A group of chemical compounds composed of multiple aromatic or benzenoid (benzene-like) closed rings containing only carbon and hydrogen are called polycyclic aromatic compounds (PACs) or also known as polycyclic aromatic hydrocarbons (PAHs). PACs are a main subgroup of the aromatic hydrocarbons and are categorized under hydrocarbons. The simplest examples of such chemical compounds that have 2 aromatic rings – the naphthalene and biphenyls, and have three aromatic rings – anthracene and the phenanthrene.
Polycyclic aromatic compounds (PACs) are also known as â€œpolynuclear aromatic hydrocarbonsâ€ that contain many fused-ring molecules that share single or multiple sides and consists of delocalized electrons. PACs that consist of atoms other than carbon or hydrogen-like oxygen, sulfur, or nitrogen inside the ring structure are called Heterocyclic PACs. Some examples are oxy-PAHs, nitro-PAHs, and alkyl-PAHs.
PACs are environmental pollutants either occurring naturally or synthesized as a single chemical or it follows complex mixtures. Although the main emphasis is avoiding contact with PACs, the molecules are also useful for making dyes, medicines, pesticides, and plastics. The earliest forms of life require PACs for abiotic synthesis of materials because they are estimated to be the tentative starting materials. Medicine and pesticide industries use manufactured PACs. Urban air has 10 times more PAC levels than rural air. Lower PAH levels are found in the drinking water supplies in some parts of America. The US (United States) commercially produce naphthalene for making other chemicals and mothballs.
PAHs are lipophilic, non-polar, and uncharged organic molecules having distinctive properties because of the delocalized electrons present in aromatic rings. They are generated through various natural or anthropogenic reactions. Generally, they are environmentally persistent because of the low solubility in water. Large PAHs are insoluble in an aqueous medium while smaller PAHs having 2- or 3-ring shows solubility in an aqueous solution that logarithmically decreases as the molecular mass increases. Large PAHs too are less soluble in lipids and organic solvents. Large PAHs exist in the solid-state while smaller PAHs with 2-, 3- and 4-ring exists in the gaseous state and are sufficiently volatile. Larger PAHs are strongly colored except pure solid PAHs that may be colorless, white, pale yellow, or green.
The two main sources of PACs are natural and man-made. PACs originated from these sources shows binding to or formation of small particles in the air. Natural PAC containing substances are fossil fuels like charcoal, coal, crude oil or oil deposits, dung, garbage or trash, gasoline, petroleum, tobacco, wood, etc. PACs are also released in the environment through naturally occurring events like fire in the forest or volcanic eruptions, etc. Some examples are engines and incinerators or when biomass is burned in forest fires. The elevated levels of PACs are found at some former coal-gasification sites. When the burning process is less efficient then more PACs are given off in the surrounding environment.
Humans are usually exposed to a complex mixture of PACs through breathing polluted air & water, and absorption to skin from coming in contact with contaminated food or tainted soil. For example – cigarette or tobacco smoke, coal tar, emissions from fossil fuels, forest fires, fumes from asphalt roads, motor vehicle dust & smoke, secondhand smoke, wildfire, wood smoke, and volcanoes. Energy production, industrial processes, transportation, and burning of open trash are few other sources of exposure. Many cosmetics and shampoos contain PACs because they are made up of coal tar. PAHs formation in chargrilled meat, smoked fish, and other foods is possible because of high-temperature cooking or settling of PAH particles in the air. Chargrilled meat and smoked fish have higher PACs while other foods like fruits, grains, refined vegetable oil, roasted coffee, roasted peanuts, and vegetables have low levels of PACs.
PAHs are disease-causing contaminants that are widely spread throughout the environment which includes air, soil, water, etc. They are environmentally persistent for months or years without breaking down easily. Thus, leads to ever-growing problems over time because of their persistent, bioaccumulative, and toxic nature. Human health gets affected by low-level environmental exposure of PAHs, which is unknown but depends on three factors that are â€“ how much PAH entered the body, exposure time, and body response. Thus, the various effects are classified into short-term and long-term exposure. We do not have any clear evidence for PAHs causing short-term health effects but other compounds commonly found with PAHs may cause short-term symptoms like breathing passages irritation, confusion, diarrhea, eye irritation, nausea, and vomiting. Cataracts, jaundice, damage to kidney and liver are part of long-term health effects. PAH naphthalene on continuous skin contact results in skin inflammation and redness. When large amounts of naphthalene are respired or swallowed then the breakdown of red blood cells occurs in the human body. Long-term and low-level exposure to some PAHs and their specific mixtures are carcinogenic which means cancer-causing. USEPA, the United States Environmental Protection Agency, identified seven PAHs as human carcinogens (cancer-causing agents). Some examples of these carcinogens are benzo-anthracene, benzopyrene (usually causes cancer in animals), and chrysene. When animal experiments were performed then it was observed that certain PAHs can cause adverse harm for the developmental, genetic, reproductive systems, and overall immune; therefore, identified as mutagenic and teratogenic substances. Breast milk of women living near harmful waste sites contains low PAHs level which is harmful to the infant and mother both. Childhood asthma and lowering down of IQ are experienced on prenatal exposure to these chemicals. However, evaluation of the major PAHs stated no dangerous effects on health, thus further research is required for understanding the overall effects of various PAHs.
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