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close this bookConducting Environmental Impact Assessment in Developing Countries (United Nations University, 1999, 375 p.)
close this folder9. Emerging developments in EIA
close this folder9.4 Environmental risk assessments
View the document9.4.1 What is environmental risk assessment?
Open this folder and view contents9.4.2 Terminology associated with ERA
View the document9.4.3 ERA and the project cycle
View the document9.4.4 ERA builds upon EIA
View the document9.4.5 Basic approach to ERA
View the document9.4.6 Characterization of risk
View the document9.4.7 Risk comparison
View the document9.4.8 Quantitative risk assessments
View the document9.4.9 Risk communication
View the document9.4.10 Risk management
Open this folder and view contents9.4.11 Guidelines for disaster management planning

9.4.7 Risk comparison

Risk assessments typically involve analysis of a specific substance or problem area. Another way to use risk assessment is to compare risks from multiple problems. Various expressions allow for comparison of risks during comparative risk assessment.

• Probability of frequency of events causing one or more immediate fatalities. One of the most valuable potential contributions of ERA is an idea of the range of possible outcomes and their respective probabilities. Although it is possible to present results in the form of a mean or medium, giving some idea of the distribution is also important in most cases.

• Chance of death for an individual within a specified population in each year, or risk of death per person per year.

• Number of deaths from lifetime exposure. This is expressed as excess deaths per million persons. For example, the chance of a person in the United States dying from cancer is 1 in 4 or 250,000 in 1 million. A lifetime exposure to a carcinogen can be predicted from cancer potency data (derived from animal tests) to cause, say, 10 additional deaths per million (105 risk) for a total of 250,010 deaths per million.

• Loss of life expectancy considers the age at which death occurs.

• Deaths per tonne of product, or per facility, are expressions that, while often encountered, are not recommended.

There is, of course, no risk-free lifestyle or location, but often the natural background level of risks is unrecognized. If the new risk is relatively small in relation to that already and unavoidably accepted, then a more "correct'' perception may occur. The number of days per year that an air pollutant exceeds the ambient air quality standard for the project site may be 10 at present and 15 after the project begins operation. The reduction of better-than-standard days from 355 to 350 offers a useful comparison. If the excess cancer deaths or days of pollution were obviously nontrivial, the comparisons would also serve to more closely correlate perception and objective assessment results more closely.

Risks of alternative technologies, sites, or projects that might provide the same economic growth benefits are a useful comparison. For example, a hydroelectric dam could be compared with a coal-fired power plant as to the different risks to human health and the environment. Improving a rail transport system for hazardous chemicals could be compared to truck transport over highways. In the case of a pulp and paper mill, the disposal of organic wastes may be compared in three ways: oxidation ponds, discharge to a stream, and use as irrigation water.

Familiar risks are an important reference point in interpreting new risks but only if they are truly comparable (i.e., they must not compare risks that have opposite aspects of the psychological factors previously mentioned, such as voluntary/involuntary). Comparison of the same risk at two different times is particularly effective (e.g., before and after installation of some pollution abatement equipment). If a standard exists, the new risk can be expressed as a percentage of the allowable amount of contaminant. If the same industrial process is used at another location, the actual experience can be used as a comparison for the predicted new risk. Risks from pesticide residues in food can be compared with risks from naturally occurring carcinogens such as aflatoxin in peanuts.

Benefits of development projects are usually so large and obvious that the accompanying risk may be acceptable if those at risk share the benefits. The risks to health and environment of not developing are also great (i.e., continued poverty, workplace risks elsewhere). The risks of a dam for irrigation should be compared with the benefits of additional agricultural production. The risks from sulphur oxide particulate pollutants from a coal-fired power plant contrast with the benefits of increased electricity. The risks of improved roads should be offset by the benefits of productivity and communication.

A final, useful type of comparison is with the costs and benefits of risk reduction. Risk assessment will usually recommend changes in the project in order to lower unacceptable risks. Where this decision is made, the remaining task is to find the most cost-effective means of achieving the goal. Usually, risk levels are sought that provide the lowest total or nett risk for the least cost: a sort of optimum allocation of risk-reduction efforts among different sources and target populations.