Introduction

We will conduct environmental health risk assessment of exposure to different toxins. In each of these situations, the goal of the exercise is to identify whether the hazard posed by a toxin present in the water, air, soil or food will likely to cause harm. The health related "harms" can be either cancerous or non-cancerous. For non-cancerous harms, the aim is to identify if compared with a level of exposure that is considered to cause minimal or no harm (that dose is referred to as reference dose, or acceptable daily intake, or tolerable daily intake, or no or low observed adverse effect level), the level of exposure a person is likely to have based on some exposure scenario. For cancerous health effects, a sufficiently low level is not possible, we assume some harm will inevitably result. The goal here is to identify what would be the level where an excess of one case of cancer per million will be observed. This is in the form of a slope of a dose-response curve where the dose is plotted on the X axis of a graph and the harm in the form of "cases of disease" or probability of cases is plotted on the Y axis. Then, based on the available values, a linear model is proposed. This linear model is then extrapolated to a zero exposure level. The idea here is to identify a "slope of the curve" that can be multiplied with a level of exposure in the form of mg/Kg/day to estimate the probability of cancer for exposure to the toxin. The higher the probability, the greater the risk, as a theoretical minimum of "no harmful exposure" is not applicable.

Methods

Environmental health risk assessment consists of five  steps as follows:
  1. Issue identification. -- Find out and describe the issue we are about to explore.
  2. Hazard identification. -- identify the hazard from (a) animal experiments, (b) epidemiological studies, (c) chemical structures. The main sources of hazard identification are  hazard inventories, ATSDR documentation, EPA archives,  epidemiological studies, meta analyses or meta analyses we can conduct ourselves, and other sources. At the end of the hazard identification step, we should be able to answer the questions about (i) what is the toxin? (ii) where can it be found? (iii) how are humans exposed to the toxin? (iv) what happens to the toxin in the body; (v) what health effects are associated with it for animals and humans? This is a the starting point of any environmental health risk assessment process.
  3. Exposure assessment. -- this is local, that is, for the particular population group, we will identify what is the level of exposure that a local person is being exposed. This can be done by (1) sampling the media: air, water, soil, food, and other sources to which a person is exposed and which can lead to an entry into the person's body or be applied on the skin; (2) obtaining body tissue and analysing (skin clippings, mucous membrane clippings, hair samples, nail samples, tear, nasal discharge, saliva, sweat, blood, urine, and faecal samples, sample of inspired air from bronchoscopy, and other sampling methods). 
  4. Dose-response assessment. -- Dose-response assessment is done to obtain a level of exposure that will lead to disease or protect; precisely quantify the level at which a certain level of disease or health state is expected. For non-cancer causing exposure, either the level till "no observed adverse effect exists" is estimated (NOAEL), or the level at which the first signs of adverse effect occurs is estimated (LOAEL, the lowest observed adverse effect level). Based on the source of the information and how much uncertainty is associated with this measurement, the analyst the uses an uncertainty factor to estimate the reference dose or acceptable daily intake of the toxin. This is given as NOAEL divided by the uncertainty factor in terms of daily intake as milligram or microgram/day. For cancer causing toxins, the estimation is based on the slope of the dose-response curve. As well as that, the least level of the toxin at which cancerous effects are observed are also estimated.
  5. Risk characterisation. -- In the risk characterisation step, we compare the exposure level in terms of daily intake, and the reference dose that we have obtained in the dose-response assessment stage, having identified the toxin and its known health effects for those health effects. We then assess whether based on exposure data, if we estimate the exposure level per day, does this value exceed or fall short of the Rfd we estimated in the third step. If the levels are lower than Rfd, then we can claim that even though we are exposed to the toxin under consideration, the chances of health effects are low. But if that exceeds the Rfd, then the chances of health effect in this particular scenario is high.
Let us illustrate these principles using four examples.
We will make the following assumptions about adults: