Matching exposure and health metrics
- The text on this page is taken from an equivalent page of the IEHIAS-project.
Estimation of the impacts of any hazard on human health requires the use of some form of exposure-response function to translate our estimates of exposure in the study population into estimates of health effect. The exposure-response functions used for this purpose typically come from previous epidemiological or toxicological studies.
If we are to use the results of these studies, it is vital that the measures used to describe both exposures and health outcomes match those being used in the assessment. For example, if studies of a specific cancer have measured health outcome only in terms of mortality, we cannot easily use the exposure-response functions to estimate morbity effects. Likewise, if exposures to air pollution have been measured in terms of fine particulates (e.g. PM10), we cannot directly apply the functions to other air pollutants, or even other particulate fractions. The way in which exposures and health outcomes have been measured in these studies thus exerts an important constraint on the design of an assessment.
In principle, the best way of dealing with this dilemma is to adapt the assessment to match the available exposure-response functions, by changing the exposure metrics or health outcomes that are to be used. The danger, however, is that this may greatly bias the assessment, by limiting it only to those exposures and effects that have been well-studied. Moreover, the exposure metrics used in many studies are not necessarily selected with the purposes of assessment in mind, but are often governed by practical considerations (such as ease and reliability of measurement). Thus, many epidemiological studies have used proxies such as distance from source as their exposure measure, because direct estimation of individual pollutants is not feasible, or because it is believed that this better represents the complex exposure mixture. These proxies may not be appropriate as a basis for assessment, because they may not reflect the changes that occur under the scenarios of interest.
The alternative is to try to adapt the available exposure-response functions in some way to match the needs of the assessment. In some instances this may be done using established conversion formulae (e.g. the ratio between indoor and outdoor air pollution, or between PM10 and PM2.5 concentrations), based on field studies. Great care is nevertheless needed because these relationships are rarely universal and may not apply equally across the study population. Expert judgements might also be used to provide an approximation of the relevant exposure-response function based on the evidence from analogous situations, or from first principles (see the example of ultrafines, referenced below). Where such adapatations are used, it is advisable to disaggregate the assessment as much as possible, so that the available exposure-response functions can be used for at least some of the impacts, and approximations can be limited only to those elements for which well-established functions do not exist.
More generally, it is advisable always to explore and review the available exposure-response functions as early as possible in the assessment process - preferably during issue-framing - so that any implications can be dealt with before the study design is too advanced.
- Knol, A.B., de Hartog, J.J., Boogaard, H., Slottje, P., van der Sluijs, J.P., Lebret, E., Cassee, F.R., Wardekker, J.A., Ayres, J.G., Borm, P.J., Brunekreef, B., Donaldson, K., Forastiere, F., Holgate, S.T., Kreyling, W.G., Nemery, B., Pekkanen, J., Stone, V., Wichmann, E. and Hoek, G. 2010 Expert elicitation on ultrafine particles: likelihood of health effects and causal pathways. Particle and Fibre Toxicology 6:19. doi:10.1186/1743-8977-6-19