Persistent organic pollutants

Persistent organic pollutants (POPs) are found in trace quantities in all areas of the environment. They are all toxic and accumulate in humans and plants. POPs do not readily break down in the environment with half-lives in soils the order of years, although they may be transformed both physically and chemically over long periods.

Over recent years there has been a growing interest in these pollutants and in particular their potential chronic toxicity and impacts on human health. This is reflected by the recent international agreement to reduce releases of these chemicals under the UN/ECE Persistent Organic Pollutants Protocol (detailed above) and their consideration for air quality standards by the Expert Panel for Air Quality Standards (EPAQS). The detailed methodology for the compilation of these inventories depends on the combination of emission factors gathered from a range of sources and production statistics used elsewhere in the emission inventory or developed for the specific sector concerned.

The UK NAEI does not include emission estimates for some POPs since many have been banned in the UK for a number of years. Table 6.2 below indicates the years in which particular POPs were banned or use severely restricted, and whether the listed POPs are included in the NAEI.

Table 6.2 POPs Included/Not Included in the NAEI and Corresponding Year of Ban¹

¹Hexachlorobenzene and pentachlorophenol are also emitted from other sources as well as being or having been active ingredients in pesticides.

² Use of pentachlorophenol is severely restricted rather than banned absolutely.

³NA indicates that a year of ban is not applicable as the compound was never approved for use in the UK as a pesticide.

Polycyclic aromatic hydrocarbons (PAHs)

The speciated PAH inventory was first compiled for the 1996 emissions inventory (see "Speciated PAH Inventory for the UK" Wenborn MJ 1999), and has allowed a more detailed understanding of the PAH emissions in the UK.

There have been several pollutant classifications relating to PAHs. Although there are a vast number of PAHs, the NAEI inventory focuses on sixteen PAHs. These 16 PAHs have been designated by the United States Environmental Protection Agency (USEPA) as compounds of interest under a suggested procedure for reporting test measurement results (USEPA 1988). The estimated emissions for individual compounds are given in Appendix 5. A subset of this includes six of the PAHs identified by the International Agency for Research on Cancer (IARC) as probable or possible human carcinogens (IARC 1987). In addition, the Borneff 6 PAHs have been used in some EC emission inventory compilations. A further subset of PAHs are those to be used as indicators for the purposes of emissions inventories under the UN/ECE’s Persistent Organic Pollutants Protocol. These classifications are give in the following table.

Table 6.3 The USEPA 16 PAH Primary Pollutants, and other PAH Subsets.

The main environmental impact of PAHs relate to their health effects, focusing on their carcinogenic properties. The most potent carcinogens have been shown to be benzo[a]anthracene, benzo[a]pyrene and dibenz[ah]anthracene (APARG 1996). The semi-volatile property of PAHs makes them highly mobile throughout the environment via deposition and re-volatilisation between air, soil and water bodies. It is possible that a proportion of PAHs released in the UK are deposited in the oceans and/or subject to long range transport making them a widespread environmental problem.

EPAQS published in 1999 a recommendation for an air quality standard for PAHs based on the use of benzo[a]pyrene as an indicator of the overall carcinogenicity of the PAHs present in the atmosphere. Emissions of benzo[a]pyrene and the total of the 16 PAH’s are summarised in Table 6.4. Aluminium production and anode baking (carried out for the aluminium industry) is the largest source of PAH emissions in the UK, contributing 64 % of the total emissions of the 16 PAHs in 1995. Emissions from anode baking then fell considerably, comprising only 36% of the 16 PAH emission for 1998. Table 6.4 shows a reduction in emissions of benzo[a]pyrene from anode baking from 44% of the national total in 1995 to 16% in 1998. As a consequence of investment in abatement equipment following from the authorisation regime implementing the Environmental Protection Act 1990, one of the anode baking plants has dramatically reduced its emissions and the other will follow shortly.

Anode baking is currently the largest single source of the emissions for eight of the individual PAHs studied. The largest sources of emission for the remaining eight PAHs considered here are: road transport, domestic coal combustion, domestic wood combustion and creosote production. For many of these compounds, the anode baking source sector still accounts for a significant percentage of the total emission.

