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Risks From BSE Via Environmental Pathways

A Summary of Risk Assessment Studies carried out by the Environment Agency

JUNE 1997

This summary document was produced by the Environment Agency.

The summary is published to co-incide with a press conference to publish the results the Agency's work to assess the risks from BSE via environmental pathways on Wednesday 25 June 1997 at the Institution of Civil Engineers in London.

The Environment Agency was assisted in this work by specialist risk assessment consultants DNV Technica.

The Environment Agency is grateful to the Government's independent advisory committee on BSE, the Spongiform Encephalopathy Advisory Committee (SEAC), and to its Chairman Professor John Pattison, for advice in relation to this work.

Requests for further copies of this summary, or for the risk assessment reports referred to in it, should be made by telephone to the Environment Agency on 01454 624400 and asking for public enquiries, or in writing to:

Environment Agency
Rio House
Waterside Drive
Aztec West
Bristol BS12 4UD



1.1 Since March 1996, new measures introduced in the UK to eradicate Bovine Spongiform Encephalopathy (BSE) have resulted in the production of large amounts of cattle-derived waste material. The major contributor to the waste arising is the Government's scheme for culling and disposing of all cattle over the age of 30 months, the 'Over Thirty Month Scheme' (OTMS).

The Environment Agency, as the body responsible for the regulation and management of waste disposal in England and Wales, has been examining potential options for the disposal of these cattle-derived wastes in a manner which is acceptable for both public health and the environment.

As part of this responsibility, the Agency has carried out several assessments to quantify the risk of BSE infection being transmitted to humans as a result of potential means of disposal and their associated practices.

The Agency's risk assessment work incorporates the expert advice of the Government's independent advisory committee, the Spongiform Encephalopathy Advisory Committee (SEAC), and has been carried out in conjunction with specialist risk assessment consultants DNV Technica.

1.2 The Environment Agency has been considering a number of options for disposal of OTMS cattle showing no signs of BSE infection, namely:

incineration of carcasses of OTMS cattle

  • incineration of the products of rendering OTMS cattle ie meat and bonemeal (MBM) and tallow, including the possibility of using coal-fired power stations
  • land filling of OTMS carcasses
  • land filling of MBM

Under regulations laid down by the Ministry of Agriculture, Fisheries and Food, the carcasses of confirmed BSE cases are destroyed by incineration in specially designated animal incinerators. This practice is covered by the Agency's risk assessment.

1.3 The Environment Agency has carried out five separate studies which together provide a basis for assessing the risks of human infection by the BSE agent from the potential disposal options for OTMS material outlined above, and from incineration of confirmed BSE cases. The methodology and risk calculations used are set out in the following reports which are available from the Agency:

An overview of the risks of BSE via environmental pathways This study provides a framework for all the risk assessment work and includes much of the detail of the assumptions and methodology used.

Risks from burning rendered products from the Over Thirty Month Scheme in power stations

Risks from disposing of BSE infected cattle in animal carcase incinerators This study also considers the risks of incinerating 'clean' OTMS carcases.

An assessment of the risk from BSE carcases already in landfill sites

Thruxted Mill rendering plant : risk assessment of waste water disposal options This study was carried out and presented toa Public Inquiry in January 1997 to determine a planning application for a waste water disposal scheme at the plant.

All the above studies are based on the assumption that consumption of a sufficient quantity of BSE infectivity could cause infection in humans. Risk is measured in terms of how close the most exposed individual might come to receiving, in one year, the dose needed to cause infection. In reality, the risk to most individuals will be well below the level assigned to the most exposed person.

Assumptions about the dose needed to cause infection in humans are based on the advice of SEAC. In applying this advice, however, the Agency has erred strongly on the side of caution and assumed the required dose to be lower than even SEAC suggests, and that infectivity can be accumulated in the body although there is no medical evidence to suggest this is likely.

Likewise, very cautious assumptions regarding human behaviour and plant operational practices have been made.

In total, therefore, such assumptions will have produced a higher estimate of risk than will be the case in reality.

