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Other Routes of Exposure
In addition to the common routes of environmental, occupational, and medical exposure (oral, respiratory, and dermal), other routes of exposure may be used for medical purposes. Many pharmaceuticals are administered by parenteral routes, that is, by injection into the body usually via syringe and hollow needle.
Intradermal injections are made directly into the skin, just under the stratum corneum. Tissue reactions are minimal and absorption is usually slow. If the injection is beneath the skin, the route is referred to as a subcutaneous injection. Since the subcutaneous tissue is quite vascular, absorption into the systemic circulation is generally rapid. Tissue sensitivity is also high and thus irritating substances may induce pain and an inflammatory reaction.
Many pharmaceuticals, especially antibiotics and vaccines are administered directly into muscle tissue (the intramuscular route). It is an easy procedure and the muscle tissue is less likely to become inflamed compared to subcutaneous tissue. Absorption from muscle is about the same as from subcutaneous tissue.
Substances may be made directly into large blood vessels when they are irritating or when an immediate action is desired, such as anesthesia. These are known as intravenous or intraarterial routes depending on whether the vessel is a vein or artery.
Parenteral injections may also be made directly into body cavities, rarely in humans but frequently in laboratory animal studies. Injection into the abdominal cavity is known as intraperitoneal injection. If it is injected directly into the chest cavity, it is referred to as an intrapleural injection. Since the pleura and peritoneum has minimal blood vessels, irritation is usually minimal and absorption is relatively slow.
Implantation is another route of exposure of increasing concern. A large number of pharmaceuticals and medical devices are now implanted in various areas of the body. Implants may be used to allow slow, time-release of a substance (e.g., hormones). In other cases, no absorption is desired, such as for implanted medical devices and materials (e.g., artificial lens, tendons and joints, and cosmetic reconstruction).
Some materials enter the body via skin penetration as the result of accidents or violence (weapons, etc.). The absorption in these cases is highly dependent on the nature of the substance. Metallic objects (such as bullets) may be poorly absorbed whereas more soluble materials that are thrust through the skin and into the body from accidents may be absorbed rapidly into the circulation.
Novel methods of introducing substances into specific areas of the body are often used in medicine. For example, conjunctival instillations (eye drops) are used for treatment of ocular conditions where high concentrations are needed on the outer surface of the eye, not possible by other routes.
Therapy for certain conditions require that a substance be deposited in body openings where high concentrations and slow release may be needed while keeping systemic absorption to a minimum. For these substances, the pharmaceutical agent is suspended in a poorly absorbed material such as beeswax with the material known as a suppository. The usual locations for use of suppositories are the rectum and vagina.
 
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