For carcinogens, there is no concept of a threshold dose limit. This is because cancerous processes are thought to start at a molecular level and therefore in theory, there cannot be a lower limit of the concentration or amount of an agent that can lead to the develop of cancer. For many chemicals such as chloroform and arsenic can have both carcinogenic and non-carcinogenic effects. Therefore, there is an increasing trend to identify non-carcinogenic effects that can in turn lead to cancers, for example change in methylation patterns, or formation of DNA adducts. Also, in a bid to bring together the concepts of non-cancerous and cancerous effects into a common framework, a concept of using reference dose or reference concentrations has emerged.
Step 4: Risk Characterisation -- construct an Rfd for the specific population who are exposed and compare with what is known
In the fourth step of risk characterisation, we put together the concepts of exposure assessment and dose response assessment and characterise for the most critical or sensitive or significant outcome, estimate the risk for each level of the dose that we estimated for the level of exposure. Again, this difers between carcinogenic and non-carcinogenic effects:
- If the effect is a cancer, then we estimate the dosage at which the probability of that cancer at the incidence of 1 in a million (1 per 1, 000, 000). This is done as follows:
Carcinogenic Risk = Slope factor (mg/kg/day) * lifetime or 70 year exposure dose of the exposure per kg of body weight (mg/kg/day)
Then compare based on the current exposure level if the current exposure is at a level that can cause cancer. Based on a single hit model where we say that there is no theoretical lower threshold level for cancer causation, we may call for elimination for that toxin altogether.
Else, if the effect is non-carcinogenic, then we estimate the exposure over a given period through which a health effect can occur, and compare this level of exposure with the Rfd that we estimated in the dose-response step. We consider here three time periods:
- Acute effects. -- If the effects are acute, then we consider an appropriate time period through which we want to study this (days to a few weeks)
- Intermediate effects -- the time period will be between a few weeks to a year
- Chronic effects -- the time period of exposure will vary between a year to lifetime (say 70 years of exposure)
Step 5: Risk mitigation -- devise ways to mitigate the health risks
These data are then used for setting policies or setting action agenda for environmental health risk management.
In the next section (part II of our EHRA), we will use the examples of PBDE and Chlorination in water to assess environmental health risks that could arise from them and examine whether they make sense.