Most toxic substances can be classified as irritants, anesthetics and narcotics, systemic poisons, sensitizers, carcinogens, mutagens, and/or teratogenic substances. Systemic poisons may be further segregated into the categories of hepatotoxic agents, nephrotoxic agents, neurotoxic agents, agents which act on the blood or hematopoietic system, and agents which damage the lung.


Irritants are substances with the ability to cause inflammation or chemical burns of the eyes, skin, nose, throat, lungs, and other tissues of the body in which they may come in contact. Some substances such as strong acids (e.g., sulfuric acid, hydrochloric acid, hydrofluoric acid, or nitric acid) may be irritating to the point of being corrosive when concentrated, and may quickly cause second and third degree chemical burns upon contact with the skin or eyes. If inhaled as a gas, vapor, fume, mist, or dust, they may cause severe lung injury, and if ingested, can seriously damage the mouth, throat, stomach, and/or intestinal tract. Yet other irritants may have milder effects and may only cause reddening of the skin or eyes after contact.

Some of the most common irritants are organic or hydrocarbon fuels which can dissolve natural oils in the skin and cause dermatitis. After repeated or prolonged contact, these will dry the skin to the point that it may become cracked, inflamed and possibly infected. These same materials often cause irritation of the eyes and possibly loss of vision upon contact of the epithelium, a clear thin membrane that covers the surface of the cornea.

Entry into the lungs of many hydrocarbons and some organic liquids that are irritants may cause chemical pneumonia or pneumonitis together with pulmonary edema (filling of the lungs with fluid), hemorrhage (bleeding), and tissue necrosis (death of living tissue). Since entry of liquids into the lungs usually involves aspiration when a victim who has accidentally ingested the substance vomits, the first aid instructions for such substances typically recommend against intentionally inducing vomiting. They also are likely to mention that the effects of aspiration into the lungs may not appear for several hours or even days after the exposure has taken place.


Simple asphyxiants are typically non-toxic gases that may cause injury by inhalation only if they are present in air in such high concentrations that they displace and exclude the oxygen needed to maintain consciousness and life. A good example is nitrogen, a gas that makes up about 78% of the air we breath and which is perfectly harmless at this level as a component of air. If additional nitrogen or another such simple asphyxiant were added to the air to the point that the normal oxygen concentration of approximately 21 percent by volume was significantly reduced, however, the situation could become life-threatening. Another example: A tank of Argon, an inert gas, ruptures in a enclosed building. The gas released from the tank displaces the air (oxygen) around it causing anyone in the effected area to become unable to breathe because of lack of oxygen. Methane, a common decomposition chemical, can accumulate in confined spaces or low-lying areas (gulleys), causing those to walk there to asphyxiate.

Chemical asphyxiants are substances that in one way or another prevent the body from using the oxygen it takes in and are often highly toxic substances. One common, classic example is carbon monoxide which combines with and ties up the component of blood (hemoglobin) that transports oxygen from our lungs to other organs. If too much of the hemoglobin becomes unavailable for carrying oxygen, a person may pass out and eventually die.


1st Stage: 20.9 - 19.5 % oxygen by volume, increase pulse and breathing rate with disturbed muscular coordination.

2nd Stage: 12 - 19.5 % oxygen by volume, faulty judgment, rapid fatigue, and insensitivity to pain.

3rd Stage: 10 - 6 % oxygen by volume, nausea and vomiting, collapse, and permanent brain damage.

4th Stage: Less than 6 % by volume, convulsion, breathing stopped, and death.


Numerous hydrocarbon and organic compounds classifies as hazardous materials, including some alcohols, act on the body by depressing the central nervous system (CNS). Early symptoms of exposure to these substances include dizziness, drowsiness, weakness, fatigue, and lack of coordination. Severe exposures may lead to unconsciousness, paralysis of the respiratory system, and possibly death.


A few hazardous materials are sensitizers and cause sensitization. What this means is that some people who are exposed to one of these materials may not be abnormally affected the first time but may experience significant and possibly dangerous effects even in the presence of very low levels of the contaminant if ever exposed again. Victims become extremely allergic to the material and possibly others of a similar nature.


  • Hepatotoxic agents - materials that cause liver damage.
  • Nephrotoxic agents - materials that cause kidney damage.
  • Neurotoxic agents - substances that in one way or another impact the nervous system and possibly cause neurological damage.
  • Carinogens - substances that may incite or produce cancer within some part of the body.
  • Mutagens - can produce changes in the genetic materials of cells.
  • Teratogenic - materials may have adverse effects on sperm, eggs, and/or fetal tissue.
  • Hematopoietic system (blood) - Some chemicals can effect the blood and the blood forming tissues.


Acute effect - Adverse health effects that are usually caused by an exposure of short duration. (minutes, hours, days) An adverse effect on a human or animal body resulting from a single exposure with symptoms developing almost immediately or shortly after exposure occurs.

Chronic effects - Effects that occur after a longer period of time (months, years) An adverse effect on a human or animal body resulting from repeated low level exposure, with symptoms that develop slowly over a long period of time or that reoccur frequently.

