As you can see in Table \ref{577947}, for inorganic As levels around 10 - 100 micrograms/L in the sediments and assuming that this would be the same exposure to inorganic arsenic, the excess risk of Diabetes over and above the baseline risk for both Europeans and Maori would be under 1%. However, the risks would be between 2-3 times greater for Maori in the Waipapa and the Wairakei drain regions (see Table \ref{524268}). While such high levels of exposure would need to be addressed, bringing down the exposure to under 10 micrograms/L would benefit all people in the region. This is an example where you can set the upper limit of accepted exposure to a chemical contaminant for this population. You can see that this estimate was derived taking into account that the dose response relationship was linear. This may not be as such for all situations and people will also have different baseline disease rates.
In all environmental exposure, when we will conduct an environmental health risk assessment, this will be the goal of our analysis. We will first assess the epidemiology of the exposure and diseases that can be caused due to the exposure. Then we will use the information obtained from the epidemiology of the exposure and association with the resultant disease to conduct an environmental health risk assessment.
Depending on the nature of the disease, cancer or non-cancer, we will have different ways to characterise the risk. The risk characterisation exercise will be used to determine a "safe" dose for those exposure that lead to non-cancerous health effect, and a rough estimate of what dosage will lead to a minimal risk for cancerous health effects.
Summary
Thus, we have examined the concepts of environmental health risk assessment for different types of diseases and exposure. This process is localised and contextualised for the specific purpose for which we will conduct this exercise. This process requires that we should understand principles of epidemiology and toxicology (how the toxins act in the body and how the body in turn acts on the toxins). We use literature review to identify the different health effects and choose one health outcome of the mix. Alternatively, we can start with a specific health outcome in perspective (as we conducted in this exercise). If we start with a range of health outcomes, then we would need to identify one particular health hazard to continue with the remaining steps of EHRA. The health outcome can depend on how we conceptualise the public health threat. It can be a common disease, or it can be a cancer as the most important and action-worthy outcome, or it can be a disease that is most sensitive to the exposure. In order to conduct the hazard identification, we should know the chemical and physical properties of the toxin, and we should also know how that toxin is produced in the environment. We should have an idea as to how the toxin enters the human body or how as humans, we are exposed to the toxin. Once we have identified the toxin and the specific health outcome we should then combine epidemiological information and data we can collect in the community or from the literature to identify the excess risk that would incur in presence of the environmental agent. This would allow us to decide what level of exposure might be manageable if the exposure has resulted in non-cancer health effects, and what would be the state of the excess health risks if the exposure were to result in cancer.