The environment affects human health both negatively and positively.  On the one hand it is the source for, and a setting in which exposures occur to, hazards of many different kinds: physical hazards such as earthquakes, storms, floods, fire or traffic accidents; biological hazards such as vector-borne diseases and pathogens (e.g. malaria, lymes disease, BSE); and chemical hazards, such as pollutants in the air, water or soil.  On the other hand, the environment provides most of the basic resources needed for human life (e.g. water and food), as well as many of the amenities which add to our well-being (e.g. places for recreation, relaxation and mental stimulation).

Many of the issues that merit integrated impact assessments involve some combination of these hazards and benefits.  Likewise, the vast majority of policies and technologies affecting the environment (whether deliberately or accidentally) have the potential for adverse and beneficial effects.  Hazards and benefits are nevertheless different in their expression.  They lead to different types of health consequence: hazards typically cause clinically detectable injury or harm; benefits are commonly more subtle in their effect, influencing our socio-psychological state and mental well-being.  In many cases, also, they affect different people, in different areas or social groups.   Hence, many issues involve both winners and losers.

In doing assessments, therefore, we need to make sure that we have considered and included the benefits as well as the hazards that might arise, and in both cases have properly delineated the populations that might be affected.  Otherwise, the assessment will be biased.  Considering these very different types of impact, however, also means that we must choose our outcome indicators withspecial care.  It is not always sufficient to measure the impacts only in terms of the burden (or absence) of disease.  Instead, we need meaures that allow us to sum and compare the negative and beneficial outcomes - for example, monetary measures or indicators of overall well-being.