Once ingested, a chemical may be absorbed across the wall of the gastrointestinal tract into the bloodstream. Most of a chemical absorbed in this way is carried first to the liver. Unless a substance is injected directly into the body, the lung is usually the most rapid means of entry into the bloodstream. For example, gaseous anesthetics act very rapidly. And inhaled toxins, such as the fumes released by burning plastics, can have rapid and catastrophic effects.
Skin is usually a defence against toxic substances, but it can also be a point of entry.
For example, the pesticide parathion is absorbed through the skin into the bloodstream. In a recent case, a scientist was poisoned by mercury absorbed through laboratory gloves. The effects of a toxin will depend on the dose and how the body responds to the dose. Some substances are poorly absorbed by the body and may be excreted rapidly, while others may be stored and build up in tissues over time. Animals are used as models to predict the effects of human exposure to a chemical.