Exposure assessment is the second step; can be done either through surveys, monitoring, or statistical modelling. Surveys and monitoring are two tools where direct measurements as to the exposure are conducted. The limitations of this approach is that, these may not be accurate and may result in under assessment. Surveys and monitoring are also expensive. An alternative is to use statistical modelling of the exposure. This requires that the person who is conducting the statistical modelling must know the source and dispersion of the chemical or the exposure variable through the media through which it spreads. The process is desktop based modelling and so is relatively inexpensive and can be used to develop possible exposure scenario. One can use this to develop high exposure scenarios.
In case of PBDE, we see that the government has declared that 74 groundwater samples have recently been taken around
Ĺhakea, up from 26 samples taken in December last year. This suggests that they have been monitoring the situation and that a "safe" groundwater level of PBDE is agreed upon. They also found that six water wells serving eight properties have returned results with levels of foam chemicals at or above the drinking water guidelines. Here, we see that they have directly sampled the water and measured the level of PBDE in them. When you need environmental monitoring, you need to sample the environmental sites and you need to appropriately select the sites. For example, when we sample air quality, we need to keep in mind that air quality can vary between industrial places, places where we have heavy traffic, and places such as residential areas whose emission profiles are different from industries and traffic related emissions (highways). So, a point to keep in mind as to the media that we must sample:
- We can sample from air, water, or soil directly using chemical analytical processes
- We should also sample for metabolic by-products or direct samples from humans in the area who are believed to be exposed. This is biological sampling and this is how we estimate the dose people are exposed to. We can sample exhaled air, blood, urine, faeces, spinal fluid, tissue samples such as skin clippings, clippings from mucous membrane, saliva, hair. Hair is particularly important as for heavy metals, hair sampling can provide us with long term exposure scenario. In all such situations, we need to keep in mind the toxicokinetics and toxicodynamics of the chemical and the body that interact with each other. As an example, breathalyzer tests that police use for DUI are examples of biological sampling of exhaled air.
- For food, we must assess the dietary intake; so a dietician or a nutritional epidemiologist will be able to help you with these investigations. Several ways to collect information about diet, such as food frequency questionnaires, and often direct measurement of cooking patterns and asking about food basket to assess what food items are consumed at various times of the day (such as breakfast, morning tea, lunch, afternoon tea, evening tea, dinner) are important for understanding the dietary patterns. Increasingly, use of apps to measure food items and food on the plate is used.
To give you an example of dose assessment is how we estimated exposure to inorganic arsenic in our study of arsenic toxicity in West Bengal. We collected the blood and urine samples from study participants, and sent the samples after preserving the samples to keep their temperature low in liquid nitrogen to a laboratory at the University of Washington in Seattle in the United States for further analysis. Later, we used atomic absorption spectrometry at a laboratory based in Calcutta in India to measure the urine and blood samples. We also collected dietary information about their food intake to estimate how much arsenic might have entered their body through food. For water intake, we assessed that they consumed about 2L of water everyday.
Step 3: Dose-response assessment -- assess at what dose the effects occur
Dose response assessment is the third step where we assess the association between an exposure (and that translated to dose of the toxin in the body) and the appearance of the adverse health effects. In case of PBDE, our aim of dose response assessment will be to ascertain at what concentration of PBDE in the environment do the health effects (say lowering of Iq for children, or other effects). These concepts differ depending on whether the effects are cancerous or non-cancerous. If a chemical is known to cause cancerous effect on humans, then:
- We assess its potential to cause cancer qualitatively by analysing human epidemiological studies or animal studies.
- We quantitatively assess its potential to cause cancer by studying its slope parameter.
We critically appraise all available animal and human studies and based on the appraisal, we arrive at one of the following conclusions: