic rings of carbon and hydrogen atoms. They are classified in several ways, including molecular weight: low-molecular-weight PAHs (LMW-PAHs) contain two or three aromatic rings, while high-molecular-weight PAHs (HMW-PAHs) have four or more rings. They can also be divided into alternant PAHs (composed entirely of six-carbon benzene rings) and non-alternant PAHs (featuring six-carbon benzene rings linked to rings with fewer than six carbon atoms) [31].  
Another standard classification distinguishes PAHs by their origin: petrogenic or pyrogenic. The first group is associated with the petroleum industry and other petroleum products. PAHs generated from petroleum have low molecular weight and two or three rings. The second group comes from incomplete combustion. Heat breaks down organic molecules during combustion, forming water and carbon dioxide. However, incomplete combustion produces more minor organic compounds that recombine into new substances, including PAHs. This group primarily includes PAHs that can be formed during food processing. Most pyrogenic sources of PAHs are complex compounds consisting mainly of four, five, and six rings [32; 33; 34].  
PAHs are colorless or pale yellow [27]. The atoms of carbon and hydrogen build them up and form benzene rings (at least 2), sharing a carbon atom. They are combined in linear, cluster, or angular arrangements [35]. Due to their intricate structure and relatively high molecular weight, PAHs have low volatility and are solid at room temperature. The widespread presence of PAHs exposes humans to them in various ways: externally (through the skin) and internally (inhalation, ingestion). According to [PERSON] [36], more than 70% of non-smokers' exposure to PAHs results from consuming contaminated products. [PERSON] [37] suggests that this figure could reach 88-98%, with only about 5% linked to plant materials [38].  
Low-molecular-weight polycyclic aromatic hydrocarbons (LMW PAHs) are volatile compounds with a relatively low toxicity. In contrast, high-molecular-weight PAHs (HMW PAHs) are more stable and exhibit higher toxicity due to their resistance to nucleophilic attack, attributed to the dense \(\pi\) electrons in their aromatic rings. As the molecular weight increases, solubility in water diminishes, while both melting and boiling points rise owing to enhanced lipophilicity (as indicated by the octanol-water partition coefficient). This increased lipophilicity also facilitates the bioaccumulation of these compounds within living organisms [27; 32; 33; 34].