molecules to escape from the liquid or solid phase into the gaseous phase) is the most important characteristic of a chemical to consider. The higher the vapor pressure, the greater the potential concentration of the chemical in the air. For example, acetone (with a vapor pressure of 180 millimeters of mercury (mmHg) at 20 °C) could reach an equilibrium concentration in air of 240,000 parts per million (ppm), or 24%. (This value is approximated by dividing the vapor pressure of the chemical by the atmospheric pressure—760 mmHg—and multiplying by 1,000,000 to convert to ppm.) Fortunately, the ventilation present in most laboratories prevents an equilibrium concentration from developing in the breathing zone of the laboratory worker.
Even very low vapor pressure chemicals can be 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 about 13 milligrams per cubic meter (mg/m3). The TLV for mercury is 0.05 mg/m3, more than 2 orders of magnitude lower.
The vapor pressure of a chemical increases with temperature; therefore, heating of solvents or reaction mixtures increases the potential for high airborne concentrations. Also, a spilled volatile chemical can evaporate very quickly because of its large surface area, creating a significant exposure potential. It is clear that careful handling of volatile chemicals is very important; keeping containers tightly closed or covered and using volatiles in fume hoods are techniques that should be used to avoid unnecessary exposure to inhaled chemicals.
Certain types of particulate materials can also present the potential for airborne exposures. If a material has a very low density or a very small particle size, it will tend to remain airborne for a considerable time. For example, the very fine dust cloud generated by emptying a low-density particulate (e.g., vermiculite) into a secondary container will take a long time to settle out, and these particles can be inhaled. Such operations should therefore be carried out in a fume hood.
Operations that generate aerosols (suspensions of microscopic droplets in air), such as vigorous boiling, high-speed blending, or bubbling gas through a liquid, increase the potential for exposure via inhalation. Consequently, these and other such operations on toxic chemicals should also be carried out in a hood.
Contact with the skin is a frequent mode of chemical injury in the laboratory. Many chemicals can injure the skin directly. Skin irritation and allergic skin reactions are a common result of contact with certain types of chemicals. Corrosive chemicals can cause severe burns when they come in contact with the skin. In addition to causing local toxic effects, many chemicals are absorbed through the skin in sufficient quantity to produce systemic toxicity. The main avenues by which chemicals enter the body through the skin are the hair follicles, sebaceous glands, sweat glands, and cuts or abrasions of the outer layer. Absorption of chemicals through the skin depends on a number of factors, including chemical concentration, chemical reactivity, and the solubility of the chemical in fat and water. Absorption is also dependent on the condition of the skin, the part of the body exposed, and duration of contact. Differences in skin structure affect the degree to which chemicals can be absorbed. In general, toxicants cross thin skin (e.g., scrotum) much more easily than thick skin (e.g., palms). When skin is damaged, penetration of chemicals increases. Acids and alkalis can injure the skin and increase its permeability. Burns and skin diseases are the most common examples of skin damage that can increase penetration. Also, hydrated skin absorbs chemicals better than dehydrated skin. Some chemicals such as dimethyl sulfoxide can actually increase the penetration of chemicals through the skin by increasing its permeability.
Contact of chemicals with the eyes is of particular concern because these organs are so sensitive to irritants. Few substances are innocuous in contact with the eyes; most are painful and irritating, and a considerable number are capable of causing burns and loss of vision. Alkaline materials, phenols, and strong acids are particularly corrosive and can cause permanent loss of vision. Because the eyes contain many blood vessels, they also can be a route for the rapid absorption of many chemicals.
Many of the chemicals used in the laboratory are extremely hazardous if they enter the mouth and are swallowed. The gastrointestinal tract, which consists of the mouth, esophagus, stomach, and small and large intestines, can be thought of as a tube of variable diameter (about 5 m in length) with a large surface area (about 200 m2) for absorption. Toxicants that enter the gastrointestinal tract must be absorbed into the blood to produce a systemic injury. Sometimes a chemical is caustic or irritating to the gastrointestinal tract tissue itself. Absorption of toxicants can take place along the entire gastrointestinal tract, even in the mouth, and depends on many factors, including the physical properties of the chemical and the speed at which it dissolves. Absorption increases with surface area, permeability, and residence time in various segments of the