1.4 The broad conclusion which the Agency has drawn from its assessments is that, for all of the disposal options considered, the risk of human infection by the BSE agent is extremely small.

In all cases, the results show that in one year the most exposed individual would be unlikely to consume, from environmental sources, more than a minute fraction - significantly less than one millionth part - of the dose of BSE infectivity needed to cause infection in humans. This is equivalent to a risk of less than one in one million, the level which the Chief Medical Officer has suggested may be neglected. By comparison, the risk of dying from cancer is about one in three hundred per year; the risk of being killed in a road accident one in ten thousand per year; and of dying in a railway accident, one in five hundred thousand per year. In reality, the risk to the general public of being infected by BSE from environmental sources will be well below the level assigned to the most exposed individual, and will in all probability be zero.

1.5 In relation to waste disposal, the Environment Agency has concluded, therefore, that there is no significant risk to public health or the environment associated with any of the options considered for the disposal of cattle-derived waste products.

1.6 The results of the Environment Agency's assessment of the risk of BSE infection being transmitted to humans via the waste disposal route are being published to ensure that the basis for any decisions by the Agency regarding waste disposal is clear and in the public domain. The Agency believes the results of these assessments will help to inform the public and all those with an interest in the eradication of BSE and with measures associated with that aim. Interest groups include national and local Government, medical and health experts, regulatory bodies, the waste disposal industry and environmental interest groups.

1.7 The Agency's conclusions do not constitute any form of approval for proposed operations to dispose of cattle-derived waste. Any application for authorisation under the relevant legislation to dispose of such waste will be considered strictly on its individual merits and within the terms of appropriate regulatory controls.

The Agency will also want to take account of other factors such as the potential disposal capacity offered by different disposal routes; likely operational difficulties; and the desirability of energy recovery where it is practicable. The latter view is in line with the requirements of the UK's national waste strategy and the Government's objective of achieving sustainable development.

The Agency will inform, and invite comment from, the public on any application received, and will take all views expressed into account before coming to a decision. The Agency will not grant approval for waste disposal unless it is fully satisfied that all the relevant statutory conditions are met. The Agency will use the results of its risk assessment to inform the decision-making process.

On existing plant, the Agency will require trials to be carried out before considering applications for longer term disposal operations in order to demonstrate that the proposed operation will properly protect the environment and public health. Equally, new plant will require comprehensive commissioning trials. Trials could only proceed after the Agency had issued an authorisation following the procedures laid down by statute.


2.1 Bovine Spongiform Encephalopathy (BSE) is a fatal neurological disease of cattle. The distinctive feature of the disease is the development of sponge-like holes in the brain tissue, leading to a deteriorating mental condition which has prompted the popular term 'mad cow disease'. The incubation period for the disease is typically five years, but can range from 30 months to 10 years

2.2 BSE was first diagnosed in 1985 and the annual number of confirmed cases in the UK reached a peak of 36,700 in 1992. It was at this point that the Government introduced a ban on the inclusion of animal protein in feedstuffs for ruminants (eg cattle) on the basis of epidemiological evidence that BSE was a feed-borne infection, probably caused by the inclusion in cattle-feed of inadequately inactivated material derived from scrapie-infected sheep. (Prior to 1988, it had been the general practice to include meat and bonemeal rendered from the remains of cattle and other mammals in ruminant feedstuffs). Since the introduction of the feed ban the number of confirmed cases of BSE has declined, to 22,000 in 1995, which is consistent with the incubation period for the disease and the introduction of the ban, supporting the evidence that feedstuffs were the source of the infection.

2.3 In March 1996, the Government reported a possible connection between BSE and a new form of the human Spongiform condition Creutzfeldt Jakob Disease (CJD). Concerns over the possible spread of CJD led the European Commission to ban the export of beef and associated products from the UK.

Partly as a consequence of EC requirements that all relevant animal waste be 'destroyed' (EC Regulation 716/96), the UK Government introduced a number of controls, the most significant being the 'Over Thirty Month Scheme' (OTMS). Under the OTMS, all cattle over the age of 30 months from farms in the UK are presently slaughtered and sent for disposal. No material from the OTMS is allowed to enter the food chain.

2.4 The introduction of the OTMS presented a major challenge for the Environment Agency, which has wide ranging statutory responsibilities under the Environmental Protection Act 1990 (EPA 90) for regulating the disposal of waste in Engalnd and Wales in a manner which is acceptable for public health and the environment.

The Agency set up a cross-functional team comprising staff with expertise in water and waste regulation, Integrated Pollution Control (IPC) of large industrial processes (such as incinerators and power stations) and operational management. The team's responsibility has been to evaluate both the regulatory and public issues raised by the BSE control regime.

2.5 As an immediate first step, the Environment Agency identified the relevant operations which might give rise to cattle-derived wastes reaching the environment (eg abattoirs, butchers, rendering plants, land filling operations, incineration plants etc).and sought the advice of SEAC as to their acceptability. Advice published by SEAC in June 1996 (MAFF news release 198/96) to the effect that such practices could continue proved invaluable to the Agency in pursuing its duties in the immediately ensuing period.

The Agency recognised, however, the desirability of being able to present clear data about the risks to public health and the environment associated with the disposal of OTMS waste, and embarked (using advice from SEAC) on the development and application of a detailed methodology to quantify those risks.

The results of this work are helping the Agency to improve its own understanding of relevant waste disposal issues, to inform policy in this area and to enable it to target its regulatory resources to minimise the risk to public health and the environment.

It is the first time, as far as the Agency is aware, that a risk assessment study of this kind has been carried out in relation to BSE.


3.1 The mathematical assessment of any risk relies on the combination of a range of different factors, some of which might be precisely defined or quantified, whilst others must be based on assumptions about the real world, about how people behave, and about how complete or accurate certain pieces of information are.

In the case of BSE, science does not yet have all the answers, and while a possible link between BSE and new variant CJD has been suggested, the medical evidence is not conclusive. For example, the nature of the BSE agent itself is still unclear. It is unlike a conventional virus or bacteria in that it stimulates no immune response in the host, and is relatively resistant to inactivation by heat, chemical disinfection or radiation. Nor is it yet possible to diagnose the disease until the clinical signs appear, or to confirm the diagnosis expect by inspection of brain tissue under the microscope.

The dominant theory is that the BSE agent is a distorted form of 'prion' protein which causes other normal proteins to distort similarly. In this way the infection spreads through the animal from the point of origin, eventually reaching the brain.

3.2 It is against this background of incomplete scientific understanding of BSE that the Environment Agency has approached the task of quantifying the risks of human infection from the disposal of cattle-derived wastes.

The Agency's approach has been an extremely cautious one. The following examples serve to illustrate the conservative nature of the exercise.

  • On the advice of SEAC, it is assumed that the 'species barrier' between cattle and humans (the term is used to describe the relative susceptability to infection of different species) is a factor of 10, with a >range between 1 and 10,000.
  • A 'species barrier' of 1 would assume that it takes the same amount of infectivity to infect humans as cattle, while SEAC's advice that the barrier is 10 assumes a human infective dose 10 times larger than that which would infect cattle.
  • It is assumed that there are, within the OTMS cattle, a number of undiagnosed cases equivalent to approximately 5% of those being presented for slaughter under the scheme, and that each one of these is fully infected. This is far higher incidence of undiagnosed cases than is likely to be the case in reality.
  • It is assumed that rendering reduces infectivity by a factor of 50, although in reality if may be more.
  • That incineration at the high temperatures achieved in incinerators and power stations reduces infectivity by a factor of 1 million, rather than destroying it completely.
  • That the presence of any amino acids in the ash remaining after incineration indicates the presence of prions and hence infectivity.
  • That some material accidentally escapes full treatment.

The Environment Agency believes that the outcome of such a cautious assessment is highly likely to have led to overstatement of the real risks involved.


