1.    Chemical Toxicity

Toxicology is the study of the adverse effects of chemical and physical agents on living organisms. Toxicity is the ability of a substance to cause damage to living tissue, impairment of the central nervous system, severe illness or in extreme cases, death, when ingested, inhaled or absorbed by the skin.

Toxicity hazard is the probability that injury will occur considering the manner in which the substance is used.

2.    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 sufficient amount.  The toxic potency of a chemical is defined by the relationship between the dose (the amount) of the chemical and the response that produced in a biological system.

3.    Routes of Entry into the Body

There are four main routes by which hazardous chemicals enter the body:

Inhalation by way of the respiratory tract.  This is most important in terms of severity.
Skin or eye absorption.
Ingestion through eating or smoking with contaminated hands, or in contaminated work areas.
Injection by contaminated needles or sharps (i.e., pipettes, broken glass)

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 either 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 include:  pesticides, carbon disulfide, phenol, carbon tetrachloride, dioxane, mercury, thallium compounds, xylene, hydrogen cyanide.

4.    Health Effects

Acute poisoning is characterized by rapid absorption of the substance and the 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 exposures of a duration measured in days, months, or years.  Symptoms may not be immediately apparent.  Examples:  lead or mercury poisoning and pesticide exposure.

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.  Examples:  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.  Examples:  arsenic affects the blood, nervous system, liver, kidneys and skin; benzene affects the 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.  Examples:  heavy metals.

Synergistic responses:  When two or more hazardous material exposures occur, the resulting effect can be greater than the effect of the individual exposures.  This is called a synergistic or potentiating effect.  Example:   exposure to both alcohol and chlorinated solvent.

5.    Other Factors Affecting Toxicity

Rate of entry and route of exposure;  that is, how fast is the toxic dose delivered and by what means.  Age can affect the capacity to repair tissue damage.   Previous exposures can lead to tolerance, increased sensitivity or make no difference.

State of health, physical condition, and life style, can affect the toxic response.   Pre-existing disease can result in increased sensitivity.

Environmental factors such as temperature and pressure may also affect the exposed individual, as well as host factors, including genetic predisposition and the sex of the exposed individual.

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