Kim Duquette Blog

Toxicology is one of the oldest fields of scientific study and involves the detection, effects and antidotes of poisons. A poison is defined as a substance that causes structural damage or functional disturbance in the body. Depending on the dose, almost any substance can be considered a poison. Water, for instance, if ingested in great quantities can cause a condition known as hyponatremia, which is a dilution of the blood’s electrolytes that can lead to swelling of the brain, fluid in the lungs, and death.

Routes of Entry

The way a substance enters the body, known as the route of entry, can have a major impact on the toxin’s actions inside the body. The routes of entry include inhalation, ingestion, absorption and injection.

Inhaling a substance into the lungs is an easy way to be exposed to a chemical. Inhaling a gas or vapour introduces it into the bloodstream, and therefore the brain, very quickly. When something is inhaled, absorption into the bloodstream starts in the sinuses and continues in the lungs. If a corrosive substance is inhaled, the damage can be immediate and irreparable.

The ingestion of a poison is usually caused by poor hygiene. Not washing the hands properly after handling toxic materials and then eating a meal or smoking a cigarette can increase the chances of accidental exposure.

Many chemicals can be absorbed through the skin upon direct contact; for example, organic solvents pass through the skin very easily, dissolving the fat bodies under the skin’s surface and causing the skin to become dry and cracked. Chemicals in vapour form can also be absorbed by the eyes and cause irritation and/or damage.

Injection refers to the entry of a toxin into the blood stream by direct blood contact. This can be through a needle or by exposing a cut or scrape to a toxin. Persons working with blood borne pathogens have an increased risk of exposure and must take precautions to protect themselves. Intravenous drug use where needles are shared can also expose individuals to infectious blood borne pathogens, such as Hepatitis and HIV.

Response to Exposure

Not everyone reacts the same way to the same amount of a toxin. In order to get some idea of the strength of a toxin, a statistical method is used. The toxicity of a compound is usually expressed on an SDS as an LD50 (Lethal Dose 50%), which is the amount of a toxin required to kill 50% of the test population. Lethal Dose measurements are indicators of the acute response to a toxin. An acute response is an effect that has a rapid onset, severe symptoms (including death), and a short course. A much more difficult response to measure is the chronic (long term) response to exposures to small amounts of a toxin. If one is exposed to small amounts over a long period of time, it is almost impossible to determine when the threshold dose will be crossed and effects will occur. A third type of response is the latent response. Latency is seen when someone is exposed to a chemical for a short period of time and then the exposure stops, and many years later an occupational disease develops.

Occupational Exposure Limits (OELs)

Because it is nearly impossible to predict how an individual will react to a small amount of a toxin, industrial hygienists use human and animal testing to determine exposure limits. These exposure levels represent conditions that nearly all workers may be repeatedly exposed to, day after day, with no adverse health effects. These exposure limits are listed in Ontario Regulation 833, Control of Exposure to Biological or Chemical Agents.

The Occupational Exposure Limits include:

  • Time-Weighted Average Limit (TWA)
  • Short-Term Exposure Limit (STEL)
  • Ceiling Limit (C)


NORCAT offers an online Toxicology program for $29.95.