To evaluate the risks of adverse health affects from chemicals, one must be aware of the routes of entry into the body, duration of exposure, toxicity of the chemical, exposure limits, and odor threshold of the chemical. This section explains these principles and describes how employees can eliminate or reduce chemical exposure in the workplace.
Inhalation is the most frequent route of chemical exposure in the lab. Exposure by this route can produce poisoning by absorption through the mucous membrane of the nasal passage, mouth, throat, and lungs and can seriously damage these tissues. Inhaled gases or vapors may pass rapidly into the capillaries of the lungs and be carried into the circulatory system.
10.1.2 Skin Contact
Contact with the skin is the second most frequent route of chemical exposure. A common result of skin contact is localized irritation. Many materials are absorbed through the skin quickly and may cause systemic poisoning. The main routes of entry for chemicals through the skin are the hair follicles, sebaceous glands, sweat glands, and cuts or abrasions of the outer layers of the skin. The follicles and the glands are abundantly supplied with blood vessels, which facilitate the absorption of many chemicals.
Skin and eye contact with chemicals can be avoided by the use of appropriate protective equipment. See Section 6, Personal Protective Equipment.
Many of the chemicals used in the lab are extremely dangerous if they enter the mouth and are swallowed. In addition, many chemicals may damage the tissues of the mouth, nose, throat, lungs, and gastrointestinal tract producing systemic poisoning if absorbed through these tissues.
To prevent entry of chemicals into the mouth, lab workers should wear gloves and wash their hands immediately after use of any chemical substance and before leaving the lab. Do not store or consume food and drinks in the lab. Mouth pipetting is prohibited.
Exposure to chemicals by injection seldom occurs in the lab. However, it can inadvertently occur through mechanical injury from glass or metal contaminated with chemicals, when chemicals are handled in syringes or during animal research.See Section 12.12, Cytotoxic Agents.
One method used for assessing the acute toxicity of a chemical is by determining its lethal dose (LD50). The LD50 is the quantity of material ingested, injected, or applied to the skin in a single dose that will cause the death of 50 percent of the test animals in which it is tested. It is usually expressed in milligrams per kilogram of body weight. The smaller the number, the more acutely toxic the chemical.
Lethal concentration (LC50) is the concentration in air of a material which, when inhaled over a specified length of time, causes the death of 50 percent of the animals in which it is tested.
Chemical toxicity can be acute or chronic or both. An acute response occurs when an employee is exposed to a significant quantity of a chemical for a short duration such as a spill or splash. The effects are usually severe, such as chemical burns, respiratory distress, or nausea. Chronic responses are defined by exposures to chemicals for an extended period of time, e.g., continually handling volatile chemicals outside the chemical fume hood. Chronic health effects may include cancer, necrosis, and damage to the immune system.
The American Conference of Governmental Industrial Hygienists (ACGIH) has developed exposure limits for airborne concentrations for many industrial chemicals. These limits are used as guidelines when evaluating occupational exposures. The limits, which are called Threshold Limit Values (TLVs) and Short-Term Exposure Limits (STELs), are maximum airborne concentrations either reported in parts per million (ppm) or milligrams per cubic meter (mg/m3) for an eight-hour or 15-minute exposure, respectively. Exposures within these limits are considered safe for the majority of the population.
In addition, some chemicals have a ceiling limit (a concentration that cannot be exceeded at any time during the work period).
In the State of Washington, the ACGIH TLVs have been adopted under the Washington Industrial Safety and Health Act (WISHA) and are known as Permissible Exposure Limits (PELs). These values are enforced by the Department of Labor and Industries. For some chemicals, the PELs may be lower than the recommended TLVs. The PELs are listed in the individual chemical summaries found at the EH&S website:
The odor threshold is the lowest concentration of a chemical at which an odor can be detected. The odor threshold may vary and is dependent upon individual sensitivity. The odor for a substance may also change depending upon the concentration. For example, butyl acetate has a sweet odor at low concentrations and a banana odor at high concentrations.
Chemicals which have odor thresholds above their PELs are said to exhibit poor warning properties. In other words, once the chemical is detected, the employee has been exposed above the PEL. Prolonged exposure to these chemicals at noticeable concentrations should be avoided.
Chemicals commonly used at the Hutchinson Center that have odor thresholds higher than their PELs are acetonitrile, which smells ether-like; chloroform, which smells suffocatingly sweet; methylene chloride, which has a strong solvent smell; and hydrazine, which smells ammonia-like. Chemicals with good warning properties can be detected long before their vapor concentrations are toxic; examples include phenol, formaldehyde, acetic acid, xylene, and toluene.