Other major sources of all the PAHs are industrial and domestic coal combustion and domestic wood combustion. Collectively vehicles are a significant source of many PAHs with cars burning leaded petrol being the largest subgroup, reflecting the higher emission factors resulting from not having catalysts.

Wood treatment is a significant source of some of the lighter PAHs such as acenapthene, fluorene and anthracene. Emissions from bitumen production and use have not been estimated due to a lack of emission data. It is possible that bitumen use is a significant source of benzo[a]pyrene and other PAHs. Increased measurement of PAHs by both industry and regulators, particularly in the aluminium sector, has allowed improvements in the precision of the emission estimates.

Overall, the high level of uncertainty associated with the PAH inventory is largely due to the lack of actual measurements from potentially significant sources. To significantly improve the quality of the inventory requires the measurement of emission factors and detailed assessment of the processes involved in the following areas:

• road vehicles both diesel and petrol driven

• off road fuel use

• coke production;

• creosote use;

• open burning;

• other domestic and industrial combustion.

Figure 6.1 Spatially Disaggregated UK Emissions of Benzo[a]pyrene

Table 6.4 - Summary of UK Emissions of PAHs (The emissions for individual compounds are given in Appendix 5)

Notes

1. The PAHs selected are listed above in Table 6.3
2. "Aluminium Production" includes production by the pre-baked anode and Soderberg processes.
3. "Anode baking" is the manufacture of carbon anodes for aluminium smelting
4. "Industrial Wood Combustion" includes the combustion of treated and non-treated wood.
5. "Domestic Coal Combustion" includes the use of anthracite, coke and Solid Smokeless Fuel
6. "Industrial Coal Combustion" includes Commercial and Institutional
7. "Other Sources" includes the electricity supply industry, waste incineration and iron & steel works.
8. Benzo[a]pyrene

Dioxins and Furans (PCDD/F)

The term "dioxin" is used to refer to the polychlorinated dibenzo-p-dioxins (PCDD) and "furan" is used for polychlorinated dibenzofurans (PCDF). Although there are 210 PCDD/F compounds in total, the emissions of concern are those of the 17 congeners, 7 PCDDs and 10 PCDFs, of concern as defined by the NATO/CCMS (Committee on the Challenges of Modern Society 1988) international toxic equivalent (I-TEQ) scheme. TEQ schemes weight the toxicity of the less toxic congeners as fractions of the toxicity of 2,3,7,8-TCDD, the most toxic congener. The inventory is in terms of the sum of the weighted emissions expressed as ‘I-TEQs’ which are widely used in UK and European legislation. However more recently the World Health Organisation (WHO) has suggested a modification to the values used to calculate the toxic equivalents for some of the PCDDs and PCDFs. They have also suggested that there is value in using a similar approach for the PCBs which have dioxin like toxicity and combining the PCDD/F and PCB TEQs together. This will be included in the 1999 NAEI dataset. The International and the WHO toxic equivalence factors (TEFs) for PCDD/Fs are shown in Table 6.5

Table 6.5 The International and the WHO Toxic Equivalence Factors for PCDD/Fs

(the differences are highlighted)

¹ NATO/CCMS (1988)

² WHO (1998)

PCDD/Fs have been shown to possess a number of toxicological properties. The major concern is centred on their possible role in immunological and reproductive effects. The main sources of PCDD/Fs are thermal processes, but they can also be released to the environment from some chemical processes.

PCDD/Fs can arise from any thermal process where chlorine, in any form is present. For example, coal and other solid fuels contain trace amounts of chlorine compounds which can under certain combustion conditions result in PCDD/F formation. In addition PCDD/Fs can themselves be present in the feed stock material, or chlorinated impurities may be introduced into the feed stock of some thermal processes. The amount of chlorine required for PCDD/F formation may be small and consequently many processes have the potential to emit these pollutants. PCDD/Fs can also be emitted from the chemical production and use of polychlorinated aromatic pesticides and herbicides, many of which are now controlled. However, some chlorinated organic chemicals such as the wood preservative pentachlorophenol are still used in the UK and these have the potential to be sources of PCDD/Fs e.g. from the combustion of treated wood.

The PCDD/F emission estimates for 1990-1995 are derived from emission factors which have been revised to reflect changes in the degree of implementation of abatement and of industry structure as a result of measures taken by industry.

