d withBased d withBased on this figure, a 70-kg person drinking 2L of water per day would be exposed to about 1 mg/day of chlorine in drinking water.
Step 5: Risk characterisation
We will characterise the risk of possible eczema brought about by necrosis of tissues as a result of drinking of chlorinated drinking water as provided by the Christchurch City council in two ways:
- We will first estimate the amount of water to be consumed to meet the Rfd standard of no observed adverse effect
- We will also estimate the relative value between the exposure to chlorine through drinking water and the Rfd.
Risk characterisation scenario 1: How much water must a 70-kg person consume per day to develop skin necrosis and eczema?
The maximum concentration at which tissue necrosis resulting in eczema are likely to manifest, the Rfd is 0.144 mg/Kg/day. This means, for a 70-kg person, he or she would need to consume: 0.144 * 70 mg of chlorine a day = 10 mg of chlorine a day. If the city council water supply is providing the person 0.5 mg of chlorine per litre of water, this would mean the person would have to consume 10/0.5 = 20 litres of water per day. This is far excess of the average daily intake of water for most people (2L per day).
Question for you: How much water would a person weighing 60-kg need to consume per day to develop skin irritation and eczema from skin necrosis?
Risk characterisation scenario 2: What is the relationship between Rfd and the exposure in terms of mg/Kg of body weight/day assuming a 70-kg individual consuming 2L of water per day?
If an individual weighing 70 kg were to consume 2L of water per day and each litre of water were to contain 0.5 mg of chlorine, then that would be equivalent of 0.5 * 2 / 70 = 0.0142 mg/Kg of body weight/day of exposure to chlorine in drinking water. Based on an Rfd of 0.144 mg/Kg of body weight/day, this amounts to 9% of the Rfd.
Risk Mitigation
Based on these measurements, we may state that at 0.5 mg/L of dissolved chlorine in the drinking water, there is very little risk of health effects such as eczema for the adults. Other health effects on children such as respiratory illnesses can occur but we have not estimated those risks. Therefore, from a risk mitigation perspective, one may continue to monitor this situation and study for any new health effect that may arise as a result of consumption of the drinking water with this level of chlorination.
Discussion
Each of these scenarios and modellings has introduced us to the possibilities of what might happen if we were to be exposed to toxins in our environment through our drinking water. The PFOA which is present in fire-fighting foams can leach into the groundwater and if groundwater happens to be our sole source of drinking water supply, then we can conduct a safety or health risk assessment to test the likelihood that such contamination would lead to endocrine disruption such as damage to thyroid gland as a result of persistent presence in the body for a long time. We found when we estimated the concentrations that we would have to consume about 20 litres of water a day to accrue sufficient toxins to damage the liver or the thyroid gland. However, there are other health effects that might result as a result of excess hydration. In the second example, we saw that if arsenic concentrations were at or exceeded 50 micrograms/L of drinking water, this had serious implications for both skin diseases and for cancer of bladder. Therefore, our strategy would be remove inorganic arsenic from drinking water source. In the third example, we saw that presence of chlorine in drinking water in the concentrations that the City Council has added is not going to harm us in any way unless we drink 20 litres of water per day and that, at most common scenarios, this presents about 9% of our reference dose.
A few points to note in the process. First, we build the scenarios based on the reports or what may be relevant to people or communities. Second, we bring in information from chemistry, physics, toxicology, animal experiments, and epidemiology to understand the hazards at the hazard identification stage. We also refer to established studies or relevant evidence based information sources to identify the Rfd for non-cancer health effects (notably IRIS information system of ATSDR or EPA). Finally, we build scenarios to assess the consumption levels to finally characterise the risk.
An advantage about these estimations is that, an environmental health risk assessment (EHRA) brings together knowledge from several fields: experiments, epidemiology, toxicology, and risk assessment in one framework and provides an intuitive insight into the environmental health related problem at hand. For many toxins, while we may know the chemical properties, it does not follow that the toxin in question will behave exactly similar to a toxin we studied, hence animal experiments are needed to study health effects in animals that can be translated to human effects. Epidemiological studies such as case control studies and cohort studies provide insights into the exposures and outcomes. The animal experiments and epidemiological studies together may also provide a comprehensive dose-response relationships such that this enables use to estimate the NOAEL or LOAEL levels. This is important in case of occupational exposures where the levels of exposure may be higher than what happens in everyday settings in the environment. Thus the exposure level of a farmer or gardener in the field to a pesticide such as glyphosate is different from the exposure of a person who suspects glyphosate in cereals, and therefore exposure assessment and scenarios need to be developed separately. Besides, people respond differently to different chemicals or toxins. This is where gene-environment interactions between toxins and the environmental variables become important. Hence, toxicology is important for EHRA. Lastly, we need to produce some easy to understand way to make sense of the diverse data from the literature, from dose response studies as well as the data obtained locally as exposure. Hence, we have two approaches to make sense: how much do we need to consume to reach the point of disease, and how well are we tracking with respect to what we know about the reference dosage?
A few points where EHRA does not do so well or where the results need to be taken with a pinch of salt must be mentioned. First, a single number does not tell us the whole story about everyone who is exposed in the community that we are analysing. However, there is no better way to ascertain the risks of every individual in the community or population we are interested to study. Second, each person is different with respect to genetic make up and environmental exposures. So, the idea that an individual who is 70 kilograms, and consumes 2L of water per day may be one scenario, but this may not apply in each case, and therefore each person may have different story to tell. Third, we must take into account the toxicokinetics and the toxicodynamics of the toxins we are considering and the pathways. Toxicokinetics refer to the way the toxin interacts with the body and toxicodynamics relates to the way the body reacts to the toxin. In most cases, these knowledge are incomplete but we will still need to work on the basis of what we know or the best of our knowledge. Therefore, there are several disadvantages of this approach but nevertheless as these serve to arrive at best estimates, an EHRA with some conservative estimates is an important toolbox to have.
Precautionary Principle
A comprehensive EHRA is essential for environmental health risk management from a public health standpoint as it allows the environmental health professional to apply the precautionary principles. Precautionary principle states that for an event that raises the potential of harm to human health or environment, precautionary measures should be taken even if the full relationship of cause and effect between what may have caused the harm and the effect is not clear clear \cite{Kriebel2001}. Environmental epidemiological studies provide part of the evidence base for precautionary principles, that is, using environmental epidemiological studies, we can link health states with exposure, even if the causal mechanism may not be clear. Toxicological studies, using animal data provide insights into biological pathways that can translate the risk in the environment to the health effect. EHRA as an integrative process enables the findings from epidemiological studies, exposure assessments, and toxicological studies to a framework where the meaning of the epidemiological investigations can be translated so that some level of precision can be used to drive a decision as to whether precautionary actions are needed at that point or whether the action can be delayed. We can illustrate this with the three cases we described here. In the first case, while precautionary principles on the basis of the association between PFOA and endocrine disruption observed in experimental animals and epidemiological studies might be justified, when we conducted EHRA based on the exposure scenarios, we found that compared with immediate action to ban fire-fighting foams, it'd be better to monitor the situation closely. In the second scenario, epidemiological studies suggested an elevated risk, but EHRA provided insights into the magnitude of the elevated risks and would justify establishment of precautionary principles to take immediate actions. In the third scenario, no action was warranted even though some residents would complain of immediate reactions to further lower the chlorine level in the water supplied. Instead, chlorination of water would be viewed as an application of precautionary principle to address the biological hazards.