Persistent Organic Pollutants (POPs)
Kevin Jones
ECG bulletin July 2001
ECG bulletin July 2001
Persistent Organic Pollutants (POPs)
For the second talk in the half-day symposium at this year’s DGL, Professor Kevin Jones from Lancaster University reviewed the environmental chemistry of ‘Persistent Organic Pollutants’.
What are Persistent Organic Pollutants (POPs)?
There are many thousands of POP chemicals and these often originate from certain classes of chemicals (e.g. the polychlorinated biphenyls). POPs are persistent in the environment, having long half-lives in soils, sediments, air or biota. There seems to be no consensus regarding how long the half-life in a given media should be for the term ‘persistent’ to be conferred. However, in practice a POP could have a half-life of years or decades in soil/sediment and several days in the atmosphere.
POPs are typically hydrophobic and lipophilic. In aquatic systems and soils they partition strongly to solids, notably organic matter, avoiding the aqueous phase. They also partition into lipids in organisms, rather than entering the aqueous milieu of cells, and become stored in fatty tissue. This confers persistence on the chemical in the biota since metabolism is slow and POPs can therefore accumulate in food chains.
Importantly, POPs have the propensity to enter the gas phase under environmental temperatures. Hence, they may volatilise from soils, vegetation and water bodies into the atmosphere and – because of their resistance to breakdown reactions in air – travel long distances before being re-deposited. The cycle of volatilisation and deposition may be repeated many times, with the result that POPs can accumulate in an area far removed from where they were used or emitted. In the atmosphere itself, POPs can partition between particles and aerosols depending on ambient temperature and physico-chemical properties of the chemical.
The combination of stability and a propensity to form a gas under appropriate environmental conditions means that POPs are subject to long-range atmospheric transport. A resistance to metabolism and their inherent lipophilicity also means that POPs will accumulate in food chains.
Among the important classes of POP chemicals are many chlorinated (and brominated) aromatics, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and-furans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs) and different organochlorine pesticides (e.g. DDT and its metabolites; Toxaphene; and Chlordane). Some POPs are the accidental by-products of either combustion or the industrial synthesis of other chemicals (e.g. the PCDD/Fs). Many have been synthesised for industrial uses (e.g. PCBs, chlorinated paraffins, PBDEs) or as agrochemicals (e.g. DDT, Lindane, Chlordane). Examples of more polar POPs are the chlorinated phenols.
Certain key properties of chemicals control their fate in the environment and, if these are known, environmental chemists can make predictions about their fate and behaviour. These properties include aqueous solubility, vapour pressure, partition coefficients between water/solid (analogous to the measured octanol:water partition coefficient, Kow) and air:solid or liquid (analogous to the measured octanol:air partition coefficient, KOA; and Henry’s law constants, KAW), and half-lives in air, soil and water. Physical properties have been compiled for many POPs. However, there are often wide variations in the reported values, resulting in some uncertainty in the precise behaviour of POPs.
What are the Harmful Effects of POPs?
Because POPs can bioaccumulate and magnify in the food chain, there are concerns about their impact on the top predatory species, including humans. Probably the best-documented and clearest evidence of the effects of POPs have been in birds and marine mammals. Indeed, Rachel Carson drew attention to declining bird populations in her classic book “Silent Spring”. Various studies have revealed how organochlorines (OCs), notably DDE - a metabolic breakdown product of DDT - can affect eggshell thickness in birds of prey. Numerous subtle but far-reaching effects on the reproductive potential of fish-eating birds continue to be reported in the Great Lakes and in Europe. It is reassuring to see that with a decline in POP residues in some areas, predator populations have increased again. Examples include harbour seals in the southeast North Sea, white-tailed eagles in the Baltic and piscivorous birds in the Great Lakes.
POPs are also amongst the many chemicals implicated in the current concerns over ‘sex hormone’ or endocrine disruption in humans and wildlife. Reproductive impairment has been observed in seals in the Baltic Sea and the Dutch Wadden Sea and for Beluga whales in the St Lawrence seaway, Canada. These reproductive effects are linked to POPs, and especially to PCBs. However, an extensive array of POPs occurs and accumulates simultaneously in biota. It is therefore difficult to conclude that an effect is due to one particular chemical, a family of chemicals, their metabolites or indeed several families of chemicals acting synergistically. This makes control of the problem difficult because scientists and policy makers have been unsure which POP(s) require restriction and regulation.
