Polychlorinated Biphenyls (PCBs) represent a class of 209 synthetic organic chemicals that were extensively manufactured and utilized from 1929 until their widespread ban in the mid-1980s.1, 2, 3, 4, 5, 6, 7, 8, 9 These compounds were highly valued in industrial applications due to their exceptional chemical inertness and heat stability, making them ideal as coolants and lubricants in electrical equipment such as transformers and capacitors, and as components in plasticizers, paints, and sealants.1, 3, 7, 9 However, these very properties, which conferred industrial utility, paradoxically ensured their long-term persistence in the environment once released.1, 7, 10 Decades after their cessation of production, PCBs remain ubiquitous global contaminants, cycling extensively between atmospheric, aquatic, and terrestrial compartments, leading to their detection even in remote polar regions far from their original industrial sources.2, 4, 5, 11, 12, 13, 14 This enduring presence highlights PCBs as a significant and persistent “toxic legacy” in the global environment.4

The Marine Management Organisation (MMO) plays a pivotal role in the UK's efforts to safeguard its marine environment. As the primary regulatory body, the MMO oversees marine planning, licensing, and environmental stewardship, which includes the critical task of monitoring and managing marine pollution, such as PCB contamination.15, 16 The MMO's Strategic Renewables Unit and Marine Licensing Team are actively engaged in enhancing the accessibility of environmental monitoring reports and data, fostering collaboration through data sharing with platforms like The Crown Estate's Marine Data Exchange.16 These monitoring activities are integral to the broader Clean Safe Seas Environmental Monitoring Programme (CSEMP), formerly known as the National Marine Monitoring Programme (NMMP), which systematically collects environmental data from UK waters to provide a comprehensive overview of contaminant levels.10, 17, 18, 19 The data generated through these initiatives are essential for fulfilling the UK's mandatory reporting obligations under international agreements, notably the Oslo and Paris Convention (OSPAR) Joint Assessments and Monitoring Programme (JAMP), and also support European Commission (EC) directives and national environmental assessments.1, 7, 10, 17, 18, 19, 20, 21, 22, 23 OSPAR has specifically designated PCBs as a “List of Chemicals for Priority Action” due to their inherent persistence, bioaccumulation potential, and toxicity, with an ultimate long-term objective of achieving concentrations “close to zero” for man-made substances in the marine environment.1, 7, 18 Within OSPAR's Coordinated Environmental Monitoring Programme (CEMP), a specific subset of seven PCB congeners—CB28, CB52, CB101, CB118, CB138, CB153, and CB180—are mandated for routine monitoring in biota (fish and mussels) and sediments to track temporal trends and spatial distribution.1, 6, 7, 10, 18, 19, 21, 22, 24, 25, 26 These “indicator PCBs” were selected due to their relatively high environmental concentrations and known toxic effects, providing valuable insights into broader PCB contamination patterns.1, 7, 10, 18, 19

This report aims to provide a comprehensive comparative analysis and ranking of these seven MMO-monitored PCB congeners based on five key environmental properties: toxicity, persistence, bioaccumulation potential, bioavailability, and natural abundance. The analysis will identify underlying trends, causal relationships, and broader implications for marine environmental management. The persistent detection of PCBs decades after their ban, coupled with their extreme environmental stability, highlights that current contamination is largely a result of historical releases and the slow degradation of existing reservoirs rather than new production. This enduring presence means that the role of organizations like the MMO extends beyond preventing new releases to actively managing and understanding the long-term fate of this existing pollution. Monitoring programs are therefore crucial for tracking the slow decline of these compounds and assessing their ongoing ecological and human health impacts. Furthermore, while OSPAR's goal of “close to zero” concentrations for man-made substances is aspirational, the continued detection of PCBs even in remote areas indicates a significant gap between policy ambition and ecological reality. The extreme persistence of PCBs makes rapid environmental clearance highly improbable, suggesting that policymakers may need to consider strategies focused on risk reduction and mitigation in the short to medium term, alongside sustained monitoring to track progress towards long-term goals.

