4.C.1.3 Routes of Exposure
Exposure to chemicals in the laboratory occurs by several routes: (1) inhalation, (2) contact with skin or eyes, (3) ingestion, and (4) injection. Important features of these different pathways are detailed below.
Toxic materials that enter the body via inhalation include gases, the vapors of volatile liquids, mists and sprays of both volatile and nonvolatile liquid substances, and solid chemicals in the form of particles, fibers, and dusts. Inhalation of toxic gases and vapors produces poisoning by absorption through the mucous membranes of the mouth, throat, and lungs and also damages these tissues seriously by local action. Inhaled gases and vapors pass into the capillaries of the lungs and are carried into the circulatory system, where absorption is extremely rapid. Because of the large surface area of the lungs in humans (approximately 75 m2), they are the main site for absorption of many toxic materials.
The factors governing the absorption of gases and vapors from the respiratory tract differ significantly from those that govern the absorption of particulate substances. Factors controlling the absorption of inhaled gases and vapors include the solubility of the gas in body fluids and the reactivity of the gas with tissues and the fluid lining the respiratory tract. Gases or vapors that are highly water soluble, such as methanol, acetone, hydrogen chloride, and ammonia, dissolve predominantly in the lining of the nose and windpipe (trachea) and therefore tend to be absorbed from those regions. These sites of absorption are also potential sites of toxicity. Formaldehyde is an example of a reactive highly water-soluble vapor for which the nose is a major site of deposition. In contrast to water-soluble gases, reactive gases with low water solubility, such as ozone, phosgene, and nitrogen dioxide, penetrate farther into the respiratory tract and thus come into contact with the smaller tubes of the airways. Gases and vapors that are not water soluble but are more fat soluble, such as benzene, methylene chloride, and trichloroethylene, are not completely removed by interaction with the surfaces of the nose, trachea, and small airways. As a result, these gases penetrate the airways down into the deep lung, where they can diffuse across the thin alveoli lung tissue into the blood. The more soluble a gas is in the blood, the more it will be dissolved and transported to other organs.
For inhaled solid chemicals, an important factor in determining if and where a particle will be deposited in the respiratory tract is its size. One generalization is that the largest particles (>5 µm) are deposited primarily in the nose, smaller particles (1 to 5 µm) in the trachea and small airways, and the smallest particles in the alveoli region of the lungs. Thus, depending on the size of an inhaled particle, it will be deposited in different sections of the respiratory tract, and the location affects the local toxicity and the absorption of the material. In general, particles that are water soluble dissolve within minutes or days, and chemicals that are not water soluble but have a moderate degree of fat solubility also clear rapidly into the blood. Those that are not water soluble or highly fat soluble do not dissolve and are retained in the lungs for long periods of time. Metal oxides, asbestos, fiberglass, and silica are examples of water-insoluble inorganic particles that are retained in the lungs for years.
A number of factors affect the airborne concentrations of chemicals, but vapor pressure (the tendency of molecules to escape from the liquid or solid phase into the gaseous phase) is the most important characteristic. The higher the vapor pressure is, the greater the potential concentration of the chemical in the air. For example, acetone (with a vapor pressure of 180 mmHg at 20 °C) reaches an equilibrium concentration in air of 240,000 ppm, or approximately 24%. Fortunately, the ventilation system in most laboratories prevents an equilibrium concentration from developing in the breathing zone of laboratory personnel.
Even very low vapor pressure chemicals are dangerous if the material is highly toxic. A classic example is elemental mercury. Although the vapor pressure of mercury at room temperature is only 0.0012 mmHg, the resulting equilibrium concentration of mercury vapor is 1.58 ppm, or approximately 13 mg/m3. The TLV for mercury is 0.05 mg/m3, more than two orders of magnitude lower.
The vapor pressure of a chemical increases with temperature; therefore, heating solvents or reaction mixtures increases the potential for high airborne concentrations. Also, a spilled volatile chemical evaporates very quickly because of its large surface area, creating a significant exposure potential. Clearly, careful handling of volatile chemicals is very important; keeping containers tightly closed or covered and using volatiles in laboratory chemical hoods help avoid unnecessary exposure to inhaled chemicals.
Certain types of particulate materials also present potential for airborne exposure. If a material has a very low density or a very small particle size, it tends to remain airborne for a considerable time. For example, the very fine dust cloud generated by emptying a low-density particulate (e.g., vermiculite or nanomaterials) into a transfer vessel takes a long time to settle. Such operations should therefore be carried out in a laboratory chemical hood or in a glovebox.
Operations that generate aerosols (suspensions of microscopic droplets in air), such as vigorous boiling,