Concentration and the duration of exposure are the two main factors in addition to others which determine the degree of toxicity caused by a chemical agent.
Exposure to toxic agents is usually followed by its absorption and entry into the blood circulation. The concentration of the toxic agent in blood stream of the organisms exposed, therefore, is an important and useful parameter to assess the degree of toxicity, the rate of absorption, the rate of elimination and detoxification as blood samples can be drawn out periodically and examined for the toxic agent or its metabolites.
Based on the concentration and the duration of exposure of the organism to a toxic agent toxic exposure can be of the following types.
1. Brief Exposures to Small Concentrations of Toxic Agents below the Threshold of Toxicity:
These exposures produce little or no effect on a biological system. They are in fact useful as they help to prepare the organisms and acclimatise the system to face any further exposure to the toxic agent.
2. Repeated or Continuous Exposure to Small Concentrations of Toxic agent:
An organism may be exposed to small concentration of toxic agents being introduced repeatedly or continuously. If the rate of entry of the chemical is slightly higher than the rate of its elimination, small quantities of toxic agent gradually accumulate to reach beyond the threshold of toxicity.
Such toxicities which are caused by a gradual accumulation of small amounts of toxic agents over a prolonged period of time are known as chronic toxicity. The exposure may be repeated or continuous but the concentration of the toxic agent absorbed is very small-so small that it may often evade detection. The toxicity manifests itself only after a long period of time when the cumulative action of small doses of toxicagents cross the threshold of toxicity. Chronic toxicity is characterised by a slow and gradual development of malfunctions in the normal activity.
3. Exposure of Large Doses of Toxic Agents:
Accidental exposures or deliberate administration of large and lethal doses cause drastic symptoms of toxicity which appear suddenly and result in emergency. The concentration of toxic agent in the blood stream of the organism rises sharply well beyond the threshold limit and this is attended by rapid development of toxicity symptoms, often with a possible lethal outcome. Such a poisoning or toxicity is known as Acute Toxicity.