PCBs are unequivocally synthetic organic chemicals, entirely a product of human industrial activity, with no known natural sources in the environment.2, 3, 5, 9, 10 Their chemical structure comprises two benzene rings linked by a single carbon-carbon bond, with varying numbers (from 1 to 10) of chlorine atoms substituted onto the biphenyl molecule. This structural variability gives rise to 209 distinct individual compounds, known as congeners.1, 2, 6, 8, 10, 22, 25, 27, 28, 29, 30 Historically, PCBs were manufactured and marketed as complex mixtures of these congeners, often under trade names like Aroclor, with different mixtures characterized by varying degrees of chlorination.2, 8, 9 The chemical inertness and heat stability that made PCBs industrially valuable paradoxically ensure their long environmental residence times. This highlights a fundamental challenge in industrial chemistry: substances designed for durability and stability in specific applications can become long-term environmental pollutants once released, creating a persistent environmental burden.

The defining characteristic of PCBs is their extreme environmental persistence. They do not readily break down or biodegrade, primarily due to the robust chemical bonds within their structure that are not typically found in natural degradation pathways.1, 2, 3, 4, 6, 7, 8, 10, 14, 18, 19, 22, 30, 31, 32, 33, 34 Their stability enables them to travel vast distances through atmospheric and oceanic currents, leading to their global distribution, with measurable concentrations detected even in remote areas such as the Arctic.2, 4, 5, 8, 12, 13, 14, 27, 30 This widespread and transboundary nature of PCB contamination underscores that local releases have global consequences, and pollution in one region can impact ecosystems and human populations far away. This reinforces the necessity of international conventions, such as the Stockholm Convention, for effectively managing persistent organic pollutants.

PCBs are highly hydrophobic (low water solubility) and lipophilic (fat-soluble).6, 8, 30, 35 This property causes them to strongly adsorb to organic matter in soils and sediments, where they can remain for months to years, acting as significant environmental reservoirs.1, 5, 6, 7, 8, 12, 14, 18, 19, 22, 23, 27, 28, 30, 31, 34, 35, 36, 37, 38, 39, 40 Crucially, their lipophilicity facilitates their uptake and accumulation in the fatty tissues of living organisms (bioaccumulation). Furthermore, their resistance to metabolism leads to increasing concentrations at successively higher trophic levels within food webs, a process known as biomagnification.1, 2, 4, 5, 6, 7, 8, 13, 14, 19, 23, 27, 30, 31, 33, 34, 37, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51 This biomagnification poses particular risks to top predators, such as marine mammals, which can accumulate alarmingly high levels, often exceeding toxicity thresholds.1, 14, 23, 30, 31, 40, 46, 49

PCBs are recognized as highly toxic compounds to both animals and humans, capable of inducing a wide array of adverse health effects.1, 2, 3, 4, 7, 8, 9, 10, 14, 19, 27, 30, 33, 40, 42, 46, 49, 52 These impacts include reproductive and developmental problems, damage to the immune system (e.g., decreased thymus size, reduced immune response, increased susceptibility to infections), interference with hormone function (endocrine disruption), neurobehavioral deficits, cardiovascular diseases, and various forms of cancer.1, 2, 7, 10, 14, 19, 27, 30, 33, 40, 42, 49, 52, 53 A critical subgroup, known as “dioxin-like” PCBs (DL-PCBs), exhibit toxicological properties similar to those of dioxins. These congeners, characterized by specific chlorine substitution patterns (e.g., non-ortho or mono-ortho chlorination allowing for a planar structure), mediate their toxicity by binding to the aryl hydrocarbon receptor (AhR).1, 6, 7, 10, 14, 18, 22, 25, 27, 52, 54, 55, 56, 57, 58, 59 Toxicity Equivalency Factors (TEFs) are used to quantify the relative toxicity of DL-PCBs compared to 2,3,7,8-TCDD, enabling the calculation of total toxic equivalents (TEQs) for mixtures.54, 56, 57, 58, 59

The seven indicator PCB congeners (CB28, CB52, CB101, CB118, CB138, CB153, and CB180) are routinely monitored by the MMO and other agencies within frameworks like OSPAR.1, 6, 7, 10, 18, 19, 21, 22, 24, 25, 26

