what is a chemical?

When I tried to describe what entities were covered by toxicology, I thought ‘that’s easy, it’s chemicals’! But it is very important to note that not all chemicals are ‘bad’, that the body is composed of chemicals, and we could not live without chemicals (as well as finding many chemicals useful and convenient). There are also different terms, some used interchangeably, some with important differences and meanings. This post will try and clarify the matter, and explore how these definitions affect toxicology. It is a work in progress!

Good vs bad; an unhelpful dichotomy

Synthetic does not equal bad, natural does not equal good; both can be good, bad, harmful, beneficial or neutral)

So, there is a distinction to be made between terms like man-made (‘synthetic’), natural, exogenous, endogenous; although it is still the case that toxicity or harm doesn’t naturally follow these subdivisions, cyanide is natural (found in almonds, millet sprouts, lima beans, soy, spinach, bamboo shoots, cassava, US ATSDR).

Human activity can make new chemicals, “man-made”, but also causes existing chemicals to be re-located, and in such  a way that the substance becomes harmful (for example asbestos and cadmium). Man-made, synthetic chemicals include medicines and pesticides, by-products (e.g. persistent organochlorine pollutants (POPs) like PCBs, Furans and Dioxins).

Synthetic vs natural, is not precise enough to separate chemicals made in a factory from those atoms found in nature, small compounds made in the body (metabolism, biosynthesis), or larger hormones, mediators and even amino acids and proteins.

Are proteins chemicals? Recombinant proteins, like insulin, certainly require a safety assessment with similar aims to the toxicological assessment of small molecule drugs etc. Are stem cells chemicals? Clearly not! but they are regulated, and need to be, perhaps more like medical devices but with added aspects that are more chemical like.

In the scientific sense, a chemical is any substance or compound studied or used in chemistry. So, is water (H2O) a chemical, a substance or a compound? Water is a liquid composed mostly of water molecules, in which two atoms of hydrogen H are covalently bonded to one oxygen atom.

Toxins or Toxicants?

It seems natural that toxicology would be concerned with the effects of toxins or toxicants; perhaps it should be clearly defined as concerned with the toxic effects of chemicals or other entities, in which case the emphasis is on the toxicity.

Toxins are often taken to refer to natural poisons, like snake venom and bacterial products like botulinum toxin (interestingly cosmetic Botox is often given as an example of a high risk entity which can by safely (low hazard)  injected into the face because it remains subdermally localized – if botox were to ‘go’ systemic then unacceptable toxicity might occur. Not everyone thinks that this is a sensible risk management option however!)

Toxicants is probably a good term for something that causes toxicity, but this could become rather circular…

 

what’s in a name?

Does it matter what we call these things? 

“caffeine is a vitamin” (anon.);

…although this quote was meant tongue-in-cheek, preferably a cheek filled with coffee too!, it is worth noting that caffeine is known to retard birthweight in mothers who consume more than a certain amount during pregnancy. Hazard labeling is part of this issue, for example. In some cases, the dose underpins hazard labelling, but not explicitly so: for example acetic acid is considered harmless at <10% , and is found at around 5% in vinegar; at 10-25% it is labeled as an irritant, at 25-90% as corrosive, and at >90% corrosive and flammable.l

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