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Last changed: 8/02/11
Health is subject to the influence of numerous factors from the environment. Such factors include environmental changes that derived completely or in part by human activity (social, mental, chemical, physical and biological factors) and, more broadly, genetic (hereditary) dispositions. The environmental influence when more narrowly defined incorporates all those chemical, biological, and physical effects which can have a negative impact on health.
The risk arising from these impacts depends from the exposure (strength of dose or concentration, duration of the exposure), the characteristics of the substances or other harmful factors, and the sensitivity of the people exposed.
Questions of actual, probable, and possible exposure plays an important role in such considerations. Actual exposure can theoretically be determined for individuals, but it loses its validity if applied to groups. Depending on the desired degree of protection sought, both average exposure and worst-case scenario can be taken as the basis of determination. Exposure estimation on the basis of known or assumed distribution (probabilistics) delivers results which are more realistic, but requires more data.
If adequate knowledge of the relationship between exposure and effect exists (dose-effect curve), the expected effect in case of a certain exposure can be quantitatively predicted. It must be taken into account that age, gender and health impairments can lead to differing effects at the same level of exposure. For this reason, “certainty factors” or, as the case may be, “uncertainty factors” in respect of assumed individual variations, and “extrapolation factors” in respect of the transfer to other situations, are introduced. From a concentration or dose with an ongoing observable effect, or from one where the effect has ceased to be observable, it becomes possible by means of these factors to derive a safe or at least justifiable level of concentration or dose (cf. guideline values for interior air quality).
In an ideal case, a dose or concentration can be defined below which no deleterious effects can occur. This initially sounds clear and simple, but it is unfortunately anything but. It is not possible to unambiguously determine which effects are harmful and which not, and the fact that no effects are observed does not necessarily mean that other investigative methods or other boundary values would also show the same results.
It is not possible to say whether there are particular doses or concentrations of potentially hazardous substances or influences which are free of undesired effects in all conceivable circumstances. In the cases of particulate matter and ozone, it has been shown that the concentration-effect relation at no point demonstrates an “effect threshold”, even at very low levels of concentration. This means that some level of risk always exists. Therefore, it cannot be a practicable goal to eliminate every risk whatever; the objective can only be to keep the residual risk as low as possible. This is also true of carcinogenic substances.
In practice, particularly defined values, which may take the form of threshold, guideline, or reference values, provide a frame of reference. In the case of threshold and guideline values, the basic considerations mentioned above are typically taken into account. Therefore it can be assumed that health perspectives have played a part in their definition. Reference values however only describe the “normal” state of affairs, thus giving no direct information on the harmfulness or non-harmfulness of a substance or environmental influence. “Normal” in this case means what applies to 95% of the population; in the case of the remaining 5% a “non-normal” exposure is postulated, by which it is assumed that such measurement results should be avoidable as they do not occur in the overwhelming majority of the population.
When it comes to subjective risk assessment it is a matter of course that the assumed or actual ability to decide for oneself whether or not to undergo a particular risk plays a significant role. It is often astounding to which extent high levels of risk (through obesity, lack of exercise, smoking) are courted more or less by choice, whereas “extraneous” risks (e.g. those brought about by “industry”) are given a disproportionate amount of attention.
In conclusion, one needs to accept that every assessment must necessarily be based on preconditions and assumptions, not all of which are firmly grounded in objectivity. Therefore, it is a prerequisite for any transparent assessment that it contains information about the underlying reasoning.