Although the ultimate aim of assessment is to give measures of health effects and their associated impacts, these are not always the only things of concern.  If we want to have prior warning of potential health effects, for example, we might want information from earlier in the causal chain - e.g. on exposures or environmental concentrations.  If we want to highlight the original causes of health effects, and places where we might take preventative action, then we might look earlier still, at source activities and emissions.

It is therefore useful to recognise three key types of indicator, dependent on their position in the causal chain:

1. Endpoint indicators, which give a measures of health effect or impact.  These include not only direct expressions of health outcome (e.g. in terms of total mortality or morbidity), but also more synoptic measures such as disease- or quality-adjusted life years (DALYs and QALYs) and monetary cost.
2. Midpoint indicators, representing dose, exposure or environmental concentrations.  While these are usually derived from the use of some form of modelling, they may in some cases include indicators derived from biomonitoring.
3. Source indicators, representing emissions, source intensity or more general measures of socio-economic activity.   

If assessments are to give full insight into risks to human health, a suite of indicators may thus be necessary, spanning the causal chain. In the case of an assessment of the health effects of air pollution, for example, these might include measures of:

• Road traffic volumes, to represent source activities;
• Annual emissions of PM_2.5 (or some other marker) to represent releases into the environment;
• Mean annual concentrations of PM_2.5 to represent environmental conditions;
• Estimated mean annual exposures to ambient PM_2.5 to represent exposures;
• Excess mortality attributable to air pollution, to represent health effects.