All PCBs are inherently toxic, capable of inducing a range of adverse effects including genotoxicity, immune suppression, and endocrine disruption.7, 10 The toxicity of individual PCB congeners is highly dependent on their specific molecular structure, particularly the number and position of chlorine atoms.9, 30, 55 A critical distinction in PCB toxicology lies between “dioxin-like” (DL-PCBs) and “non-dioxin-like” (NDL-PCBs). DL-PCBs, characterized by specific chlorine substitution patterns (e.g., non-ortho or mono-ortho chlorination that allows a planar molecular configuration), exhibit toxicity similar to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by binding to the aryl hydrocarbon receptor (AhR).1, 6, 7, 10, 14, 18, 22, 25, 27, 52, 54, 55, 56, 57, 58, 59 NDL-PCBs, typically having chlorines in the ortho positions, do not generally interact with the AhR and exert their toxicity through different mechanisms.52, 54

Among the seven MMO-monitored congeners, CB118 (2,3',4,4',5-Pentachlorobiphenyl) is the only one consistently classified as a dioxin-like PCB (mono-ortho).1, 7, 10, 18, 23, 27 Consequently, CB118 is explicitly identified as the “most toxic dioxin-like compound” among the monitored PCBs.1, 7, 10, 19, 36 Its WHO 2005/2022 Toxic Equivalency Factor (TEF) is 0.00003.56, 57 Monitoring data indicate that CB118 concentrations continue to exceed target threshold levels (Environmental Assessment Criteria, EAC) in some UK marine areas, signifying an ongoing risk of adverse effects on marine organisms.10, 19, 36 This consistent exceedance of environmental targets for CB118, despite overall PCB declines, highlights the disproportionate risk posed by this specific congener due to its higher potency. This suggests that monitoring programs must prioritize the assessment and management of DL-PCBs, particularly CB118, to effectively mitigate ecological and human health risks.

The other six monitored congeners (CB28, CB52, CB101, CB138, CB153, CB180) are generally considered non-dioxin-like.1, 6, 25 While they do not act via the AhR pathway, they are still capable of causing various adverse health effects.2, 53, 60 For instance, lower chlorinated congeners like CB28, CB52, and CB101 have been associated with an increased risk of respiratory infections in infants following prenatal exposure.53 CB153, a hexachlorobiphenyl, has been shown to induce accelerated apoptotic cell death in human liver and kidney cells.60 It is also frequently used as a marker for total PCB exposure due to its prevalence.53 The precise ranking of toxicity among these NDL-PCBs is complex and often species-dependent. For example, CB28 was found to be the most toxic to green algae (/P. subcapitata/), while CB153 was the most toxic to abalone haemocytes (/H. tuberculata/), demonstrating that a universal toxicity ranking can be challenging without specifying the biological endpoint and species.52 This indicates that while TEFs provide a standardized measure for dioxin-like activity, the actual ecological impact of PCBs is more nuanced. Different congeners might have varying toxic effects on different organisms or through different mechanisms, necessitating a comprehensive risk assessment that considers a broader range of toxicological endpoints and species-specific sensitivities.

Table 1: PCB Congener Toxicity Ranking (Increasing Order)

This table provides a clear, ordered summary of the relative toxicity for the MMO-monitored PCB congeners. It distinguishes between dioxin-like and non-dioxin-like PCBs, a fundamental classification in understanding their health impacts. The inclusion of TEF values for the dioxin-like congener (CB118) offers a quantitative basis for its higher ranking, while qualitative remarks address the complexities and species-specific effects observed for non-dioxin-like PCBs. This structured presentation helps to quickly identify the congeners of highest toxicological concern.

The persistence of PCBs is a defining characteristic, stemming from their extreme stability and resistance to biodegradation.1, 2, 3, 4, 6, 7, 8, 10, 14, 18, 19, 22, 30, 31, 32, 33, 34 This resistance to degradation generally increases with the degree of chlorination; congeners with more chlorine atoms are typically more resistant to environmental breakdown.4, 8, 33, 62 While PCBs can undergo photodegradation in the vapor phase with reported half-lives ranging from 10 days to 1.5 years 4, 8, their persistence in other environmental compartments is significantly longer. In sediments, half-lives can span from months to years 6, 22, with some studies indicating ranges of 2 to 25 years.32 For instance, individual PCB congeners in San Francisco Bay sediments were found to have half-lives ranging from 4 years (for PCB 18) to 30 years (for PCB 194).64

Beyond environmental compartments, the persistence of PCBs within biological systems is particularly critical. Human elimination half-lives, which reflect how long these compounds remain in the body, are notably long. For the monitored congeners, reported human half-lives include approximately 3-4 years for CB138, 4.5-5.5 years for CB118, and 7-9 years for both CB153 and CB180.65 CB101 also has a reported human half-life of around 5.4 years.65 These extended biological half-lives mean that once PCBs are accumulated by organisms, they persist within their tissues for substantial periods, contributing to a chronic toxic burden. This long-term retention in biological systems has profound implications for chronic health effects, as continuous low-level exposure combined with slow elimination means that organisms, particularly long-lived top predators, will carry a toxic burden throughout their lives.31, 40, 46, 49 This necessitates sustained biomonitoring and health studies over decades, even after the cessation of PCB production.

While general principles suggest that persistence increases with chlorination, the human elimination half-lives reveal a nuance: CB138, a hexachlorobiphenyl, has a shorter human half-life (3-4 years) than CB118, a pentachlorobiphenyl (4.5-5.5 years).65 This indicates that biological persistence is not solely dictated by the number of chlorine atoms but also by specific metabolic pathways and elimination processes in organisms. The lower chlorinated congeners, CB28 (trichlorobiphenyl) and CB52 (tetrachlorobiphenyl), are generally considered less persistent than the higher chlorinated ones, though specific human half-lives for these are not provided in the available information.8, 33 The pervasive presence of PCBs in marine sediments, much of which entered the sea before regulatory controls were fully in place, highlights that “removing these sediments from the sea is not a practicable option”.19 This underscores the futility of complete removal and emphasizes that natural attenuation, through slow degradation, is the primary long-term “remediation” pathway for much of the existing contamination. This reinforces the importance of preventing the resuspension of contaminated sediments and managing legacy sources to minimize further releases, as the environment's capacity to naturally process these compounds is extremely slow.

Table 2: PCB Congener Persistence Ranking (Increasing Order)

This table provides a structured overview of the persistence of the MMO-monitored PCB congeners, a critical factor for understanding their long-term environmental impact. By ranking them in increasing order of persistence, primarily based on reported human elimination half-lives and general principles of chlorination, the table helps to identify which congeners will remain in the environment and biota for the longest periods. The inclusion of human half-lives emphasizes the long-term retention in biological systems, which is highly relevant for human health risk assessment and biomonitoring efforts.

Bioaccumulation describes the process by which an organism absorbs a contaminant from its food or environment at a rate faster than it can excrete it, leading to a net increase in the contaminant's concentration within its tissues over time.8, 45 A related and critical process in environmental toxicology is biomagnification, where the concentration of a contaminant increases progressively at successively higher trophic levels within a food web.1, 4, 5, 6, 7, 8, 13, 14, 19, 23, 27, 30, 31, 33, 34, 39, 40, 41, 44, 45, 46, 47, 48, 49, 50, 51 PCBs, being highly lipophilic (fat-soluble), readily accumulate in the fatty tissues of organisms and are efficiently transferred and magnified through marine food chains.1, 2, 4, 5, 6, 7, 8, 13, 14, 19, 23, 27, 30, 31, 33, 34, 39, 40, 45, 46, 47, 48, 49, 51 This means that top marine predators, such as whales, dolphins, orcas, and seals, are disproportionately affected, accumulating levels that can exceed toxicity thresholds and severely impact their health and reproductive success.1, 30, 31, 46, 49 This phenomenon poses a severe threat to marine biodiversity and ecosystem health, even when overall environmental concentrations may appear low.

The octanol-water partition coefficient (Log Kow) is a widely accepted and crucial indicator for predicting the bioaccumulation tendency of lipophilic organic compounds. A higher Log Kow value generally correlates with higher lipid solubility and, consequently, a greater potential for bioaccumulation.8, 12, 32, 35, 40, 43, 47, 48, 66, 67 The Log Kow values for the seven indicator PCBs demonstrate a clear trend:

  CB28: 5.67

  CB52: 5.84

  CB101: 6.38

  CB118: 6.74

  CB138: 6.83

  CB153: 6.92

  CB180: 7.36 [[#ref62|62]]

Based on these values, bioaccumulation potential generally increases with the degree of chlorination. Higher chlorinated congeners, such as CB138, CB153, and CB180, are more prevalent in food and human serum due to their combined persistence and high bioaccumulation potential.65 Studies from Lake Victoria, for example, show that CB138, CB153, and CB180 are the dominant PCB congeners found in both sediment and fish samples.34, 39 Furthermore, CB153 is frequently reported as the most abundant congener in various biota.37, 61 The ability of certain organisms to serve as effective biomonitors is also evident; fish parasites, for instance, have been shown to biomagnify PCBs at significantly higher rates (e.g., Proteocephalaus sp. by 6–14 times, Ligula intestinalis by 8–16 times) than their piscine hosts.34, 39 This highlights that specific organisms can provide a concentrated signal of environmental contamination that might be difficult to detect otherwise.

Table 3: PCB Congener Bioaccumulation Potential Ranking (Increasing Order, based on Log Kow)

This table directly fulfills the request for ranking by bioaccumulation potential, utilizing the widely accepted quantitative metric of Log Kow. The clear progression of Log Kow values with increasing chlorination visually demonstrates a fundamental chemical property that governs the environmental fate and accumulation of these compounds. This quantitative relationship aligns with observed environmental prevalence, where higher Log Kow congeners like CB138, CB153, and CB180 are frequently dominant in contaminated biota.

Bioavailability refers to the fraction of a chemical that is available for uptake by an organism from its surrounding environment.6, 27, 52, 58, 66, 68 The bioavailability of PCBs in marine environments is a complex property influenced by various factors, including their physical-chemical properties, the environmental matrix (water, sediment, food), and the organism's uptake pathways. PCBs have low aqueous solubilities, which generally decrease with increasing chlorination or molecular mass.8, 35 They are also strongly sorbed by organic matter in soils and sediments.8, 35 Critically, truly dissolved PCBs are more bioavailable for biological uptake compared to those associated with organic material (dissolved or particulate).69

Uptake pathways vary by organism size and type. For unicellular organisms like protozoa and phytoplankton, diffusion through cellular membranes is often the dominant uptake pathway, and this can occur faster than ingestive uptake.14, 52, 69 In contrast, for larger organisms such as fish and marine mammals, the primary pathway for PCB accumulation is the ingestion of contaminated food.1, 5, 7, 13, 23, 27, 33, 34, 39, 42, 43, 47, 48, 68, 69 Dermal and gill uptake are considered minor contributors to overall PCB uptake in larger marine organisms.69

The “aging” of PCBs in sediments can significantly diminish their bioavailability over time, as they become more strongly bound and less accessible for desorption.32 This means that even if large quantities of PCBs are present in sediments, their actual threat to marine life may decrease as they become kinetically trapped. However, this also highlights the importance of preventing sediment disturbance (e.g., dredging, bottom trawling) which could re-mobilize these “aged” PCBs and increase their bioavailability.

Congener-specific bioavailability is nuanced. Lower chlorinated PCBs (e.g., CB28, CB52) are generally more water-soluble and volatile 5, 8, 33, 35, suggesting higher bioavailability from the dissolved water phase or air. However, they also partition rapidly from water into sediment or are taken up by biota.18 Conversely, higher chlorinated PCBs (e.g., CB153, CB180), with their higher Log Kow values, are more strongly sorbed to sediments 8, 35 and less volatile.5, 33 This implies that their direct bioavailability from the dissolved water phase might be lower. This is supported by a study showing that bioavailability from sediment (estimated by 24-h Tenax-aided desorption) decreased with increasing Log Kow, with CB52 showing higher bioavailability than CB153.32 This suggests that while highly chlorinated PCBs have a strong potential to accumulate in fatty tissues once absorbed (high Log Kow), their release from environmental sinks like aged sediments can be lower due to strong binding. Despite this, their primary exposure pathway shifts to dietary ingestion, where their high lipophilicity drives significant accumulation.1, 5, 7, 13, 33, 34, 42, 43, 68, 69

Table 4: PCB Congener Bioavailability Ranking (Increasing Order)

This table provides a ranking of PCB congener bioavailability, acknowledging the complexities and context-dependency of this property. The ranking is primarily inferred from Log Kow values, with lower Log Kow suggesting higher water solubility and thus potentially greater bioavailability from the dissolved water phase. It also highlights that while higher chlorinated PCBs may have lower bioavailability from sediments due to strong binding, their primary uptake route shifts to dietary ingestion due to their high lipophilicity. This nuanced presentation is crucial for understanding how different congeners interact with the marine environment and are taken up by organisms.

PCBs are entirely synthetic organic chemicals; there are no known natural sources of PCBs in the environment.2, 3, 5, 9, 10 Their existence is solely a result of human industrial activity, with their initial synthesis occurring in 1929 and industrial production commencing in the 1930s.2, 6, 7, 8, 30 Consequently, any detection of PCBs in the environment, regardless of congener, is a direct indicator of anthropogenic pollution. Their presence stems from various human activities, including industrial discharges, leaks from electrical equipment, improper disposal practices, and atmospheric transport resulting from incineration or volatilization from contaminated materials.2, 4, 5, 7, 13, 23, 27, 30, 39, 40, 49, 58 This fundamental characteristic means that environmental quality targets for PCBs can realistically aim for “close to zero” concentrations, as there is no natural background level to consider.

Table 5: PCB Congener Natural Abundance

This table directly addresses the query's requirement for natural abundance and reinforces a fundamental characteristic of PCBs as purely anthropogenic pollutants. Explicitly stating “Zero” for all congeners underscores that any detection in the environment is a direct result of human activity, simplifying source attribution and emphasizing the need for strict regulatory controls.

The comprehensive assessment of the seven MMO-monitored PCB congeners reveals a complex interplay of physical-chemical properties and toxicological profiles that collectively determine their environmental risk. Generally, a positive correlation exists between the degree of chlorination, Log Kow values, persistence, and bioaccumulation potential. This means that higher chlorinated PCBs, such as CB138, CB153, and CB180, tend to exhibit higher Log Kow values, greater environmental and biological persistence, and consequently, a higher potential for bioaccumulation and biomagnification.8, 33, 34, 39, 62, 65 These congeners are frequently observed to be dominant in environmental biota and human tissues.34, 37, 39, 53, 61, 62, 63, 65 Conversely, lower chlorinated PCBs (e.g., CB28, CB52, CB101) are generally less persistent and bioaccumulative 8, 33, but their higher volatility and water solubility can influence their transport and initial bioavailability from air or water.5, 8, 33

Toxicity, however, does not always follow a simple linear correlation with chlorination level or persistence. The presence of a “dioxin-like” structure, particularly planarity, is a key determinant of high toxicity, exemplified by CB118. Despite being a pentachlorobiphenyl, CB118 is consistently identified as the most toxic among the monitored congeners due to its dioxin-like activity.1, 7, 10, 14, 18, 19, 22, 27, 36, 52, 54, 55, 56, 57, 58, 59 Its Log Kow (6.74) is lower than that of CB153 (6.92) and CB180 (7.36) 62, demonstrating that inherent toxicity is not directly proportional to bioaccumulation potential across all congeners. This highlights the necessity for multi-criteria assessment in risk evaluation.

A significant aspect of PCB environmental behavior is the complex relationship between persistence, bioaccumulation, and bioavailability. Highly persistent and highly chlorinated PCBs, such as CB153 and CB180, tend to exhibit lower water solubility and a stronger affinity for binding to sediments.5, 6, 8, 12, 32, 35 This means that their bioavailability from the dissolved water phase might be lower. However, their overall environmental impact remains high due to their prolonged residence times and efficient bioaccumulation through the food chain. This presents a critical nuance: while these congeners have a high potential to accumulate in fatty tissues (high Log Kow), their release from environmental sinks like aged sediments can be lower due to strong binding. This suggests that the primary exposure pathway for these highly hydrophobic compounds shifts from direct uptake from water to dietary ingestion, particularly for top predators.

The observed inverse relationship between Log Kow and bioavailability from aged sediments, where higher chlorinated congeners like CB153 show reduced desorption compared to lower chlorinated ones like CB52 32, is a crucial finding. This implies that while highly chlorinated PCBs are highly bioaccumulative once ingested, their release from long-term sediment reservoirs might be slower. This phenomenon, where the most persistent and bioaccumulative congeners can become kinetically trapped in sediments, has significant implications for remediation strategies. It suggests that natural attenuation might slowly reduce the risk from deeply buried and aged sediments. However, it also emphasizes the importance of preventing sediment disturbance, such as through dredging or bottom trawling, which could re-mobilize these “aged” PCBs and increase their bioavailability.

