Guidelines for Environmental Risk Assessment and Management
[This document refers, in a number of instances, to the then Department of the Environment, Transport and the Regions (DETR). The text of this document has not been updated since the transfer of environmental protection functions to Defra.]
Chapter 2
A framework for environmental risk assessment and management
2.1 An overall framework
A pragmatic approach to environmental risk assessment can transform what may sometimes appear to be an extremely detailed, complex and resource-intensive process into a practical aid to decision-making. Figure 2.1 provides a framework for a tiered approach to environmental risk assessment and management where the level of effort put into assessing each risk is proportionate to its priority (in relation to other risks) and its complexity (in relation to an understanding of the likely impacts). This framework, the principal elements of which are described in more detail in Chapters 4 to 8, also illustrates:
- the importance of correctly defining the actual problem at hand;
- the need to screen and prioritise all risks before quantification;
- the need to consider all risks in the options appraisal stage; and
- the iterative nature of the process.
The remainder of this chapter sets out some of the generic aspects of this framework, and in particular those elements which need to be considered at a number of points in the risk assessment process, albeit at differing levels of detail.
2.2 Key stages in each tier of environmental risk assessment
Stage 1: Hazard identification
These guidelines define hazard as a property or situation that in particular circumstances could lead to harm. This may be determined by properties or circumstances and could include, for example, the release of chlorofluorocarbons (CFCs), a tidal surge along a stretch of the coast, a dry summer leading to low river flows, or the planting of a genetically modified crop. Where risk assessment is to be applied at the policy level, the hazard may be as broad as the adverse impacts of road transport on the environment, or the adverse impacts of induced climate change from the contribution of fossil fuel-derived carbon dioxide emissions.
The identification of hazards, both in the problem formulation stage (Chapter 4), and in subsequent tiers in the process, will have an important bearing on the breadth of the overall assessment and the credibility of the final output.
One common pitfall in establishing the hazard is to overlook secondary hazards that may arise. For example, during a river flood, sediments may be deposited on agricultural land in the flood plain. If these sediments were to be contaminated, they might pose an additional hazard.
Stage 2: Identification of consequences
The potential consequences that may arise from any given hazard are inherent to that hazard. Although the full range of potential consequences must be considered at this stage, no account is taken of likely exposure and therefore likely consequences. For example, while the potential consequences of a discharge of toxic metals to a watercourse may be self-evident, a flood may have additional, non-obvious consequences such as pollution arising from an over-stretched sewerage system, or loss of habitats due to river scouring.
These examples serve to highlight why it is necessary to take a broad look at the potential environmental damage that may occur, if only to be clear why some potential consequences are rejected for further assessment.
Stage 3: Estimation of the magnitude of consequences
The consequences of a particular hazard may be actual or potential harm to human health, property or the natural environment (the issue of probability of occurrence is covered below). The magnitude of such consequences can be determined in different ways depending on whether they are being considered as part of a risk screening process, or as part of a more detailed quantification of risk. At all stages of risk assessment several key features need to be considered, as described below.
The spatial scale of the consequences
The geographical scale of harm resulting from an environmental impact will often extend considerably beyond the boundaries of the source of the hazard. Failure to consider this at an early stage may result in the scope of the risk assessment being too limited. For example, a major accident in a chemical plant is likely to have significant effects on the environment well beyond the perimeter of the site.
The temporal scale of the consequences
The duration of the harm that results may raise issues of intergenerational equity (see Section 1.5), or may be so prolonged that the damage can be assumed to be permanent and the environment beyond recovery. For example, should the release of a genetically modified crop result in extensive cross-breeding with adjacent indigenous flora, any harmful environmental impacts could extend far into the future.
The time to onset of the consequences
A further factor to consider is how quickly harmful effects might be seen. Standard economic techniques tend to discount impacts that will happen in the future but sustainable development emphasises the need to protect the interests of future generations. Risk assessment and management must therefore pay as much attention to long-term problems as to the more immediate risks. For example, the spillage of a solvent on porous ground may not result in an impact on the underlying aquifer for decades. Once realised, however, the duration of the harm is likely to be of the order of decades and will compromise the value of that aquifer as a source of water for future generations.
