Humans come into contact with harmful contaminants in many ways. We may:
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- eat food that carries toxin residues
- touch contaminated soils
- absorb radiation or chemical vapors through our skin
- drink polluted water
- inhale gases and particulates as we breathe
- Determining how much exposure of a contaminant is safe is a multi step process that involves using many different sources of information
- Chemical and Physical Properties of the toxin
- The form in which humans usually come into contact with it
- The amount that comes into contact with humans
- Major routes of exposure
- Human physiology (how the body works)
- Human toxicology (how the body uses and responds to chemicals)
- Once an internal dose of a chemical is absorbed into the bloodstream, it becomes distributed among various tissues, fluids, and organs, a process called partition.
- The contaminant can be stored, transported, metabolized, or excreted.
- Many contaminants that are highly soluble in water are excreted relatively quickly,
- Some, such as mercury, cadmium, and lead, bind tightly to specific organs.
- Agents that are not highly soluble in water, such as pesticides and insecticides, tend to move into fatty tissues and bioaccumulate.
The portion of an internal dose that actually reaches a biologically sensitive site within the body is called the delivered dose.
- To calculate delivered doses, researchers start by mapping how toxic substances move through the body and how they react with various types of tissues.
- The delivered dose is the measurement most closely related to expected harms from exposure
- To calculate the delivered dose researches measure blood concentrations of a toxin using PBPK (Physiologically-Based Pharmacokinetic) models.
- This approach simulates the time and the path that a contaminant will take to go through the different tissues in humans
- They do this by dividing the body into a series of compartments based on how quickly they take up and release the substance.
- Using known values for physical functions like breathing, this model estimates how quickly a toxin will move through a human body and how much will be stored, metabolized, and excreted at various stages.
It’s important to note that these models make assumptions, estimates and judgments on sometime limited information
- This makes it hard to predict the health effects of a toxin