In toxicology, understanding how a chemical enters the body is critical for assessing health risk. There are four primary routes of exposure: inhalation, dermal (skin) absorption, ingestion, and injection. Each route affects the rate of absorption, distribution, and ultimately the toxicity of a chemical substance.
Inhalation
Inhalation is often the most significant route of occupational exposure. Airborne chemicals — including gases, vapors, dust, fumes, and mists — can be inhaled and rapidly absorbed through the lungs into the bloodstream. The large surface area of the lungs and their rich blood supply make inhalation a highly efficient absorption route. Occupational exposure limits (OELs) such as OSHA’s Permissible Exposure Limits (PELs) and ACGIH Threshold Limit Values (TLVs) are primarily designed around inhalation exposure.
Dermal Absorption
The skin acts as a barrier to most chemicals, but lipophilic (fat-soluble) substances can penetrate through intact skin. Factors affecting dermal absorption include the chemical’s molecular weight, lipid solubility, skin condition, and the body surface area exposed. Some chemicals — such as organophosphate pesticides and certain solvents — are readily absorbed through the skin and pose significant occupational hazards even when inhalation exposure is controlled.
Ingestion
Ingestion typically occurs through hand-to-mouth contact or accidental contamination of food or water. In occupational settings, good hygiene practices — such as washing hands before eating and not eating or drinking in work areas — are essential to prevent incidental ingestion of chemicals. Ingested chemicals are absorbed through the gastrointestinal tract and may undergo first-pass metabolism in the liver before entering systemic circulation.
Injection
Injection bypasses normal skin barrier and GI absorption, delivering chemicals directly into the bloodstream or tissues. While rare in occupational settings, needlestick injuries can be a concern in laboratories and healthcare settings. Injection results in the most rapid and complete absorption of any exposure route.
