In real world, what we mean by exposure assessment can be quite complex. In exposure assessment for local issues, you will need to actually conduct local measurements in the media where you think an individual may be exposed. Such measurements will need to take into account the following:
- The medium in which the substance or the exposure variable is present. For air pollutants, you will need to measure the criteria pollutants (that is Particulate matters of various sizes e.g., PM2.5, or PM5, or PM10, carbon dioxide, oxides of sulphur and nitrogen, ozone); if you were to measure exposure to climate change related variables, you would have to measure pollutants in the air, temperature, rainfall, humidity (as temperature, rainfall, and humidity are the three indicators of climate change over time); if the media were water borne infectious agents, or water quality, you would have to measure pollutants in water;
- You would have to take into account the time period through which a person would be exposed to such media and then aggregate these measurements over all periods of time the person would be exposed. Indeed, for arsenic exposure, this is how measurements are conducted in epidemiological studies. For example, if we know that a person uses water from a particular well that has about 50 micrograms/L of arsenic in it and he has consumed about 1 Litre of water per day from that well over 20 years, we would use that measurement. While cumulative measurements are used for some risk estimates, for relative risk based estimates, use daily or rate of consumption rather than cumulative exposure.
- In workplace settings, we use time-weighted average. For example, if a labourer works for 8 hours a shift for two days at a particular area of the plant, then we would multiply the average hourly exposure of the exposure agent with eight to get a sense of his daily exposure
- Just external exposure is insufficient as an exposure agent upon entering the body undergoes metabolic transformation. When it does so, the dose that is presented at the site of action is different from the exposure. Also, in particular with Arsenic toxicity, when arsenic undergoes a metabolic transformation, it forms MMA3 (mono-methyl arsonous acid). This product is toxic.
Step III: Dose Response Assessment
This will be different for diabetes and bladder cancer. For diabetes, we will try to find out what would be the maximum acceptable dose that we can allow. This is the reason, for non-cancer diseases, we refer to environmental health risk assessment as health safety assessment. Here, the ultimate purpose is to test as to how much of the exposure can be safe for humans even though we know that whatever may be the exposure, there will be some harm. Is it possible to identify a certain level of exposure at which the health effect will not be produced? Whatever that value may be, we will set that value as a maximum allowable amount of exposure. Therefore, when we use dose response studies for non-cancer health outcomes, we would aim for dose response studies at different levels to test whether attributable risk would have exceeded the expected or frequency or incidence of the disease in the non-exposed population.
What does the dose-response relationship between exposure to inorganic arsenic and diabetes (Type II diabetes) look like?
Wang et.al. (2013) conducted a meta-analysis on the association between exposure to inorganic arsenic through drinking water and the risk of development of diabetes (Type II DM); while they did not provide a table of data, as part of the meta analysis, they provided a dose-response chart as follows (Figure \ref{688210}):