In addition to their reproductive effects, many POPs are known or suspected carcinogens. Polynuclear aromatic hydrocarbons (PAHs) and (PCDD/Fs are perhaps the most obvious examples. POPs also affect the immune system of the top predators, enhancing their susceptibility to disease and patterns of behaviour. Clearly, the concerns over adverse health effects of POPs in humans and wildlife provide the impetus for research on the sources, environmental fate and food chain transfer properties of these chemicals.
Sources, Measurement and Trends of POPs
Sources
For agrochemical POPs the source is clear - the deliberate application to crops and soils. However, despite their deliberate manufacture, data on the total amount entering the environment and regional/global usage patterns of agrochemical POPs is highly uncertain and often poorly known.
Other POPs have been deliberately manufactured but have multiple diverse and diffuse uses. PCBs, for example, were first synthesised in the late 1920s and have been used in many ‘open’ and ‘closed’ applications. It has been estimated that global production to date is of the order of 106 tonnes, but information on the breakdown of usage is very limited, making it very difficult to derive estimates for historical and contemporary sources.
This issue is compounded in the case of accidentally formed and released POPs. PCDD/Fs/ PCDD/Fs enter the environment from a variety of combustion sources, from metal refining, and as impurities in other, deliberately manufactured chlorinated compounds, such as pentachlorophenol (PCP) and PCBs. It is clearly of fundamental importance to identify the dominant sources to the environment if source reduction measures are to be effective.
National efforts have been directed towards compiling PCDD/F source inventories. However, these are difficult and costly to measure and based on very limited information. There is a lack of consensus on whether primary (combustion) or secondary (e.g. volatilisation from soil; chemical usage) sources dominate atmospheric emissions, and there are discrepancies between the national/regional emission and deposition estimates. Basic information is still lacking, e.g. there is disagreement about the natural formation mechanisms for PCDD/Fs, which may be responsible for a pre-industrial environmental burden of these compounds, or whether their occurrence in the contemporary environment results purely from industrial activity in the last two hundred years.
Measurements
Environmental organic chemists have access to very sensitive and selective analytical methods for measuring POPs. For example, PCDD/Fs are released into the environment in ultratrace amounts; the national atmospheric emission to the UK is only ~0.5 kg åTEQ annually. If this output were dispersed evenly throughout the UK atmosphere (volume ~1014 m3), air concentrations would be of the order of femtograms per cubic metre. Current high resolution gas chromatography-high resolution mass spectrometry (HRGC-HRMS) techniques allow detection of a few femtograms. Consequently, if air samples of just ~500 m3 are taken, then a wide array of compounds can be detected. Sensitivity at this level can present problems because samples can become contaminated by residues in glassware, solvents, or laboratory air that come into contact with the sample. Modern methods of analysis benefit greatly from the availability of 13C and deuterated analogues of the POPs, making possible precise and sensitive quantitation by isotope dilution-MS techniques.
Because of the great sensitivity of electron capture and MS detectors, POPs can now be routinely detected in the full array of environmental media at trace levels; they are ubiquitous in the modern environment even in areas far removed from sources. However, detection needs to considered in the context of the amount emitted and the persistence of the POP. Detection does not automatically signal that there is a ‘POPs problem’.
Trends and Environmental Recycling of POPs
Many classes of POPs (e.g. PCBs, OC pesticides, chlorobenzenes) are subject to a similar broad trend in their usage/emission to the environment:
For some ‘newer’ POPs a similar ‘pattern’ may have been observed, but have been more compressed in time. Examples are the brominated flame-retardants, used extensively in insulating material for electrical equipment and the chlorinated benzyltoluenes used as PCB substitutes.
Recent research on the recycling of POPs indicates:
Regional and global POPs cycling
Air-surface exchange of POPs occurs in response to temperature changes. Persistent, semi-volatile compounds can participate in repeated air-surface exchanges and as a consequence move from one area of the globe to another. Emission to air will tend to occur primarily in ‘global source areas’ where the POP is used or released. For example, DDT usage has been extensive through the tropics and the high temperatures there will mean greater volatilisation rates of DDT than somewhere cooler.
Under cool conditions, gas phase POPs can partition back to the earth’s surface, while in rainy areas wet deposition will deposit POPs to the surface. Rates of re-release from the surface will be slower in colder polar or high altitude regions. Hence, it has been proposed that on a regional or global scale POPs can potentially migrate from warmer to cooler areas and become ‘fractionated’ on latitudinal or altitudinal gradients. Over the last few years there has been an increasing interest in studying and testing this hypothesis of ‘global fractionation’ or ‘cold condensation’ for POPs, with evidence steadily accumulating for its occurrence for selected compounds with a certain range of physico-chemical properties. Indeed, concern has been expressed that the polar regions will become ‘global sinks’ for POPs released/used elsewhere on Earth. The Canadian government has been funding research programmes on Arctic pollution, largely arising from concerns that native people living in the far north and dependent on animal fat-rich diets (marine mammal meat/blubber and fish and vulnerable top predator wildlife species, e.g. wolf, polar bear) are prone to particularly high POPs exposure.
Compounds need to have properties in a certain narrow window to be susceptible to long-range transport and to accumulate in the oceans and land surfaces of the poles. However, it should be appreciated that complex array of processes operate at the global scale to affect the cycling and long-term fate of POPs. Amongst these are:
These factors combine to reduce the threat of accumulation in the polar regions. Indeed the current (very limited) database suggests that whilst global fractionation is occurring, residues of many POPs are declining in polar regions, but at slower rates than in tropical and temperate latitudes. Much more research is required on these issues to fully elucidate the processes involved and to monitor trends.
Attention, then, is beginning to focus on constructing regional and global scale budgets, inventories and models for POPs. These are, of necessity, rather simplistic at present and have substantial gaps in information and data. Foremost amongst the deficiencies of current regional/global cycling models are:
KEVIN JONES
Lancaster University,
June 2001
For the second talk in the half-day symposium at this year’s DGL, Professor Kevin Jones from Lancaster University reviewed the environmental chemistry of ‘Persistent Organic Pollutants’.
What are Persistent Organic Pollutants (POPs)?
There are many thousands of POP chemicals and these often originate from certain classes of chemicals (e.g. the polychlorinated biphenyls). POPs are persistent in the environment, having long half-lives in soils, sediments, air or biota. There seems to be no consensus regarding how long the half-life in a given media should be for the term ‘persistent’ to be conferred. However, in practice a POP could have a half-life of years or decades in soil/sediment and several days in the atmosphere.
POPs are typically hydrophobic and lipophilic. In aquatic systems and soils they partition strongly to solids, notably organic matter, avoiding the aqueous phase. They also partition into lipids in organisms, rather than entering the aqueous milieu of cells, and become stored in fatty tissue. This confers persistence on the chemical in the biota since metabolism is slow and POPs can therefore accumulate in food chains.
Importantly, POPs have the propensity to enter the gas phase under environmental temperatures. Hence, they may volatilise from soils, vegetation and water bodies into the atmosphere and – because of their resistance to breakdown reactions in air – travel long distances before being re-deposited. The cycle of volatilisation and deposition may be repeated many times, with the result that POPs can accumulate in an area far removed from where they were used or emitted. In the atmosphere itself, POPs can partition between particles and aerosols depending on ambient temperature and physico-chemical properties of the chemical.
The combination of stability and a propensity to form a gas under appropriate environmental conditions means that POPs are subject to long-range atmospheric transport. A resistance to metabolism and their inherent lipophilicity also means that POPs will accumulate in food chains.
Among the important classes of POP chemicals are many chlorinated (and brominated) aromatics, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and-furans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs) and different organochlorine pesticides (e.g. DDT and its metabolites; Toxaphene; and Chlordane). Some POPs are the accidental by-products of either combustion or the industrial synthesis of other chemicals (e.g. the PCDD/Fs). Many have been synthesised for industrial uses (e.g. PCBs, chlorinated paraffins, PBDEs) or as agrochemicals (e.g. DDT, Lindane, Chlordane). Examples of more polar POPs are the chlorinated phenols.
Certain key properties of chemicals control their fate in the environment and, if these are known, environmental chemists can make predictions about their fate and behaviour. These properties include aqueous solubility, vapour pressure, partition coefficients between water/solid (analogous to the measured octanol:water partition coefficient, Kow) and air:solid or liquid (analogous to the measured octanol:air partition coefficient, KOA; and Henry’s law constants, KAW), and half-lives in air, soil and water. Physical properties have been compiled for many POPs. However, there are often wide variations in the reported values, resulting in some uncertainty in the precise behaviour of POPs.
What are the Harmful Effects of POPs?
Because POPs can bioaccumulate and magnify in the food chain, there are concerns about their impact on the top predatory species, including humans. Probably the best-documented and clearest evidence of the effects of POPs have been in birds and marine mammals. Indeed, Rachel Carson drew attention to declining bird populations in her classic book “Silent Spring”. Various studies have revealed how organochlorines (OCs), notably DDE - a metabolic breakdown product of DDT - can affect eggshell thickness in birds of prey. Numerous subtle but far-reaching effects on the reproductive potential of fish-eating birds continue to be reported in the Great Lakes and in Europe. It is reassuring to see that with a decline in POP residues in some areas, predator populations have increased again. Examples include harbour seals in the southeast North Sea, white-tailed eagles in the Baltic and piscivorous birds in the Great Lakes.
