6 - TOXICOLOGY
Section Contents
| 6.1 | CHEMICAL TOXICITY |
| 6.2 | DOSE-RESPONSE RELATIONSHIPS |
| 6.3 | ROUTES OF ENTRY INTO THE BODY |
| 6.4 | TYPES OF EFFECTS |
| 6.5 | OTHER FACTORS AFFECTING TOXICITY |
- CHEMICAL TOXICITY
Toxicology is the study of the nature and action of poisons. Toxicity is the ability of a chemical molecule or compound to produce injury once it reaches a susceptible site in or on the body. A toxicity hazard is the probability that injury will occur considering the manner in which the substance is used.
- DOSE-RESPONSE RELATIONSHIPS
The potential toxicity (harmful action) inherent in a substance is manifest only when that substance comes in contact with a living biological system. A chemical normally thought of as "harmless" will evoke a toxic response if added to a biological system in a sufficient amount. The toxic potency of a chemical is thus ultimately defined by the relationship between the dose (the amount) of the chemical and the response that is produced in a biological system.
- ROUTES OF ENTRY INTO THE BODY
There are three main routes by which hazardous chemicals enter the body:
- Absorption through the respiratory tract through inhalation. This is usually the most important in terms of severity.
- Absorption through the skin. This accounts for most occupational disease (dermatitis).
- Absorption through the digestive tract. Can occur through eating, smoking, or applying cosmetics with contaminated hands or in contaminated work areas.
Most exposure standards, Threshold Limit Values (TLVs) and Permissible Exposure Limits (PELs), are based on the inhalation route of exposure. They are normally expressed in terms of parts per million (ppm) or milligrams per cubic meter (mg/m3) concentration in air.
If a significant route of exposure for a substance is through skin contact, the MSDS will have a "skin" notation. Examples: pesticides, carbon disulfide, carbon tetrachloride, dioxane, mercury, thallium compounds, xylene, hydrogen cyanide.
- TYPES OF EFFECTS
Acute poisoning is characterized by rapid absorption of the substance, and the effect of exposure is sudden and severe. Normally, a single large exposure is involved. Examples: carbon monoxide or cyanide poisoning.
Chronic poisoning is characterized by prolonged or repeated low level exposures of a duration measured in days, months or years. Symptoms may not be immediately apparent. Examples: lead, mercury, or pesticide poisoning.
Local refers to the site of action of an agent and means the action takes place at the point or area of contact. The site may be skin, mucous membranes, the respiratory tract, gastrointestinal system, eyes, etc. Absorption does not necessarily occur as in the case of strong acids or alkalis.
Systemic refers to a site of action other than the point of contact and presupposes absorption has taken place. For example, an inhaled material may act on the liver. Other examples: arsenic affects the blood, nervous system, liver, kidneys and skin; benzene affects bone marrow.
Cumulative poisons are characterized by materials that tend to build up in the body as a result of numerous chronic exposures. The effects are not seen until a critical body burden is reached. Example: heavy metals.
Substances in Combination: When two or more hazardous materials are present at the same time, the resulting effect can be greater than the effect predicted based on the individual substances. This is called a synergistic or potentiating effect. Example: exposure to alcohol and chlorinated solvents.
- OTHER FACTORS AFFECTING TOXICITY
Rate of entry and route of exposure (how fast the toxic dose is delivered and by what means) may affect toxicity. Age can affect the capacity to repair tissue damage. Previous exposure can lead to tolerance, increased sensitivity, or make no difference. State of health, physical condition and life style can affect the toxic response. Preexisting disease can result in increased sensitivity. Environmental factors such as temperature and pressure are factors, as are host factors including genetic predisposition and the gender of the exposed individual.