Poisons and toxins bypass the body’s natural defenses through specific pathways known as routes of exposure. A poison is any substance that can cause illness, injury, or death when introduced into the body in a sufficient dose. Understanding the entry mechanism is the first step in assessing potential harm and determining the appropriate response. The four primary routes of entry—ingestion, inhalation, absorption, and injection—govern how quickly a toxin reaches the systemic circulation to affect target organs.
Ingestion Through the Digestive System
Poisoning by ingestion occurs when a toxic substance is swallowed and enters the gastrointestinal (GI) tract. While the stomach’s acidic environment begins to break down the material, the majority of absorption into the bloodstream takes place in the small intestine. The small intestine features a massive surface area due to its folds, villi, and microvilli, enabling efficient uptake.
The rate and extent of absorption depend heavily on the substance’s chemical properties, such as solubility, and whether the stomach is full or empty. Once absorbed, the blood carrying the toxins travels directly to the liver via the portal vein. The liver metabolizes these compounds in a process called the first-pass effect, potentially reducing their toxicity before they circulate. Common examples include improperly stored medications, household cleaning products, contaminated food, or toxic plant materials.
Breathing Toxic Substances
Inhalation is a rapid route of exposure because it introduces substances directly into the body’s gas exchange system. When toxic gases, vapors, or particulate matter are breathed in, they travel down the respiratory tract to the lungs. The lungs contain millions of tiny air sacs called alveoli, which are surrounded by a dense network of capillaries. This alveolar-capillary membrane is extremely thin and has an enormous surface area, facilitating the rapid transfer of substances directly into the bloodstream.
Gases like carbon monoxide (CO) are dangerous because they quickly bind to hemoglobin, displacing oxygen and causing tissue hypoxia. Exposure to volatile organic compounds (VOCs) found in paints and solvents, or chemical fumes, also follows this pathway. The lack of a “first-pass” filter, unlike the liver provides for ingested toxins, means inhaled poisons quickly reach high concentrations in the brain and other organs.
Absorption Through the Skin
Dermal absorption involves the passage of a poison through the skin and into the circulatory system. The outermost layer, the stratum corneum, acts as the primary barrier, composed of tightly packed, dead cells embedded in a lipid matrix. The skin is most easily penetrated by lipophilic, or fat-soluble, substances, especially those with a small molecular weight. Chemicals with these properties can dissolve through the lipid layers of the stratum corneum and enter the dermis.
Factors such as prolonged contact time, increased skin temperature, or damage to the skin barrier (like cuts or rashes) significantly increase the rate of absorption. Highly potent chemicals, such as organophosphorus pesticides and industrial solvents, are known to cause systemic poisoning following skin contact.
Direct Entry via Puncture or Injection
The most immediate route of entry is through a direct physical breach of the body’s protective barriers, typically the skin. This pathway bypasses natural filtration systems, allowing the toxic substance to enter the tissue or bloodstream almost instantly. Intentional injections, such as intravenous administration, are designed to deliver a substance directly into the vein, resulting in the quickest onset of systemic effects.
Unintentional direct entry occurs through means involving a physical puncture. Examples include animal or insect bites that inject venom, or a puncture wound from a contaminated object like a rusty nail or discarded syringe. The depth of the breach determines the speed of absorption. A substance delivered directly into a muscle (intramuscular) or a vein (intravenous) is absorbed much faster than one placed just under the skin (subcutaneous).