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Hazard, risk, human health and pesticides

Basic principles of toxicology

Toxicology is the study of adverse effects of substances on living organisms

Toxicology is the study of adverse effects of substances on living organisms. Toxicology work enables the the hazard level of a given pesticide to be determined and underpins both the assessment and management of risk associated with that pesticide.

How the human body responds to a toxic chemical (i.e., the toxic effect) depends upon the:

  • dose received (i.e. the amount of the chemical taken into the body); the
  • chemical and physical properties of the chemical (where the chemical properties refer to how the substance reacts chemically and the physical properties are the measurable aspects of the substance that don’t change e.g. boiling point, colour, smell); the 
  • exposure situation (e.g. is the person indoors or outdoors; what type of clothing is being worn i.e. would the clothing protect the person from the chemical or increase exposure?); and the 
  • susceptibility of the person (e.g. is the person ill or weak or have a larger or smaller body size e.g. adult or child)

The interaction of the above four factors will determine the toxic effect.

For example:

Dose Received: In general, if a man drinks one glass of wine, it will have less affect on him than if he drinks five glasses of wine in the same period of time. Thus the size of the dose (one glass versus five glasses) has a direct impact on the effects.

Chemical and physical properties: However, if he drinks one glass of pure methanol the toxic effect on his body will far exceed the potential toxic effect on his body, of one glass of wine (which contains ethanol). This is because  – although ethanol and methanol are from the same broad chemical class - methanol has different chemical and physical properties to wine  and thus interacts inside the body in a different (and much more serious) way.

Exposure Situation: If a person drinks three glasses of wine with a large meal, absorption will be slower and thus the likelihood of being affected will be lower than if they drink three glasses of wine on an empty stomach. Exposure is also related to the route of absorption – pouring a large amount of wine on the skin has a negligible potential for toxic effect as opposed to pouring it down the throat because much less alcohol is absorbed through the skin than via the digestive tract.

Susceptibility of the person: Finally, the individual characteristics, the susceptibility, of the person comes into account. For example:

  • If a small child drinks one glass of wine he/she will be more affected than if an adult drinks one glass of wine, because amongst other factors, the child has a smaller body size and a less developed liver and so processes the same dose less effectively.

  • Some people tolerate alcohol better than others due to having higher levels of the enzymes that break down alcohol in the blood, so they can take higher doses before they have the same level of toxic effect. 

  • Other people are allergic to alcohol and so their bodies react in a different and more serious way to a non-allergic person, when given the same dose of alcohol.

Understanding toxic effects is therefore a complex interaction of many factors.

Hazard, risk, human health and pesticides | Basic principles of toxicology | Assessing the risk to human health | Glossary - human health section