When a major accident or other rare event causes a significant spill or discharge of a toxic material into the environment, the general public or nearby workers may be exposed to relatively high levels of one or more toxic contaminants until such time as they escape, are rescued from contaminated locations, or the contaminant itself becomes diluted below hazardous levels.

These short-term, rare exposures in the sense there will be long periods of time between repeated exposures (if they reoccur at all) are referred to as acute exposures. Not all acute exposures, of course, need involve high concentrations of toxic materials. A small spill or discharge may produce low levels of contamination yet still be of an acute nature.

Many chemicals will not persist for long periods of time in the environment, or at least in those parts of the environment of concern, while others may remain present for weeks, months, or years. Materials that do not persist for long periods are unlikely to pose long-term chronic hazards in the event of a major spill or discharge. There is controversy over what constitutes a "safe level" especially for non-work related exposures such as in a residential area, where exposure is not limited to eight hours per day and continues on through weekends. Nor has it been proven that these "safe levels" are appropriate for populations that have been previously and acutely exposed and sensitized from previous exposure. Employeers have a duty to protect the health of their workers, but this duty does not apply to nearby residents and passersby.

Great uncertainty also exists concerning multiple chemical exposures and any possible synergistic reactions and what is the appropriate "safe level." OSHA PELs have been known to be wrong. For example, Methylene Chloride once had an OSHA PEL of 500 parts per million (ppm), and now, over a period of ten years, that PEL has been revised downward to 25 ppm. 1,3 Butadiene once had an OSHA PEL of 1,000 ppm, and this has now been reduced to 1ppm.

Chemicals which are relatively inert and which do not degrade, react, vaporize, or dissolve freely may pose health hazards for extended periods of time within a localized environment and may require additional planning to address long-term chronic exposure hazards to the public. Examples include heavy metals (lead, cadmium, chromium) and various chlorinated hydrocarbons such as DDT, trichloroethylene, and PCBs.


In considering the effects of toxic exposures, it is necessary to understand that the duration of an exposure can be as important as the level of exposure in determining the outcome. The body has a capacity to cope with the intake of many contaminants as a certain rate. Below a certain threshold rate of intake or absorption, the body has an ability to excrete or somehow convert the contaminant to a harmless substance, so toxic effects may be minimal or nonexistent. For example, arsenic is commonly found in all human bodies at low levels. It is only when the level exceeds the safe threshold due to excessive intake that symptoms of toxicity become apparent.

The rate at which a contaminant enters the body by inhalation is a function of the concentration of the contaminant in the air, and the specific properties of the contaminant. Higher concentrations in air obviously lead to higher rates of intake or absorption into bodily tissues.

The potential for toxic effects via skin absorption is a function of the amount of toxic material that contacts the body, the properties of the material, and the length of time it is permitted to remain in contact.

Toxic effects via ingestion can also be a function of the amount or rate of intake over a period of time. Small doses of certain poisons ingested hours or days apart may not be harmful, but taken the total amount all at once may be deadly. Other poisons may accumulate in the body such that small doses taken over time may buildup to a fatal dose.

The reason that chronic exposure to low levels of toxic materials commonly found in the environment does not often cause widespread health problems is likely that the rate of intake is below the threshold at which health effects become apparent.

Major spills or releases of toxic materials may pose a significant threat to public health because the resulting contaminant concentrations in the local area may be so high that only a moment or two of exposure is sufficient to produce severe health problems due to an excessive body burden of contamination. This is particularly true where large amounts of toxic gases or vapors are released into the air. Most at risk in such situations are emergency response personnel who enter contaminated areas without adequate personnel protective clothing and respiratory devices in attempts to contain or otherwise mitigate the impacts of the spill.


Toxicology is based on the dose-response relationship. This relates the amount of a substance (the dose) given to a test animal to the effect shown by the animal (the response).

The simplest study relates the percentage of test animals which die (mortality) to the dose given. The dose is usually expressed in mg/kg (for ingestion or inoculation), in mg/m3 (for skin exposure), or in parts-per-million (ppm). The response is expressed in percent of animals which have died.

The LD50 is the lethal dose of a substance that caused the death of 50% of a group of laboratory animals used in experiments. The important thing to remember for hazard awareness is that the smaller an LD50 is, the more toxic the chemical is likely to be. Also bear in mind that many of the laboratory animals died at various levels of exposure to the chemical before the point was reached where half had died. Some people will get ill at levels below the LD50.


For more information about health effects of chemicals, try the following government-sponsored websites:

healthfinder - Department of Health and Human Services' gateway to medical journals, news, databases, libraries, state agencies, educational sites, organizations, and support groups www.healthfinder.gov

National Institutes of Health - federal health information resources, clinical-trial databases, consumer health publications, and an index of health conditions being investigated by the government. www.nih.gov

National Library of Medicine features MedLine, a free database of citations and abstracts from 3,900 medical journals www.nlm.nih.gov