Local and Systemic Effects

Local effects from chemical exposure occur at the site of contact, i.e., eye irritation, skin burns or blistering, respiratory distress, or pulmonary edema. Systemic effects occur at a location distant from the point of contact, i.e., liver, CNS, heart, or kidneys. These effects may occur years after a single high level of exposure, or as the result of chronic exposure. Systemic effects are often more difficult to trace to their cause, and can include organ damage, respiratory diseases, and other illnesses in an exposed population.

Certain toxic substances produce their long-term effects by altering the genetic code, or DNA, which tells the body’s cells to perform certain activities. Three categories of effects can result from such substances:

A carcinogenic effect is an increase in an individual’s risk of contracting cancer.
- (a) It has been evaluated by the International Agency for Research on Cancer (IARC), and found to be a carcinogen or potential carcinogen; or
- (b) It is listed as a carcinogen or potential carcinogen in the Annual Report on Carcinogens published by the National Toxicology Program (NTP) (latest edition); or
- (c) It is regulated by OSHA as a carcinogen.
A mutagenic effect is a permanent change in the genetic material (DNA), which may be passed along to later generations.
A teratogenic effect is an increased risk that a developing embryo will have physical defects.

Determining what level of exposure causes these effects requires laboratory research under controlled conditions. Even then, results must be extrapolated from laboratory animals to humans. That is, scientists must make assumptions and apply formulas to decide what their experiments tell them about human exposures.

Another way to classify a hazardous substance or chemical is by their physiological effects. This classification includes the following major groups:

Irritants are chemicals which are not corrosive, but which cause a reversible inflammatory effect on living tissue by chemical action at the site of contact. A chemical is a skin irritant if, when tested on the intact skin of albino rabbits by the methods of 16 CFR 1500.41 for 4 hours exposure or by other appropriate techniques, it results in an empirical score of five or more. A chemical is an eye irritant if so determined under the procedure listed in 16 CFR 1500.42 or other appropriate techniques.
Corrosives are chemicals that cause visible destruction of, or irreversible alterations in, living tissue by chemical action at the site of contact. For example, a chemical is considered to be corrosive if, when tested on the intact skin of albino rabbits by the method described by the U.S. Department of Transportation in appendix A to 49 CFR part 173, it destroys or changes irreversibly the structure of the tissue at the site of contact following an exposure period of 4 hours. This term shall not refer to action on inanimate surfaces.
Asphyxiants can be physical or chemical. Physical asphyxiants are gases or vapors that dilute or displace oxygen normally in the atmosphere. (Vapors from flammable and combustible liquids displace oxygen in the environment, being heavier than air.) Chemical asphyxiants are chemicals that prevent the cells from taking up or transferring oxygen in the body or to the tissues. Carbon monoxide is a well-known asphyxiant, which chemically “ties up” the hemoglobin in the blood so that the body’s metabolism slows and stops.
Central Nervous System (CNS) Depressants affect the nervous system. This broad category includes vapors from most anesthetic gases, depressants, and organic solvents (a general category that includes most household cleaners as well as many paints, glues, and adhesives). Some CNS depressants produce a feeling of dizziness or giddiness. More severe effects (including death) can also result.
Systemic Toxicants dramatically affect specific organ systems. For example, mercury vapor, which Victorian hat makers had to inhale regularly when mercury was used in making hats, causes a serious nervous system disorder which could lead to insanity. (The “Mad Hatter” in Alice in Wonderland suffered from an occupational illness.)
Sensitizers are chemicals that cause a substantial proportion of exposed people or animals to develop an allergic reaction in normal tissue after repeated exposure to the chemical. (Examples are formaldehyde, poison ivy, and poison oak. Some epoxy resins and polyester resins can cause many people to have a sensitivity reaction and become ill.)

Many chemicals can have multiple effects. For example, xylene, commonly used in paint, is both an irritant and a CNS depressant.

Symptoms of toxic exposure include a broad range of reactions: chronic coughs, difficulty in breathing, skin ulcers, diarrhea, irregular heartbeat, headaches, dizziness, chest pain, sore eyes and skin, difficulty in sleeping, lack of appetite, weight loss, nausea, tremors, and many others.

However, the same symptoms can result from many other causes as well. Tracing a particular reaction to a specific source can be a challenge to even the most experienced environmental toxicologists, allergists, and industrial hygiene specialists. This is further complicated by the fact that many effects are delayed, and are apparent only later in life. The individual experiencing the symptom may no longer live near the original source, or may not even know that the exposure occurred.

At low enough exposure levels, a toxic substance will produce no observable harmful effects. As the dose increases, so does the potential for harm. For every substance—even table salt—there is a lethal dose.