4.1 An Overview of the Risks of BSE via Environmental Pathways

This study presents an overview of the possible environmental pathways so far identified by which BSE infectivity might be transmitted to humans. The environmental pathways to humans which were identified within the scope of the study can be characterised as:

  • direct ingestion by humans of material from the land eg where cattle waste has fallen or is spread
  • direct ingestion by humans of untreated water from sources contaminated eg by run-off from land-spreading, burial, leachate from landfill sites
  • inhalation of particles in the atmosphere eg resulting from burning or incineration of cattle-derived wastes.

The risk calculations show that the likelihood of the most exposed individual ingesting, in one year, sufficient material to cause infection is less than one in one million and that, in particular, the practices of incineration and land filling offer high and consistent levels of protection. It is important to note that the real risk to the general public will be well below the level of the risk to the most espoxed person.

4.2 Risks from Burning Rendered Products from the Over Thirty Month Scheme in Power Stations

4.2.1 It has been proposed that meat and bonemeal (MBM) and tallow rendered from OTMS cattle might be burned in coal fired power stations, such processes being attractive due to the potentially large incineration capacity and the potential to recover energy.

4.2.2 During the summer of 1996, the two power generators National Power and PowerGen carried out small-scale trials of burning MBM and tallow under test conditions and with strict supervisory control by the Environment Agency.

4.2.3 The normal waste streams from a combustion process such as a power station comprise emissions to the air from the stack and ash from the combustion chamber. In the power station trials, monitoring of the expected stack emissions (eg sulphur dioxide and dioxins) indicated that incineration of MBM and tallow would make no significant difference to emissions compared to normal coal-fired operation of a power station.

4.2.4 At present, no specific analytical technique exists to test for the presence of the BSE prion in any matter. There is no direct method, therefore, of detecting the prion in either emissions or ash. The prion is a type of protein. The best available technique, and the one adopted in the trials, is to analyse the particulate matter from burning and the ash from the combusion chamber for the presence of proteins by looking for certain linked sequences of amino acids.

Amino acids are the essential building blocks of all proteins. The pattern in which the acids are linked together determines the precise nature of the protein. Therefore, by careful analysis of the particulates and ash for amino acids, deductions can be made about the the presence of proteins and, therefore, of prions.

4.2.5 The analyses of ash from the power generators' trials showed that, while there were some amino acids still present, there were no complete sequences of acids. It would have been reasonable to deduce from this finding, then, that no intact proteins, and hence no intact prions, were present in the ash.

Nevertheless, on the advice of SEAC, the Agency has made the cautious assumption in its risk calculation that the presence of amino acids, albeit in incomplete sequences, might indicate the presence of some infectivity in the final ash.

4.2.6 The risk calculation shows that the likelihood of the most exposed individual ingesting sufficient infectivity as a result of the burning of rendered material from the OTMS scheme in power stations (assuming that the entire throughput of the scheme were to be disposed of by this route) is less than one in one billion years. This is equivalent to a risk more than one thousand times less likely than death by lightning. The real risk to the general public, however, will be well below the risk to the most exposed person.

The Environment Agency is formally responsible for regulation of combustion processes only in England and Wales. The Agency's risk study did not specifically consider risks in Scotland or Northern Ireland, but they are considered to be no greater than those reported as a result of this study.

4.2.7 The Environment Agency's study did not consider the risks to power station employees. as this is matter for which the Health and Safety Executive (HSE)has responsibility. The HSE has produced general guidance on occupational hazards associated with BSE materials and is currently preparing more specifice guidance for workers handling MBM in storage. or disposal situations. The Advisory Committee on Dangerous Pathogens has concluded that the risks to those working with MBM will be extremely low.

4.2.8 The results of the trials, including the Agency's environmental monitoring data and the generators' reports of monitoring and ash analyses, have been placed on the Environment Agency public registers.

4.3 Risks from Disposing of BSE-infected Cattle in Animal Carcase Incinerators

4.3.1 Since 1991, it has been the practice to incinerate the carcasses of cattle infected with BSE designated animal carcase incinerators. SEAC, in its advice of June 1996, expressed satisfaction with the practice.