POPs are also amongst the many chemicals implicated in the current concerns over ‘sex hormone’ or endocrine disruption in humans and wildlife. Reproductive impairment has been observed in seals in the Baltic Sea and the Dutch Wadden Sea and for Beluga whales in the St Lawrence seaway, Canada. These reproductive effects are linked to POPs, and especially to PCBs. However, an extensive array of POPs occurs and accumulates simultaneously in biota. It is therefore difficult to conclude that an effect is due to one particular chemical, a family of chemicals, their metabolites or indeed several families of chemicals acting synergistically. This makes control of the problem difficult because scientists and policy makers have been unsure which POP(s) require restriction and regulation.
In addition to their reproductive effects, many POPs are known or suspected carcinogens. Polynuclear aromatic hydrocarbons (PAHs) and (PCDD/Fs are perhaps the most obvious examples. POPs also affect the immune system of the top predators, enhancing their susceptibility to disease and patterns of behaviour. Clearly, the concerns over adverse health effects of POPs in humans and wildlife provide the impetus for research on the sources, environmental fate and food chain transfer properties of these chemicals.
Sources, Measurement and Trends of POPs
Sources
For agrochemical POPs the source is clear - the deliberate application to crops and soils. However, despite their deliberate manufacture, data on the total amount entering the environment and regional/global usage patterns of agrochemical POPs is highly uncertain and often poorly known.
Other POPs have been deliberately manufactured but have multiple diverse and diffuse uses. PCBs, for example, were first synthesised in the late 1920s and have been used in many ‘open’ and ‘closed’ applications. It has been estimated that global production to date is of the order of 106 tonnes, but information on the breakdown of usage is very limited, making it very difficult to derive estimates for historical and contemporary sources.
This issue is compounded in the case of accidentally formed and released POPs. PCDD/Fs/ PCDD/Fs enter the environment from a variety of combustion sources, from metal refining, and as impurities in other, deliberately manufactured chlorinated compounds, such as pentachlorophenol (PCP) and PCBs. It is clearly of fundamental importance to identify the dominant sources to the environment if source reduction measures are to be effective.
National efforts have been directed towards compiling PCDD/F source inventories. However, these are difficult and costly to measure and based on very limited information. There is a lack of consensus on whether primary (combustion) or secondary (e.g. volatilisation from soil; chemical usage) sources dominate atmospheric emissions, and there are discrepancies between the national/regional emission and deposition estimates. Basic information is still lacking, e.g. there is disagreement about the natural formation mechanisms for PCDD/Fs, which may be responsible for a pre-industrial environmental burden of these compounds, or whether their occurrence in the contemporary environment results purely from industrial activity in the last two hundred years.
Measurements
Environmental organic chemists have access to very sensitive and selective analytical methods for measuring POPs. For example, PCDD/Fs are released into the environment in ultratrace amounts; the national atmospheric emission to the UK is only ~0.5 kg åTEQ annually. If this output were dispersed evenly throughout the UK atmosphere (volume ~1014 m3), air concentrations would be of the order of femtograms per cubic metre. Current high resolution gas chromatography-high resolution mass spectrometry (HRGC-HRMS) techniques allow detection of a few femtograms. Consequently, if air samples of just ~500 m3 are taken, then a wide array of compounds can be detected. Sensitivity at this level can present problems because samples can become contaminated by residues in glassware, solvents, or laboratory air that come into contact with the sample. Modern methods of analysis benefit greatly from the availability of 13C and deuterated analogues of the POPs, making possible precise and sensitive quantitation by isotope dilution-MS techniques.
Because of the great sensitivity of electron capture and MS detectors, POPs can now be routinely detected in the full array of environmental media at trace levels; they are ubiquitous in the modern environment even in areas far removed from sources. However, detection needs to considered in the context of the amount emitted and the persistence of the POP. Detection does not automatically signal that there is a ‘POPs problem’.
Trends and Environmental Recycling of POPs
Many classes of POPs (e.g. PCBs, OC pesticides, chlorobenzenes) are subject to a similar broad trend in their usage/emission to the environment:
- synthesis and development for use earlier in the 1930s/40s,
- increasingly widespread use in Europe/North America and other industrialised regions through the 1950s and 1960s,
- concerns over environmental persistence and food chain accumulation in the late 1960s/early 1970s, resulting in restrictions in usage in Europe and North America,
- reductions in emissions in Europe, North America and other industrialised regions arising from the bans/controls in the 1970s through the 1980s and 1990s.
