Introduction
More than 100,000 chemicals are released into the global environment every year through their normal production, use, and disposal. To understand and predict the potential risk that this environmental contamination poses to humans and wildlife, our knowledge on the toxicity of a chemical must be coupled to our knowledge on how chemicals enter into and behave in the environment. The scheme shown in figure 1 illustrates the relationship between a toxicant source, its fate in the environment, its effective exposure or dose, and resulting biological effects.
Figure 1 - Environmental fate models are used to help determine
how the environment modifies exposure resulting from different sources
of toxicants.
Source: Hodgson E. (2010).
A prospective or predictive assessment
of a chemical hazard would begin by characterizing the contamination
source, modelling the chemical fate to predict exposure, and using
exposure/dose response functions to predict effects (moving from left
to right in fig. 1). A common application would be to analyse the
potential effects of a new waste discharge. A retrospective assessment
would proceed in the opposite direction starting with some observed
effect and reconstructing events to find a probable cause. Assuming
that we have reliable dose/exposure response functions, the key to a
successful use of this simple relationship is to develop a qualitative
description and quantitative model of the sources and fate of
contaminants in the environment.
Chemicals are released into the environment in many ways and rarely remain in the form, or at the location, of release. For example, agricultural chemicals used as sprays may drift from the point of application as air contaminants or enter runoff water as water contaminants. Therefore, chemicals do not necessarily remain in the same compartment into which they were first released, as a result of natural phenomena such as rainfall and winds, or even human activities, and they can travel along many pathways during their lifetime. Due to this constant change, it is very difficult to predict where chemicals will end up in the environment, i.e. their environmental fate.
A contaminant present in the environment at a given time-point and space can experience three possible outcomes:
- it can be stationary and add to the contaminant inventory and exposure at that location;
- it can be (geographically) transported to another location (within the same compartment, long distances away from the original source of emission, or between different compartments, for example by the action of precipitation or evaporation);
- it can be transformed into another chemical species.
Environmental contamination and exposure resulting from the use of a chemical is modified by the transport and transformation of the chemical in the environment. Degradation and dilution can attenuate the source emission, while processes that focus and accumulate the chemical can magnify the source emission.
The actual fate of a chemical depends on the chemical use pattern and physicochemical properties, combined with the characteristics of the environment in which it is released. The intrinsic properties of chemicals that can be used to help predict their potential movement, concentration and fate in the environment include volatility, water solubility, bioaccumulation potential and biodegradability. Establishing such properties as lipid solubility or octanol/water partition coefficient may enable preliminary estimates of uptake rate and persistence in living organisms. More specifically, the octanol/water partition coefficient (KOW) is a key property of a chemical for environmental considerations, as it is a predictor of sediment and soil adsorption and subsequent bioaccumulation in organisms and potential biomagnification by trophic transfer through the food chain. Vapour pressure is another property that may indicate whether the respiratory system is a probable route of entry. Although these intrinsic properties will influence the movement of chemicals and their fate in the environment, transport within the environment will, to a large extent, depend on the mobility of the medium into which the chemical was introduced. The chemicals present in the air or water (compartments) will be transported further than those remaining within the soil or sediment. As a result, attention from environmental agencies worldwide is focused mainly on chemical pollution of both air and water media. One well-known example of a problem resulting from chemical movement within the environment is that involving sulphur dioxide emissions from the United Kingdom being transported to countries in Scandinavia. This has caused the acidification of both water (lakes) and soil as a result of the gas being washed out of the air by water which then falls as "acid rain".
Conceptually and mathematically, the transport and fate of a contaminant in the environment is very similar to that in a living organism. Contaminants can enter in an organism or environmental system by many routes (e.g. dermal, oral, and inhalation vs. smoke stack, discharge pipe, or surface runoff). Contaminants are redistributed from their point of entry by fluid dynamics (blood flow vs. water or air movement) and intermedia transport processes such as partitioning (blood-lipid partitioning vs. water-soil partitioning) and complexation (protein binding vs. binding to natural organic matter). Contaminants are transformed in both humans and the environment to other chemicals by reactions such as hydrolysis, oxidation, and reduction. Many enzymatic processes that detoxify and activate chemicals in humans are very similar to microbial biotransformation pathways in the environment.
In fact, physiologically based pharmacokinetic models are similar to environmental fate models. In both cases, a complicated system is divided into simpler compartments and the rate of transfer between the compartments and the rate of transformation within each compartment are both estimated. The obvious difference is that environmental systems are inherently much more complex because they have more entry routes, more compartments, more variables (each with a greater range of values), and a lack of control over these variables for systematic study.
