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 5
Risk screening and prioritisation
5.1 Background
Setting priorities is important for decision-making. In environmental risk assessment and management, prioritisation may be undertaken at several stages. In the initial stages, hazards may have to be scored and ranked to prioritise those that are of most concern. Later, risks and risk management options may be scored and ranked to identify priorities for further risk assessments and for risk management decisions.
Screening and prioritisation can be applied at all levels of risk assessment and management, and across a diverse range of activities which may impact on the environment. Given the wide variety of uses, there is no single ranking or prioritisation system appropriate to all applications in environmental risk management. Nevertheless, these guidelines aim to promote consistency across a broad range of activities by highlighting the common principles of priority-setting. Priority-setting can help to promote transparency in decision-making by ensuring an explicit and justifiable basis for those decisions.
5.2 Why screen and prioritise?
In general, screening will be used to determine which hazards or risks should be investigated in more detail. Ranking each of these, based on their screening scores, will provide a priority list for further action.
In the past there has been a tendency to apply quantitative methods at the outset of a risk assessment, and thereby miss issues that are difficult to quantify. The ability to consistently screen all risks for a given problem is therefore vitally important. Risk screening (Tier 1 of the framework for environmental risk assessment in Figure 2.1) to identify and subsequently prioritise relevant risks helps to minimise unnecessary effort and reduces the chance of potentially important risks being overlooked. It also provides an auditable trail to support or explain the omission of certain risks from further consideration.
5.3 Key criteria for risk screening
Various approaches to risk screening have been developed both in the health and safety field and for environmental risk. Although they differ in their structure and the measures used to determine the priority of any risk, the key elements of the screening process reflect the framework for a full risk assessment as described in Chapter 2, but are quantified in much less detail.
Identification and magnitude of consequences
Characterising the nature of the hazard requires a consistent measure to be used and usually reflects the importance of the hazard in relation to others. For example, where the hazard is a chemical, its relative toxicity to the likely receptor organisms might be an appropriate measure. A very swift inundation by flood waters may rank higher than a gradual rise in water levels.
Exposure may not always follow on from a hazard. Screening and prioritisation can be based on an initial evaluation of likely pathways between source and effect. Factors such as the strength and direction of the wind in relation to an atmospheric release, the ability to evacuate homes in advance of flooding, and the ability of fish to move away from zones of polluted water will all affect the probability of exposure.
Probability of consequences
The likelihood of the hazard being realised can be roughly estimated using coarse indicators. For instance, the effectiveness of existing flood defences and typical meteorological conditions could be used to predict the probability of a flood.
Significance of the risk
This reflects the harm that may result if exposure to the hazard actually occurs. The screening of impacts or consequences should take account of their nature, geographical extent, timing and duration and their likely importance. Likely public concern (Section 3.3) should also be considered.
5.4 Methods for risk screening and prioritising
Depending on the risks in question, different methods for screening and prioritisation can be applied. The key to effective screening and prioritisation is consistency and transparency of approach.
Numerical approaches
Scoring systems and scales (eg low (1) to high (5)) should be designed with reference to the factors outlined in Section 5.3 and must be appropriate and meaningful to the application under study. The overall score for the risk is the product of each criterion score. The data and information used to assign scores can come from a variety of sources:
- experience of the same issue;
- experience of similar issues;
- experience elsewhere in the world (eg generic information); and
- worst-case scenario estimates.
For some environmental problems, the complexity of the issues to be addressed may be considerable. This is particularly true when risk assessment is employed to assess policy-level issues or where the sources of risk are diverse. Sometimes there may be no prior experience on which to base risk judgements and worst-case assumptions have to be made. In such cases, it may be difficult to determine the scores that should be assigned to each of the criteria listed in Section 5.3, and another approach needs to be adopted.
Expert judgement and preference elicitation have been used by many organisations as a way of screening risks and prioritising future work. The Warwick Risk Initiative has developed techniques which have subsequently been employed by the Environment Agency in screening risks to the environment from road transport. The technique involves a panel of people scoring each risk through a structured discussion. Expert groups, such as the Advisory Committee on Releases to the Environment (ACRE), are regularly used by Government to advise on the priority that should be assigned to particular risks.
5.5 Prioritising effort
Risk screening and subsequent prioritisation has a number of benefits:
- it clearly identifies why some risks will not be investigated further;
- it identifies some risks where action, as opposed to any further investigation, may be preferable; and
- it prioritises resources for the subsequent stages of risk assessment.
It is important that risks identified through screening processes as being of low priority are not discarded entirely from the remainder of the process. For instance, a future risk management option targeted at a high priority risk may also reduce risks of lesser priority. The value of this option would therefore be increased. Equally, some risk management options may worsen low priority risks.
There may be situations in which the cost of carrying out the required risk assessment would exceed the expected benefits of the intention. If this is still the case after taking all reasonable steps to reduce the costs of the risk assessment to a minimum, and taking account of the full socio-economic value of the intention, then the sensible course of action would be to decide not to proceed with the intention.
5.6 Further information
Key references
Health and Safety Executive (1997) Risk Ranking (Contract Research
Report 131/1997), Sudbury, UK, HSE Books
Presents a review of methods available for comparing and ranking risks
for priority setting in decision-making.
Swanson MB & Socha AC (1997) Chemical Ranking and Scoring:
Guidelines for Relative Assessment of Chemicals, Pensacola FL, USA,
Society of Environmental Toxicology and Chemistry
Summarises the discussions of a workshop which focused on the science
of chemical ranking. The publication focuses on measures of exposure,
ecological effects, human health effects and other chemical characteristics
to consider in ranking and scoring.
Electronic information sources
Warwick Risk Initiative internet site - www.warwick.ac.uk/statsdept/risk/index.html
Key periodicals
Risk Analysis
Defra is not responsible for the contents or reliability of the linked web sites and does not necessarily endorse the views expressed within them. Listing should not be taken as endorsement of any kind. We cannot guarantee that these links will work all of the time and we have no control over the availability of the linked pages.
Page published 2 August
2000;
Page last modified
19 September, 2002