The estimated PCDD/F emissions for 1990-1998 are summarised in Table 6.6 below.

Table 6.6 UK emissions of PCDD/Fs (grammes TEQ/year)

¹ "Coal Combustion- Industrial" includes coke

² "Coal Combustion- Domestic" includes Solid Smokeless Fuel

³ "Chemical Industry" includes fuel combustion

⁴ "MSW Incineration" includes incineration for power and as waste treatment

Previously the largest sources of PCDD/F emission was waste incineration. However, emissions have fallen rapidly from 1993 to 1998. As a result, the percentage contribution from MSW incineration to the total emission has fallen from 60% to 4% for 1993 and 1998 respectively. This significant trend has been driven by the introduction of control measures. MSW incinerators not meeting the new standards closed in the period leading up to December 1996. New designs of MSW incinerator result in lower levels of PCDD/F emissions.

The relatively low emissions from chemical incinerators reflects the use of rotary kilns and the incorporation of a secondary combustion chamber in the process to destroy organic contaminants together with the relatively low waste throughput and advanced pollution abatement equipment. Clinical waste incineration remains a significant source, contributing some 7% to the total emission. This is due to the fact that emissions from clinical waste incinerators (although showing significant reductions) have not been reducing as rapidly as the total PCDD/F total.

Emissions from power stations are fairly low because the combustion is efficient and the post combustion fly ash temperatures are rapidly reduced. The emission factors associated with industrial and domestic coal combustion are significantly higher and result in a contribution of 1% and 4% respectively from these source sectors even though the coal consumption is less. However, emissions from all three sectors have decreased with the reduction in the quantity of coal burned.

"Accidental fires and open agricultural burning" has greatly decreased since the cessation of most agricultural burning. Accidental fires now account for the great majority of this source sector. Accidental fires are currently treated as a source of constant magnitude, and consequently, the percentage contribution from this sector to the total PCDD/F emission has risen as emissions from other significant sectors have decreased. A study is presently underway to improve the emission estimates from accidental fires.

There are significant emissions from sinter plants owing more to the large gas volumes emitted than to high concentrations. Emissions from iron and steel plant are probably underestimated since only electric arc furnaces are considered. Scrap used in electric arc furnaces and secondary non-ferrous metal production will contain chlorinated impurities such as plastics and cutting oil which contribute to PCDD/F formation.

It is generally accepted that the source of PCDD/F emissions from road transport are the 1,2-dichloroethane scavengers added to leaded petrol. Over recent years both the consumption of leaded petrol, and the lead content of leaded petrol has decreased. Consequently the emissions of PCDD/F from this sector have decreased. Unleaded petrol and diesel is likely to contain only trace quantities of chlorinated impurities. The contribution in 1998 to PCDD/F emissions from road transport was 3%.

The priority areas to further improve the PCDD/F emission inventory are :

• non-ferrous metal production;
• chemical industry
• accidental fires
• wood combustion
• iron and steel

Polychlorinated biphenyls (PCBs)

PCBs are synthetic organic compounds that have mainly been used in electrical equipment as dielectric insulating media.

PCBs have been linked with subtle sub-chronic effects such as reduced male fertility and long-term behavioural and learning impairment- they are classified as probably carcinogenic to humans. Certain PCBs have been assessed as having dioxin-like effects. PCBs are extremely persistent in the environment and possess the ability to accumulate in the food chain. These compounds are highly insoluble in water but accumulate in body fat. Present human exposure is probably dominated by the accumulation through the food chain of the PCBs present in environmental reservoirs such as soils and sediments as a result of previous releases to the environment.

PCBs have not been manufactured and used in the UK for many years, but old PCB-containing equipment still exist. It is estimated that 81% of primary PCB emissions to the atmosphere are associated with such appliances. These emissions primarily arise from in-service appliances, however emissions during disposal are also considered to be significant. Large quantities of PCBs are thought to have been disposed of to landfill in the past, mainly in the form of electrical components or fragmentiser residues, but now such equipment containing PCBs are disposed of by chemical incineration. PCBs are also emitted from the soil having previously been deposited there from the air.

PCB speciation has been incorporated into the 1998 emission estimates for the first time. A summary of the total PCB emission estimates for 1990 to 1998 is given below in Table 6.7 (detailed emission estimates are give in Appendix 6). A TEQ for PCBs will be included in the 199 NAEI dataset (see Section 6.2.2 for TEQs).

