Vulnerable and susceptible groups
The populations studied as part of integrated environmental health impact assessments are never wholly uniform. How the environment impacts on the people concerned is therefore likely to vary, depending on their personal characteristics and situations. Ignoring these variations can be dangerous, both because it can lead to errors in the assessment and, more specifically, can mean that the differences in impact that occur across the population (including, in some cases, contrasts between benefits and losses) are not recognised.
Three terms are often used, sometimes interchangeably, to describe the personal and contextual factors that contribute to these variations: sensitivity, susceptibility and vulnerability. To avoid the ambiguity that thus arises, it is helpful to use only the first two terms - vulnerability and susceptibility - and to define these carefully.
Vulnerability
Vulnerability refers to the variations in exposure between individuals or groups - and thus to the potential for health effects. To a large extent, this is likely to be due to variations in the hazards themselves. However, exposure is also a function of where people live, how (and where) they spend their time, and their more general lifestyle. In the case of air pollution, for example, living near busy roads or spending long hours in transport environments tends to increase vulnerability. In the case of pesticides, living close to areas where crops are sprayed or eating foodstuffs that have been heavily treated during production, storage or processing likewise acts to increase exposure, and thus vulnerability.
In contrast, susceptibility refers to the degree to which individuals or groups may respond to a given exposure to a hazard. This can be subdivided into innate and acquired susceptibility. Innate susceptibility is to a large extent due to genetic predisposition or to incomplete development of normal (adult) physiological functions. For example, a young child may be susceptible to a given pollutant because detoxification processes are not yet fully developed. Such susceptibility is transient and disappears with age and growth. Acquired susceptibility may be due to disease, age or socioeconomic status (though It should be noted that 'socioeconomic status' is not a precise identification of a causal factor). A number of mechanisms are known to play a part (see the detailed report on Susceptible Groups, under See also).
Dealing with vulnerability and susceptibility in health impact assessments
The importance of vulnerability and susceptibility in determining the health effects of exposures to environmental hazards, such as polluants, means that they need to be taken into account in all impact assessments. This has various implications for the way in which the assessment is designed. In particular:
- study population should be segregated into appropriate sub-population groups, which can be separately analysed (e.g. on the basis of age, gender or socio-economic status);
- relevant information on factors affecting vulnerability should be available and incorporated into the assessment (e.g. by taking account of behaviour and time-activity patterns);
- relevant information on the factors affecting susceptibility should likewise be available, and incorporated (e.g. by using exposure-response functions specific to each sub-population group);
- outcomes of the assessment should be reported for the different sub-population groups, as well as for the study population as a whole, so that inequalities in impacts can be identified.
WHO for Europe. Health aspect of air pollution – answer to follow-up questions from CAFE. Report on a WHO working group meeting Bonn, Germany, 15-16 January 2004.
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