Module 3: Toxicology - Section 3: Linking Chemical Exposures and Health Effects

TOX 3.2: Factors Determining Hazard Potential

FACTORS DETERMINING HAZARD POTENTIAL

Although most chemicals have a toxic potential, the extent to which they pose a hazard to health will depend on the interplay between a number of factors.

• The physical state of the chemical:

  Chemical substances may exist as a single phase medium such as a solid, liquid or gas, or coexist in more than one form. Solids can give off hazardous gases or vapours, be changed to fumes by burning or melting (e.g. heavy metals) or be changed to airborne dusts by grinding, cutting or abrasion. Dispersed in the air they are easily inhaled and are able to exert their toxic effects. Lipid soluble liquids (e.g. phenol, aniline) are easily absorbed through the skin. Liquids can be dispersed as mists or give off vapours by industrial processes and in this way become airborne.

• The route of exposure of the chemical

  Chemical substances can enter the body through inhalation, which is by far the most common route of entry for a large number of chemicals. Absorption through the skin or via the eyes occurs either through direct skin contact or from contact with contaminated surfaces or clothing. Chemical ingestion in the workplace may occur either by chemicals settling directly on food or workers eating food with contaminated fingers.

• The exposure dose of the chemical

  The most important environmental factors determining exposure dose include the concentration, duration and frequency of exposure to the chemical concerned. Certain chemicals such as sensitisers and irritants may produce adverse local health effects from single episodes of exposure to very high concentrations of the substance. Other chemicals such as solvents and heavy metals exert their systemic health effects as a result of repeated low dose exposure to these chemicals. The presence of administrative controls such as job rotation, use of personal protective equipment and engineering controls (e.g. local exhaust ventilation) may also modify the exposure dose.

• Individual differences among exposed individuals

  There are a number of host factors that determine the extent to which chemicals are absorbed, distributed, metabolised and excreted by the body. Individual physical characteristics such as physical work load (e.g. heavy manual work underground) and skin characteristics (e.g. presence of pre-existing eczema) can increase the amount of chemical absorbed by the body. Furthermore, dietary patterns can also increase the amount of chemicals absorbed due to chemical contamination of the food chain (e.g. arsenic in fish), making it difficult to differentiate between occupational exposure and general background environmental exposure. The body size and composition (fat content) are particularly important since they determine the potential surface area available for distribution and deposition of the chemical in various parts of the body. Certain solvents (e.g. benzene) have a high affinity for fat deposition and result in increased blood levels even after exposure has ceased. There are various factors such as age, gender, and social habits that modify the manner in which the chemical is metabolised and excreted by the body. Social habits such as smoking tobacco and alcohol consumption increase the absorption and the metabolism of chemicals such as lead. Pre-existing kidney disease and old age are associated with decreased excretion of the chemical, thereby increasing the concentration of heavy metals (e.g. mercury, cadmium and lead) in the body to exert their toxic effects.

Postgraduate Diploma in Occupational Health (DOH) - Modules 3: Occupational Medicine & Toxicology (Basic) by Profs Mohamed Jeebhay and Rodney Ehrlich, Health Sciences UCT is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License. Major contributors: Mohamed Jeebhay, Rodney Ehrlich, Jonny Myers, Leslie London, Sophie Kisting, Gail Todd, Neil White. Source available from here. For any updates to the material, or more permissions beyond the scope of this license, please email healthoer@uct.ac.za or visit www.healthedu.uct.ac.za. Last updated Jan 2007.
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