The congener-specific nature of PCB risk assessment is paramount. The data clearly demonstrates that different PCB congeners possess distinct toxicities, persistence, bioaccumulation potentials, and bioavailabilities. For example, CB118 is highly toxic but not necessarily the most bioaccumulative by Log Kow, while CB153 is highly bioaccumulative and persistent but is not dioxin-like.1, 10, 18, 36, 62 This means that a uniform approach to PCB risk assessment is insufficient. Environmental management must adopt a congener-specific framework, recognizing that the most prevalent congeners might not be the most toxic, and vice-versa. This necessitates sophisticated analytical methods and targeted monitoring for specific congeners of concern, rather than relying solely on total PCB measurements.

The report also highlights the challenge of balancing ecological and human health risks. The data points to risks for both marine organisms (e.g., reproductive issues in porpoises 30, 31, 49) and human health (e.g., dietary exposure, cancer, neurological effects 1, 2, 5, 7, 13, 27, 42, 68). The critical congeners and exposure pathways might differ for these endpoints. For instance, highly bioaccumulative congeners in fish pose a direct human dietary concern, while highly toxic DL-PCBs might threaten sensitive marine species at lower concentrations. Regulatory bodies like the MMO must consider both ecological and human health endpoints when setting environmental quality standards and developing management strategies. This may necessitate multi-faceted approaches that address different risk profiles simultaneously. Protecting top marine predators from biomagnified PCBs, for example, indirectly safeguards human consumers of marine products.

The continued exceedance of Environmental Assessment Criteria (EAC) for CB118 in some areas 10, 19, 36 indicates that current concentrations still pose a risk to marine organisms despite overall PCB declines. The long half-lives in both the environment and biota necessitate sustained, long-term monitoring strategies to track the slow decline of legacy contamination.4, 6, 8, 19, 22, 31, 32, 64, 65 The significant biomagnification in top predators underscores the vulnerability of these species and the need for targeted assessments in higher trophic levels.1, 23, 30, 31, 34, 39, 40, 46, 49 The variability in observed trends, with some congeners declining while others level off or even increase in certain organisms 41, highlights that a static monitoring approach is insufficient. Monitoring programs must be dynamic and adaptable, focusing on sensitive indicators and adapting to changing congener profiles.

Based on the comprehensive assessment of the MMO-monitored PCB congeners and their environmental behavior, the following recommendations are put forth to enhance marine management strategies:

  Targeted Monitoring and Research:

      Continue and enhance congener-specific monitoring, particularly for CB118, given its persistent exceedance of Environmental Assessment Criteria (EACs) and high toxicity. This targeted approach is crucial for accurately assessing and mitigating ecological risk.[[#ref10|10]], [[#ref19|19]], [[#ref36|36]]

      Prioritize monitoring efforts in identified "hotspots," such as the English Channel, Gulf of Cadiz, and areas adjacent to industrial or urban centers, including old landfill sites, where PCB concentrations remain elevated.[[#ref19|19]], [[#ref27|27]], [[#ref28|28]], [[#ref30|30]], [[#ref36|36]], [[#ref37|37]], [[#ref61|61]], [[#ref70|70]]

      Expand biomonitoring to encompass a wider range of trophic levels, with a particular focus on long-lived top predators like marine mammals and large fish, which accumulate the highest concentrations of PCBs due to biomagnification.[[#ref1|1]], [[#ref23|23]], [[#ref30|30]], [[#ref31|31]], [[#ref34|34]], [[#ref39|39]], [[#ref40|40]], [[#ref46|46]], [[#ref49|49]] The utility of specific biomonitors, such as fish parasites, which can accumulate higher PCB levels than their hosts, should also be explored.[[#ref34|34]], [[#ref39|39]]

      Conduct further research into the long-term bioavailability of PCBs from aged marine sediments and assess the potential for their remobilization due to environmental disturbances (e.g., climate change impacts on ocean currents, increased storm activity) or human activities like dredging.[[#ref5|5]], [[#ref14|14]], [[#ref30|30]], [[#ref32|32]], [[#ref35|35]], [[#ref40|40]], [[#ref71|71]] This understanding is vital for predicting future exposure risks.

