Table 1 [from EEA, 1999] identifies and locates the 3 main clusters of causal factors usually involved in disease and ill health viz:
The "genetics" of the individual.
The “host condition” of the individual, which contains most of the factors usually described as “ lifestyle” that can affect nutritional, immune and health status. Many ‘lifestyle’ risk factors are partly determined by wider issues that are largely outside the control of the individual, such as societal choices over energy; transport; the built environment and the food manufacturing and distribution systems. This cluster also includes income, employment and housing, as well as age: sensitivities to environmental stressors vary greatly with age, for example, from day one after conception to 90+years. Host condition factors contribute to ill health both directly, by providing a specific link in the causal chain, (such as extensive use of the contraceptive pill, obesity, and late childbirth), and indirectly, by modifying the extent to which environmental stressors interfere with health.
"Environmental stressors". This cluster includes environmental exposures from air, water, food, soil, other surfaces, and consumer products, as well as the exposures to climate, radiation, noise, damp, housing, etc. that arise from some of the host conditions; factors such as transport, food and planning policies. These stressors can vary from large and ‘acute’ exposures, to small, yet continuous ’chronic’ exposures over many years. Usually, exposures are mixtures of physical, biological and chemical agents that vary with the emissions and the concentrations of the agents. Their impacts are determined by their abilities to deliver biologically effective doses, which of course vary with both the genetics and the host state of the people exposed. Knowledge about specific exposures are usually difficult to obtain but generic exposures can usually be inferred from the production of stressors, such as thousands of tons of pollutants emitted to the environments that people inhabit, or from proxy indicators such as the consumption of tobacco as a proxy measure for exposure to the 3000+ chemicals in tobacco smoke.
Figure 4 (EEA Table1): Multi-causality and Complexity
Most diseases, such as cancers, asthma and the neurological or endocrine mediated diseases are the result of different combinations of factors arising from all three clusters e.g. genes, the host condition and environmental stressors, involving a genetic/environment interaction which is mediated by the condition of the host body (e.g. foetus, child, adult) before it receives the environmental stressors. Many different and specific combinations of factors are responsible both for individual cases of disease and for particular fractions of total disease. It follows that there are many cause- effect combinations in differently configured causal chains.
Such multi-causality provides many obstacles to understanding the mechanisms and factors in the causal chain but it also provides many opportunities for removing links in the chain and thereby preventing harm, particularly where there are inter-dependencies between causal factors, such that removing even small links can reduce the impact of both those causal factors and others that may depend on them.
The simple model in is less clear cut on closer analysis because the boundaries between the 3 clusters are fuzzy: for example the environment can influence genes, and genes can influence lifestyle choices, which, in turn, affect both the host conditions and the stressors that it receives. In most cases neither “Nature” (genetics) nor “Nurture” (environment) is an independent cause of harm – both are usually necessary. [Hogben, 1933].
: Multi-causality Framework for Environment and Health .
In other words: "the genes provide the bullets but the environment provides the triggers"; and "Biology is not necessarily destiny". A more dynamic, circular and iterative illustration of these three clusters of causal factors is provided in Figure 5. Despite its simplicity, the clustering of many interacting causes into the three groups of Figure 5 can be a useful tool to help policymakers and the public respond to the complexities of the multi-causal realities that characterise the four diseases targeted in the EU Environment and Health Strategy: childhood asthma, childhood cancer, endocrine mediated diseases and neurological diseases.