Humans come into contact with harmful contaminants in many ways. We may:

  • 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

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