      Investigate the combined toxicological effects of PCB mixtures and other co-contaminants (e.g., heavy metals, polybrominated diphenyl ethers) in marine organisms, as real-world exposure scenarios involve complex chemical cocktails.[[#ref14|14]], [[#ref30|30]], [[#ref41|41]]

  Policy and Management Considerations:

      Reinforce and accelerate global efforts to identify and safely dispose of remaining PCB stockpiles and contaminated equipment, ensuring adherence to the targets set by the Stockholm Convention. The current pace of elimination is insufficient to meet established deadlines.[[#ref4|4]], [[#ref49|49]]

      Implement stricter controls and develop effective remediation strategies for known land-based sources, including landfills located near water bodies and e-waste recycling sites, to prevent further leaching of PCBs into marine environments.[[#ref1|1]], [[#ref3|3]], [[#ref4|4]], [[#ref5|5]], [[#ref23|23]], [[#ref27|27]], [[#ref30|30]], [[#ref39|39]], [[#ref42|42]], [[#ref49|49]], [[#ref58|58]]

      Develop and disseminate clear public health advisories regarding the consumption of locally caught fish and shellfish from contaminated areas, ensuring these advisories consider congener-specific risks and the biomagnification potential in different species.[[#ref5|5]], [[#ref37|37]], [[#ref46|46]], [[#ref56|56]], [[#ref61|61]], [[#ref68|68]], [[#ref72|72]], [[#ref73|73]]

      Advocate for enhanced international cooperation and information sharing on PCB monitoring data and best practices for management, recognizing the transboundary nature of these persistent pollutants.[[#ref1|1]], [[#ref4|4]], [[#ref7|7]], [[#ref17|17]], [[#ref18|18]], [[#ref19|19]], [[#ref21|21]], [[#ref22|22]], [[#ref23|23]], [[#ref26|26]]

      Ensure that environmental policies and regulations, including EACs and monitoring targets, are dynamic and adaptable, incorporating the latest scientific findings on PCB toxicity, persistence, and bioavailability. Continuous scientific research directly informs and strengthens regulatory frameworks, ensuring their effectiveness in protecting marine ecosystems and human health.

Polychlorinated Biphenyls, entirely anthropogenic in origin, continue to represent a significant and enduring threat to marine ecosystems globally. Their extreme persistence, coupled with their high bioaccumulation and biomagnification potential, ensures their continued presence in the environment and accumulation in marine food webs, particularly in top predators.

Among the seven PCB congeners routinely monitored by the Marine Management Organisation (CB28, CB52, CB101, CB118, CB138, CB153, CB180), a nuanced understanding of their properties is essential for effective management:

  Toxicity: CB118 stands out as the most toxic due to its dioxin-like properties, consistently exceeding environmental targets in some areas. Other non-dioxin-like congeners, such as CB28 and CB153, also exhibit specific toxic effects, although their overall potency is generally lower than that of dioxin-like PCBs.

  Persistence: Persistence generally increases with the degree of chlorination, with higher chlorinated congeners like CB153 and CB180 exhibiting the longest environmental and biological half-lives. These compounds will remain in marine systems and biota for many years, contributing to a long-term toxic burden.

  Bioaccumulation Potential: This property correlates directly with the octanol-water partition coefficient (Log Kow), which increases with chlorination. Consequently, CB138, CB153, and CB180 demonstrate the highest bioaccumulation potential and are frequently dominant in marine biota.

  Bioavailability: This is a complex property influenced by the environmental matrix and uptake pathways. Lower chlorinated congeners may exhibit higher bioavailability from the dissolved water phase. In contrast, higher chlorinated PCBs, while highly lipophilic, can become strongly adsorbed to sediments, potentially reducing their bioavailability from these matrices over time due to aging. However, dietary ingestion remains the primary uptake route for these compounds in higher trophic levels.

  Natural Abundance: All PCBs are purely synthetic compounds, meaning their natural abundance is zero. Any detection in the environment is a direct indicator of anthropogenic contamination.

Despite the global bans enacted decades ago, the “legacy burden” of PCBs will continue to impact marine ecosystems for decades to come. The biomagnification of these compounds, particularly in long-lived top marine predators, remains a critical concern for ecosystem health and biodiversity, as it can lead to severe health effects and population declines. The complex interplay of congener-specific properties necessitates a sophisticated, congener-specific approach to risk assessment and environmental management. This includes targeted monitoring, ongoing research into their environmental fate and effects, and continued efforts to manage existing PCB sources and prevent their release into the marine environment. The persistent presence of these compounds underscores the ongoing challenge of managing anthropogenic pollutants and the critical need for sustained vigilance and adaptive strategies in marine conservation.

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