Table 6.7 - Summary of PCB Emissions in the UK 1990-1998 (kg)

The emission inventory for PCBs is very uncertain as the information on the quantity of PCBs in electrical equipment is based on an estimate. In addition, leakage rate from these dominating sources is also estimated.

Sales of PCBs in the UK were stopped in 1986, though it is thought that they are still manufactured in some countries. The total PCB emission in 1990 was dominated by leaks from capacitors (89% of total emission), and this is the case for 1998 (80% contribution). However, not all electrical equipment containing PCBs is readily identifiable. Emissions from electrical equipment will probably continue, and will only start to fall significantly as the relevant electrical equipment is either destroyed or reaches the end of its working life.

In 1997 an Action Plan was published by DETR which laid out how the commitments made by the UK at the Third International North Sea Conference at the Hague in 1991 and in accordance with the requirements of Directive 96/59/EC. Regulations were recently introduced which will implement the Action Plan and require all holders of remaining PCB stocks in the UK to report their stocks to the relevant regulatory body by the end of July 2000. These stocks except for certain exemptions must be destroyed by the end of December 2000.

PCBs can be formed in trace amounts from chlorinated precursors in thermal processes such as scrap metal recycling. As a result, there are significant emissions from the iron and steel industrial sector, as with PCDD/Fs.

PCBs occur in sewage sludge due to their persistent nature. Not all the PCBs spread on land will volatilise but the potential for emissions to air is greater than that of landfill. The emission estimate comprises only 1% of the total and is highly uncertain. Emissions arise from waste incineration and refuse derived fuel production result from the PCB content of the waste.

Pesticide Emissions

There is little available information to enable accurate estimates of pesticide emissions to air. The emissions estimates presented here follow from significant improvements to the earlier emission estimates first made in 1996.

Despite these improvements, the confidence in the accuracy of these estimates is low. Relevant data is currently scarce with the majority of emission factors coming from the USA or Europe. The emissions factors used here will have been derived for a particular method of pesticide application and during atmospheric conditions which may not be representative of the situation in the UK. Until further data becomes available it is difficult to improve the uncertainty associated with these estimates.

Pesticide emissions to the air occur predominately through three pathways: during manufacture, during application and volatilisation after application. Tables 6.8, 6.9 and 6.10 show the estimated emissions of lindane (gamma-HCH), pentachlorophenol (PCP) and hexachlorobenzene (HCB) respectively.

Lindane (g HCH)

Lindane is applied as an insecticide and fungicide in agriculture and is used for wood preservation and in domestic and veterinary formulations. Until 1990 lindane was also used as a remedial wood treatment i.e. in a curative role rather than a preservative/preventative. However, data on quantities used for a remedial wood treatment prior to 1990 are not available.

HCH exists in several isomers, however as a result of regulation in the UK, gamma-HCH accounts for more than 99% of the total HCH use. Consequently only the gamma isomer has been considered in any detail here. The emission estimates presented in Table 6.8a were made assuming that emissions arise from: the application of g-HCH, treated wood and agricultural and domestic use. g-HCH emissions are dominated by emissions from treated wood and wood preserving sources, contributing 68% and 13% to the 1998 total emission respectively. Emissions from wood preserving are expected to fall.

Emissions from agricultural activities are also significant, accounting for around 17% of total 1998 g-HCH emissions. These emissions are based on statistics giving the use of pesticides containing lindane, obtained from the Pesticide Usage Survey Group (MAFF, 1991a,b,c; 1992a,b,c,d) The emission factors used are taken from van der Most et al (1989).

Emissions of g-HCH arising from domestic applications are thought to be comparatively small. However, usage statistics are scarce and were only available for 1988 ( DOE, 1989). Emission factors are taken from van der Most et al (1989).