4.3.2 This study uses essentially the same methodology as applied to power station burning rials in order to calculate the risk from burning infected carcasses, assuming all eight existing incinerators authorised for that purpose are in full use. There are a number of differences, however, to account for different material being incinerated, the much lower thoughput of carcase incinerators compared with power stations and the different dispersion characteristics of incinerator stack emissions.

4.3.3 The risk calculation shows that the likelihood of the most exposed individual ingesting, in one year, sufficient material to cause infection as a result of burning infected cattle in specially designated incinerators is less than one in one billion ie the same as the risk associated with power stations burning rendered OTMS material. As in other cases, the real risk to the general public will be well below the level assigned to the most exposed person.

4.3.4 In addition to the practice of burning infected carcasses, it has been proposed by the Intervention Board that OTMS carcasses might be burnt whole in purpose-built incinerators with a typical capacity of one tonne per hour, constructed singly or in groups. The risks associated with this proposed practice have been considered as part of this study, with the risks calculated on the basis of all OTMS cattle being disposed of by this route.

4.3.5 The risk calculation shows that the likelihood of the most exposed individual ingesting, in one year, sufficient material to cause infection as a result of burning OTMS carcasses in incinerators to be less than one in one billion ie the same as the risks associated with burning infected cattle. In reality, the risk to the general public will be well below this level because of lower exposure but also because the Agency has deliberately made extremely cautious assumptions about the number of undiagnosed infected cattle in the OTMS.

4.4 Assessment of the Risk from BSE Carcasses in landfill sites

4.4.1 According to MAFF figures, the carcasses of approximately 6000 BSE-infected cattle were disposed of in 59 landfill sites between 1988 and 1991. Since that time, the practice has been to incinerate infected carcasses as mentioned above. SEAC, in its advice of June 1996, concluded that the practice was likely to be satisfactory and that retrieval of the carcasses for disposal be alternative means would not be justifed.

4.4.2 Earlier this year, the Environment Agency, in order to improve its own understanding of this situation, made a preliminary assessment of the sites involved on the basis of factors such as the degree of containment offered by each site, its proximity to water courses etc. >From this initial investigation the Agency identified the six sites which it considered to be more likely to cause concern, albeit very minor, as the basis for a precautionary risk assessment.

4.4.3 The risk study of these six landfill sites differs from the Agency's other risk studies in that site specific information is available. Given the varying nature of the sites, the report necessarily concludes a range of value The calcultations show that the likelihood of the most exposed individual ingesting, in one year, sufficient material to cause infection as a result of the presence of infected carcasses in the sites assessed range from one in 10,000 million years to one in one million years, depending on local circumstances. In reality, however, the risk to the general public in each case will be well below the level of risk to the most exposed person.

4.4.4 The Agency believes that the risks assessed for these six sites probably represent the full range of the risks posed by all 59 landfill sites. Consideration is being given to whether there is a need to carry out any further detailed risk assessments on the remaining sites in due course.

4.5 Thruxted Mill Rendering Plant: Risk Assessment of Waste Water Disposal Options

4.5.1 Thruxted Mill is an animal rendering plant in Kent which handles OTMS carcasses. Quite separately, the Environment Agency had for some time been seeking improved arrangements for the disposal of waste water from the site. The previous means of disposal had been by spreading on land. A new and substantially better waste treatment system was installed on the site but disposal was to be further improved by changing to a system where the now much cleaner waste to be injected underground, a change which required planning permisssion.

4.5.2 This study, which looked at the risks associated with four different disposal options, was carried out by the Environment Agency to support the planning application, and was presented at a Public Inquiry the decision of which is awaited.

4.5.3 The risk calculations show that, for all four options, the likelihood of the most exposed individual ingesting, in one year, sufficient material to cause infection is of the order of one in one billion. The only risk that could sensibly be taken into account in this case was the risk to water sources, the rest being far too small to be realistically considered.

The real risk to the general public, however, will be well below the level of risk to the most exposed person.

The results of the study supported the Agency's case for abandoning the previous disposal arrangements and moving to the improved injection system.

Environment Agency 25 June 1997

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