Since the U.S. Environmental Protection Agency (EPA) classified 16 polycyclic aromatic hydrocarbons (PAHs) as priority pollutants in 1976, extensive research has been conducted to investigate their sources, fate, exposure levels, and health impacts. As a class of organic compounds composed of multiple aromatic rings, PAHs are widely distributed across various environmental matrices and exhibit numerous adverse health effects, including strong oxidative stress, carcinogenicity, mutagenicity, teratogenicity, and immunotoxicity. The toxicological effects of these PAHs have been confirmed through toxicological and epidemiological studies.
Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds composed of multiple aromatic rings. They are ubiquitous in the environment, formed primarily through incomplete combustion of organic matter. This can occur naturally, such as in wildfires or volcanic eruptions, and through human activities, including the burning of fossil fuels, industrial processes, and smoking. Due to their widespread presence and persistence, PAHs have garnered significant attention for their environmental and health impacts.
Sources of polycyclic aromatic hydrocarbons
The primary sources of PAHs are anthropogenic, with industrial processes, vehicle emissions, residential heating, and tobacco smoke being major contributors. Natural sources, although less significant in terms of quantity, also play a role in the environmental distribution of PAHs. Additionally, PAHs are present in grilled and smoked foods, contributing to dietary exposure.
Environmental Pollution and Impact
PAHs are persistent organic pollutants, capable of long-range transport in the atmosphere. They tend to accumulate in soil and sediments, where they can persist for years, posing risks to aquatic and terrestrial ecosystems. PAH contamination can affect the reproductive, developmental, and immune systems of wildlife. Moreover, their presence in the environment can lead to bioaccumulation and biomagnification, affecting a wide range of organisms across the food chain.
Human Health Effects and Toxicity
Exposure to PAHs can occur through inhalation, ingestion, and dermal contact. The health risks associated with PAH exposure include respiratory issues, cardiovascular diseases, and developmental defects. However, the most concerning aspect of PAH exposure is its carcinogenic potential.
Carcinogenic Mechanism and Research
Research by the International Agency for Research on Cancer (IARC), a part of the World Health Organization (WHO), has classified certain PAH compounds, such as benzo[a]pyrene, as carcinogenic to humans (Group 1). The carcinogenic mechanism involves the metabolic activation of PAHs to form DNA adducts, leading to mutations and ultimately cancer. Studies have shown that PAH exposure is associated with an increased risk of various cancers, including lung cancer, skin, bladder, and gastrointestinal cancers.
Cancer Associations
The association between PAH exposure and cancer has been well-documented in epidemiological studies. Lung and skin cancers are among the most common types linked to PAH exposure, supported by research findings published in journals such as “Cancer Research” and “Environmental Health Perspectives”. These studies highlight the role of occupational exposure to PAHs in industries such as aluminum production and coal tar distillation, as well as environmental exposure through air pollution.
Prevention and Risk Mitigation
Reducing PAH exposure involves both individual and collective efforts. At the individual level, minimizing consumption of smoked or grilled foods, avoiding tobacco smoke, and using proper protective equipment in occupational settings can decrease personal exposure. On a broader scale, implementing stricter environmental regulations, improving industrial processes to reduce emissions, and promoting cleaner sources of energy are crucial steps in mitigating PAH pollution. Public awareness and education about PAH sources and health risks also play a key role in prevention efforts.
Polycyclic Aromatic Hydrocarbon Derivatives:
Primarily airborne pollutants, these are a class of organic compounds that originate from parent PAH structures and possess a variety of functional groups or elements. In contrast to the significant attention given to PAHs, knowledge about their derivatives, including nitro-PAHs (NPAHs), oxidized PAHs (OPAHs), halogenated PAHs (XPAHs), and alkylated PAHs (APAHs), is extremely limited.
Although the environmental concentrations of these PAH derivatives may be lower than their parent PAHs, substituent groups could render them more carcinogenic, teratogenic, and mutagenic, as well as increase their mobility and bioavailability. For instance, the toxicity equivalent factor (TEF) of the alkylated derivative 7,12-dimethylbenz[a]anthracene (7,12-DMBA) is 20 times that of its parent compound and twice that of B[a]P, indicating that their low concentrations could have significant impacts on exposure toxicity. Additionally, oxidized PAHs have been identified as important components of fine particulate matter (PM2.5) in the atmosphere, with inflammatory effects. Therefore, studying PAH derivatives is of great importance.
