Module 3: Toxicology - Section 2: Classification of hazardous substances

TOX 2.2: Making sense of the multiplicity of chemical substances

Note: How can I make sense of all of these chemical substances?

There are thousands of chemical substances used in industry. The Hazardous Chemical Substances under the OHS Act lists over 700 items.

Encountering all of these chemical substances for the first time can be a dizzying experience, let alone remembering all the facts about them. At the diploma level, we do not believe you need to be able to remember a large amount of detail about many compounds.

Instead, consider the occupational health situations in which you will encounter them:

1. You see an individual worker who has had a specific exposure and is worried about the possible effects.
2. You see someone with a specific complaint , e.g. a skin rash or a blood disease, and the question arises as to whether there may be an occupational cause.
3. As part of risk assessment. You have a contract at a company and you are presented with a list of chemicals on site. You need to advise on health risks, medical surveillance, etc.
4. Preparing educational or risk communication material.
5. Advocating for stronger controls, e.g. stronger occupational exposure limits or even banning.

Parallel situations arise with environmental impacts, except that you will be dealing with community residents rather than workers.

The next question is what information you will need to solve the problem. Usually these are human, and specifically occupational, health effects, i.e. from inhalation and contact and not ingestion.

The first thing you will do is try to remember what you can about the substance. With experience, the list of what you remember will grow but will never become very long unless you are in an academic setting.

The value of a classification system is that you can probably infer quite a bit about possible effects just from the category. For example, by knowing that something is an aliphatic hydrocarbon probably gets you as far as you need to. (You may want to know how long the chain is, since this affects volatility). Even more generally, knowing that something is an organic solvent tells you, for example, that you must consider skin effects.

However, this may not be enough and you will then turn to an accessible information source to find out more about the substance. There may be an industry provided Material Safety Data Sheet available which may have what you need. However, the information on MSDSs about human health effects, particularly from long term exposures, is limited. Absence of information on a MSDS about an occupational health effect does not mean absence of an effect.

The next step will be to consult a more detailed database. This may be electronic or a textbook, and you should develop your usual sources. The typical summary will be a combination of a little bit of biochemistry, some in vitro or animal toxicology, a summary of clinical case data (acute effects) and some epidemiology of long term exposures. For the latter, a PubMed search may be useful.

For a diploma course, what you need to know are the classification systems, some of the more important chemical substances (which we shall cover or emphasise) and how to access information that you need.

Closing comment:

Chemical substances that are mined, processed or manufactured are ubiquitous and the modern world is unthinkable without them. There is tendency to assume that anything with a chemical name is toxic, and the longer or more obscure the name, the more toxic it must be. There may a useful precautionary element in this, but someone with training in occupational health should move beyond this approach. It is the purpose of this and related sections to help you to do so.

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, Rajen Naidoo, Saloshni Naidoo. 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.
Disclaimer note: Some resources and descriptions may be out-dated. For suggested updates and feedback, please contact healthoer@uct.ac.za.