Table 6.8a - Summary of g-HCH Emissions in the UK 1990-1998 (tonnes)

For completeness, the total emissions of HCH are also included here (see Table 6.8b below), although the differences are obscured due to rounding. These total HCH emissions estimates assume the worst case scenario of 1% contribution from non g isomers to the HCH total:-

Table 6.8b - Summary of Total HCH Emissions in the UK 1990-1998 (tonnes)

Pentachlorophenol (PCP)

Pentachlorophenol is used as a biocide, and is effective in destroying insect eggs. It is used in the timber and textile industries. The emission estimates given here also include emissions of sodium pentachlorophenoxide (NaPCP) and pentachlorophenyl laureate (PCPL) as well as PCP since these are also included in the proprietary formulations.

The estimated PCP emissions for 1990 to 1997 are given in Table 6.9. The largest percentage contribution to the total 1997 PCP emission arises from wood that has been treated within the last 15 years. This accounts for some 80% of the 1997 total PCP emission.

Once again it is very difficult to be certain of these estimates due to the lack of research into emission rates and limited knowledge of quantities used both in the year of the estimate and in previous years. An emission factor of 3% of the wood content per year was used- the same method used for lindane.

PCP emissions from the textile industry primarily arise from volatilisation during application as a cotton preservative. Emission factors used were based on a study of PCP emissions in the UK (Wild, 1992) who report that approximately 30% of the applied PCP is lost through volatilisation. Emissions from this sector are comparatively small.

PCP is used in the agricultural sector as the active ingredient in disinfecting wooden trays used in mushroom farming. Usage statistics are reliable coming from the Pesticide Usage Survey Group (MAFF, 1991a,b,c; 1992a,b,c,d). The emission factor assumes 30% loss due to volatilisation (Wild, 1992). Emissions from this sector are comparatively small.

Table 6.9 - Summary of PCP Emissions in the UK 1990-1998 (tonnes)

The emission inventory for PCP is very uncertain as limited emission factors are available on the release of PCP during agricultural activities and statistics are not actively collected on the extent of its usage. There is some data on release of PCP from combustion processes, but the available studies are not consistent with each other suggesting that the uncertainty may be considerable. However combustion processes are not significant sources.

Hexachlorobenzene (HCB)

Studies in the USA have identified two main industrial sources of HCB (Mumma et al, 1975) (Jacoff et al, 1986). These are the manufacture of chlorinated solvents (e.g. trichloroethylene, tetrachloroethylene and carbon tetrachloride) and the manufacture of specific pesticides where HCB remains as an impurity. HCB emissions can also result from the use of hexachloroethane tablets as a degassing agent in secondary aluminium melting (van der Most et al, 1992). HCB emissions may also arise from combustion sources, but other than waste incineration these could not be estimated though they are believed to be small.

Statistics for chlorinated solvent production in the UK are commercially confidential, hence estimates were made based on UK solvent usage data from the Solvent Industries Association and import and export statistics. Emission factors were taken from van der Most et al (1989).

Although there is no UK manufacture of pesticides that results in the production of HCB, pesticides with HCB as an impurity are still imported and used in the UK for agricultural pest control. Statistics for the use of these pesticides is provided by the Pesticide Usage Survey Group (MAFF, 1991a,b,c; 1992a,b,c,d). Emission factors used are from van der Most et al 1989.

HCB emissions in secondary aluminium melting result from the use of hexachloroethane (HCE) tablets as a degassing agent. Regulations now control the use of HCE and so very little secondary aluminium is now melted using HCE. Estimates of the quantity of degassing agent supplied and industrial expert estimates of the quantity of HCE used per tonne of aluminium melted were used to estimate the total aluminium melted using HCE. Emission factors used are from van der Most et al (1989).

Emissions from chlorinated solvent production and pesticide application are the most significant sources in the UK (Table 6.10) and in 1998 were estimated to account for 26% and 62%, respectively, of the total HCB emissions. This represents a significant change in the relative contributions to the total for 1996 where the same sectors contributed 46% and 45% respectively. This change is a direct result of the reduced emissions from the production of chlorinated solvents, however only very small changes are noted between 1997 and 1998.

Table 6.10 - Summary of HCB Emissions in the UK 1990-1998 (tonnes)

Accuracy of POPs inventories

There are major uncertainties in the POPs emission inventories due to the lack of source measurements, inconsistencies in those measurements made and the lack of collection of frequent pesticide usage quantities. In general there is greater confidence in the percentage change in emissions from 1990 -1997 than in the individual estimates. In general emission estimates of persistent organic pollutants are of no more than order of magnitude accuracy.