The skin is composed of three layers of tissues—the epidermis, dermis, and subcutaneous tissues—and is an effective barrier to many substances. The outer skin layer is the epidermis, containing five layers of cells. The stratum corneum, which is the outermost epidermal layer, consists of dead cells and is the major barrier to chemical transfer through the skin. Although nonpolar chemicals cross the skin by diffusion through the stratum corneum, no active transport exists in the dead cells of this layer. The second layer, the dermis, is thicker and is composed of loosely packed connective tissue cells in a watery matrix of collagen and elastin fibres, as well as sweat glands, hair follicles, capillaries, and lymphatic vessels. After crossing the epidermis, chemical molecules are absorbed into the circulatory system via the capillaries. The capillaries drain into venules in the subcutaneous tissue.

The stratum corneum is not very permeable to water-soluble molecules and ions, although lipid-soluble molecules do cross it to a certain extent. The permeability is directly proportional to the lipid solubility of the chemical (i.e., highly lipid-soluble chemicals are readily absorbed) and inversely proportional to the molecular weight of the chemical (i.e., the rate of absorption increases as the molecular weight of the molecule decreases).

The rate of percutaneous absorption also varies with the thickness of the stratum corneum at different sites of the body. The rate of absorption is higher for skin on the forehead, axilla, back, and abdomen than for thicker regions like the plantar surface of the foot and the palm. The condition of the skin is also important. Percutaneous absorption is faster when the skin is moist rather than dry.

Solids are not absorbed through the skin because the skin is generally not covered with liquid and because pinocytosis does not operate in dead cells. Liquid chemicals penetrate the skin largely because of their lipid solubility. Gases and certain vapours can be absorbed through the skin also, although to a much lesser extent than via inhalation.

Inhalation

The absorption of inhaled gases and vapours differs from that of aerosols and thus will be discussed separately.

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Because the same principles govern the absorption of gases and vapours, the word “gases” is used here to represent both gases and vapours. Absorption of inhaled gases takes place mainly in the lungs. Before the gases reach the lung, however, they pass through the nose, where highly water-soluble, or highly reactive, gas molecules are retained by mucosa.

Unlike intestinal and percutaneous absorption of chemicals, respiratory absorption of gases does not depend on the pH of the alveoli, because gas molecules are not ionized. It also does not depend on the lipid solubility of the gas molecules, for three reasons. First, the alveolar gas molecules are situated in close proximity to the capillaries. Second, the alveoli form a huge surface for gas absorption. Third, the time it takes for a unit of blood to go through the lungs is more than adequate for gas molecules to diffuse from the alveolar space to the blood.

Gas molecules move into the blood by partitioning, which is a gas-transfer process between two phases, such as between the air and the blood or the blood and the tissues. In partitioning, gas molecules move from a phase of high partial pressure to an adjacent phase of low partial pressure. When an individual first inhales the gas, the partial pressure of the gas is higher in the air than in the blood, driving gas molecules from the alveolar space to the blood. As more gas molecules are driven into the blood, the blood’s partial pressure is raised. Eventually the partial pressure gradient between the air and blood dissipates and gas transfer stops; equilibrium is then reached, usually before the blood leaves the lungs.

The blood carries the gas molecules to the rest of the body, where the gas is transferred from the blood to the tissue until equilibrium is reached. The blood picks up more gas molecules in the lungs, and the process continues until the gas in each tissue of the body is in equilibrium with that of the blood entering the tissue. At this time, barring biotransformation, no further net absorption of gas takes place as long as the exposure concentration remains constant. A person can breathe the gas forever and not absorb more, a unique characteristic of gas exposure.

The particle size and water solubility of an aerosol chemical are the important characteristics determining absorption of aerosols. For an aerosol to be absorbed, it must be inhaled and deposited on the respiratory tract. If not deposited, the aerosol particles are exhaled. Aerosols of less than 100 micrometres (0.004 inch) can be inhaled.

The aerosol size also determines the tendency of a particle to be deposited on a certain region of the respiratory tract. The larger aerosols (greater than five micrometres) tend to be deposited in the upper respiratory tract, while the smaller ones (less than five micrometres) have a greater chance of being deposited on deeper sites of the lung. The nose acts as a “scrubber” for larger aerosols and thus protects the lung from injury.

Once deposited, aerosol particles must dissolve in the liquid lining the respiratory tract in order to be absorbed. For most aerosols of poor water solubility, the particles are cleared from the respiratory tract by mechanical or cellular means. In the nasopharyngeal region, mechanical methods of clearance include sneezing and nose blowing for particles deposited on the anterior one-fifth of the nasal cavity. Particles deposited on the remaining portion of the nasal cavity and on the pharynx are removed by tiny hairs, called cilia, on the surface of these two regions, which beat almost continuously to move a covering layer of mucous toward the throat (mucociliary apparatus). Any particles deposited on the mucous are carried along and finally swallowed.

In the tracheobronchial region, mechanical clearance includes coughing and the mucociliary apparatus. The trachea, bronchi, and bronchioles, down to the terminal bronchioles, are covered with mucous and cilia. The mucociliary apparatus moves upward toward the larynx, where the respiratory tract joins the esophagus. The particles are eventually swallowed and may be absorbed by the gastrointestinal tract.

The alveolar region has the slowest rate of particle clearance in the entire respiratory system, unless the particles are water-soluble, in which case they are cleared readily by dissolution. Water-insoluble particles in the respiratory bronchioles and alveoli are removed by cellular means, principally by macrophages—scavenger cells that engulf cellular debris in the body by a process called phagocytosis. Once phagocytosed, macrophages that contain particles are removed by the mucociliary apparatus in the terminal bronchioles. Pinocytosis by the cells lining the alveoli probably move the free particles to the interstitial space, where they either enter the lymphatic capillaries and are carried to the bloodstream, or they undergo a long process of dissolution. It can take years for water-insoluble particles to dissolve, depending on the chemical, which is why water-insoluble particles deposited in the alveolar region tend to remain in the interstitial space for a long time and can cause serious harm.