The ability to forecast the time-scale and magnitude of the environmental impact through robust and long-term modelling is therefore valuable, particularly at the quantifiable end of the risk spectrum.
Stage 4: Estimation of the probability of the consequences
All stages to this point have assumed that realisation of the hazard will lead to environmental harm. However, the probability of the consequences occurring must also be taken into account. This has three components:
The probability of the hazard occurring
The probability of the receptors being exposed to the hazard
The probability of harm resulting from exposure to the hazard
The probability of the hazard occurring
Depending on the circumstances, assigning probabilities may be quite straightforward or may require some sophistication in approach. For example, at a screening level, it might be as simple as stating, on the basis of experience, that on a scale of 1 (low) to 5 (high) a pin-hole leak in a particular pipe in a chemical plant has a probability of, say, 4. Floods can be categorised by their return period (eg one in a hundred years) based on historical records. On the other hand, there will be situations in which it is necessary to assign a probability distribution to the likelihood of the event occurring - for example, that a non-genetically modified crop will be widely pollinated by a genetically modified crop. In many instances this information can be obtained from monitoring data, or based on 'worst-case' or 'reasonable worst-case' scenario estimates.
The probability of the receptors being exposed to the hazard
It is important to establish, at an early stage in the process, whether or not a pathway exists between the hazard and the receptor. If it can be shown that no actual or potential connection exists, then the risk requires no further attention. For example, soil contamination will not pose a risk to farm animals if the land is not used for agricultural purposes. But care is needed not to overlook less obvious pathways, or changes in future circumstances.
Having established one or more pathways, the degree of exposure via those pathways should be quantified. A range of factors will affect the probability and degree of exposure. For example, the exposure of a receptor to an atmospheric emission of sulphur dioxide will depend on the direction and strength of the prevailing wind at the time of release. The impact of a coastal flood in a tourist area may be dictated by the time of the year at which the flood occurs; the loss of property may be greater in summer when caravan parks are occupied than during the winter season when occupancy is likely to be low.
The probability of harm resulting from exposure to the hazard
Even following exposure, the likelihood of harm resulting is probabilistic and will depend on the likely susceptibility of an individual receptor to the hazard and the amount and duration of exposure. This is often simplified in terms of a dose-response relationship, which directly relates exposure to the magnitude of harm for certain receptor types. Such relationships frequently embody 'safety' or uncertainty factors to account for the extrapolation of data from experimental or generalised studies. In flood damage assessment, for example, standard depth-damage curves are used to relate the depth of flood waters to the amount of damage sustained by a building or its contents, again according to the duration of exposure to the flood waters. These relationships simplify the probabilistic nature of harm, because for any exposure, the likelihood of harm at a certain magnitude will be dependent on many individual factors. Few risk assessments allow for this level of sophistication, and the magnitude of harm is usually taken as a direct result of exposure.
Stage 5: Evaluating the significance of a risk
This stage is often referred to as risk characterisation, although this terminology tends to hide the true goal of the activities involved. Having determined the probability and magnitude of the consequences that may arise as a result of the hazard, it is important to place them in some sort of context. It is at this point, therefore, that some value judgements are made, either through reference to some pre-existing measure, such as a toxicological threshold, environmental quality standard or flood defence standard, or by reference to social, ethical, or political standards. In some circumstances, a formalised quantitative approach to determining significance may be possible, for example the tolerability of risk (TOR) framework developed by the Health and Safety Executive. In other instances, the risks of various options might be compared against one another.
2.3 Options appraisal
Having estimated the magnitude and the significance of the risks posed by the hazard(s), the options for risk management are identified and evaluated. It is important to carry out this procedure as a distinct preliminary step because ill-considered risk management strategies may otherwise result in wasted effort and expenditure on the part of the decision-maker. Options appraisal provides a framework for doing this (Chapter 8). The options that will usually be available are:
- exploring with society the acceptability, or otherwise, of the risk - this can include rejecting unacceptable risks altogether or accepting the risk being imposed;
- reducing the hazard through new technology, procedures or investment; and
- mitigating the effects, through improved environmental management techniques.
The decision on precisely which option or combination of options to choose will involve a balance of risk reduction, costs, benefits and social considerations. These issues are discussed in greater detail in Chapters 7 and 8.
Page published
2 August 2000;
Page last modified
19 October, 2002