For some ‘newer’ POPs a similar ‘pattern’ may have been observed, but have been more compressed in time. Examples are the brominated flame-retardants, used extensively in insulating material for electrical equipment and the chlorinated benzyltoluenes used as PCB substitutes.
Recent research on the recycling of POPs indicates:
- primary and secondary sources may both be supplying POPs to the atmosphere,
- dynamic temperature-controlled exchange of the POP can occur between gas and particle phases in the atmosphere,
- dry gaseous, dry particulate and wet deposition processes deliver POP to vegetation, soil and water bodies,
- the rates of exchange and net fluxes across air-surface interfaces can vary spatially and temporally and with compound physico-chemical properties,
- because POP chemicals ‘have time’ in the environment, they strive to approach multi media equilibrium status,
- net losses from the system can occur due to degradation/burial/occlusion in the soil/water body and atmospheric reactions (i.e. photolysis; OH radical reactions), which shift the air-surface equilibrium status.
Regional and global POPs cycling
Air-surface exchange of POPs occurs in response to temperature changes. Persistent, semi-volatile compounds can participate in repeated air-surface exchanges and as a consequence move from one area of the globe to another. Emission to air will tend to occur primarily in ‘global source areas’ where the POP is used or released. For example, DDT usage has been extensive through the tropics and the high temperatures there will mean greater volatilisation rates of DDT than somewhere cooler.
Under cool conditions, gas phase POPs can partition back to the earth’s surface, while in rainy areas wet deposition will deposit POPs to the surface. Rates of re-release from the surface will be slower in colder polar or high altitude regions. Hence, it has been proposed that on a regional or global scale POPs can potentially migrate from warmer to cooler areas and become ‘fractionated’ on latitudinal or altitudinal gradients. Over the last few years there has been an increasing interest in studying and testing this hypothesis of ‘global fractionation’ or ‘cold condensation’ for POPs, with evidence steadily accumulating for its occurrence for selected compounds with a certain range of physico-chemical properties. Indeed, concern has been expressed that the polar regions will become ‘global sinks’ for POPs released/used elsewhere on Earth. The Canadian government has been funding research programmes on Arctic pollution, largely arising from concerns that native people living in the far north and dependent on animal fat-rich diets (marine mammal meat/blubber and fish and vulnerable top predator wildlife species, e.g. wolf, polar bear) are prone to particularly high POPs exposure.
Compounds need to have properties in a certain narrow window to be susceptible to long-range transport and to accumulate in the oceans and land surfaces of the poles. However, it should be appreciated that complex array of processes operate at the global scale to affect the cycling and long-term fate of POPs. Amongst these are:
- ‘atmospheric dilution’, taking chemicals from source regions to ‘clean’ areas where these chemicals have not been widely used,
- physical removal processes, such as burial in soils, peat bogs and sediments, which take a proportion of the global POPs inventory out of the ‘pool’ for re-cycling,
- the physical occlusion and/or formation of bound residues in soils and sediments,
- chemical reaction processes, notably reaction with atmospheric hydroxyl radicals, which will occur at different rates in different parts of the globe,
- biologically mediated degradation in soils, sediments, the water column and the food-chain,
- rate-limitations on the air-surface exchange processes for some POPs.
These factors combine to reduce the threat of accumulation in the polar regions. Indeed the current (very limited) database suggests that whilst global fractionation is occurring, residues of many POPs are declining in polar regions, but at slower rates than in tropical and temperate latitudes. Much more research is required on these issues to fully elucidate the processes involved and to monitor trends.
Attention, then, is beginning to focus on constructing regional and global scale budgets, inventories and models for POPs. These are, of necessity, rather simplistic at present and have substantial gaps in information and data. Foremost amongst the deficiencies of current regional/global cycling models are:
- poor or non-existent information on past/present regional usage/emission of many POPs,
- a lack of understanding of how POPs cycle and become transported in the deep oceans,
- uncertainties over the rate of atmospheric reactions for different POPs, at different heights in the atmosphere and with latitude,
- uncertainties over the rates of biodegradation in real environmental settings,
- a lack of residue data in air, water, soil and sediments for huge areas of the globe, notably in Africa, Asia, the former Soviet Union and China, South America and the oceans.
KEVIN JONES
Lancaster University,
June 2001