Exposure Routes of Polycyclic Aromatic Hydrocarbon Derivatives:
Similar to parent PAHs, their derivatives can also be absorbed through various routes, including inhalation, ingestion, and skin contact in both occupational and non-occupational settings. The primary routes of human exposure to polycyclic aromatic hydrocarbons and their derivatives include:
- (1)Direct inhalation of polluted air or tobacco smoke;
- (2) Ingestion of contaminated water and certain foods;
- (3) Skin contact with contaminated soil, ash, tar, and other materials.
It’s noteworthy that some exposure scenarios may involve multiple pathways simultaneously, affecting the total dose absorbed (e.g., contact from atmospheric air or contaminated soil involving both skin contact and inhalation).
From research on parent PAHs, we understand that diet typically accounts for at least 90% of the intake in non-smoking adults, while exposure through air, water, and soil contributes only about 4% of carcinogenic PAHs. The presence of PAH derivatives in food can be traced back to several sources, including atmospheric pollution, plant absorption from contaminated soil, and technological processes used in the food industry such as drying, baking, and smoking. For example, nitro-PAHs have been detected in vegetables, fruits, spices, oils, coffee, tea, and smoked foods, while oxidized PAHs have been found in smoked meats, seafood, and milk. Moreover, inhalation exposure might also be a significant route for polycyclic aromatic hydrocarbon derivatives.
The respiratory tract is the initial site for the deposition of these substances. They can be directly inhaled as gaseous pollutants or attached to particulate matter, entering the body through the respiratory system and potentially the bloodstream. Cooking has been identified as a significant source of polycyclic aromatic hydrocarbons and their derivatives in indoor environments.
Previous studies, especially those from regions in northern and central China, have assessed exposure to nitro-PAHs and oxidized PAHs through inhalation in households and individuals, finding that the average daily exposure levels for nitro-PAHs were a few nanograms per cubic meter, and for oxidized PAHs, a few tens of nanograms per cubic meter. Daily activities such as biomass cooking and smoking are associated with higher personal exposure levels to these polycyclic aromatic hydrocarbon derivatives. Similar observations have also been made in studies from northern Thailand.
In particular, chlorinated PAH derivatives have been detected in cooking exhaust gas, with median concentrations of 19 nanograms per cubic meter. However, a previous study in urban Nepal pointed out that skin contact with dust is another major pathway for indoor exposure to nitro-PAHs and oxidized PAHs, for both adults and children. Similarly, due to the low bioavailability of PAH derivatives in particulate matter, the daily dose absorbed through skin absorption is significantly lower than inhalation.
In outdoor environments, exposure patterns for PAH derivatives such as nitro-PAHs, oxidized PAHs, and amino-PAHs have been identified through particulate and gas-phase samples. The cancer risk associated with inhalation exposure is primarily linked to fine particulate matter, while the risk from skin exposure is mainly related to lifetime exposure to coarse particulate matter. Additionally, occupational exposure to polycyclic aromatic hydrocarbon derivatives poses significant health risks. For example, in the electronic waste recycling industry, dismantling printed circuit board waste often leads to increased inhalation or skin contact with oxidized PAHs and chlorinated PAH derivatives, raising the risk of cancer for workers.
Similarly, workers at coal-fired power plants exposed to high concentrations of polycyclic aromatic hydrocarbons and their derivatives, whether in the air as gases or attached to particulate matter. Studies show that the serum concentrations of polycyclic aromatic hydrocarbons in these workers are higher than in those without such occupational exposure. Security guards working near heavy traffic also face occupational exposure, with significant increases in oxidative stress markers for oxidized PAHs after an 8-hour shift, providing evidence for this. The firefighting profession also faces exposure risks to polycyclic aromatic hydrocarbon derivatives, as pollutants pass through gaps in their personal protective equipment, exacerbating skin exposure.
Conclusion
PAHs represent a significant environmental health concern due to their ubiquity, persistence, and toxicity. Understanding the sources, environmental behavior, and health impacts of PAHs is crucial for developing effective strategies to reduce exposure and mitigate risks. Continued research and policy efforts are essential to address the challenges posed by PAHs and protect public health and the environment.