Chemicals In The Media – A Guide
They are in the air, in our homes, in our food. We are reminded by the press every day that chemicals are invading our lives. But what does this really mean? As technology advances irrepressibly, are our lives becoming ever more toxic? This guide will hopefully allow you to make an informed response to the latest news story.
Firstly, we must answer the question – what is a chemical? Despite “chemical” becoming a byword for “poison” in modern parlance, everything is actually a chemical. It is a catch-all term to describe a pure material with a unique structural combination of atoms and bonds. Companies advertising “chemical-free” products are at best misunderstanding terminology, and at worst, outright lying.
Dihydrogen monoxide spews forth from our taps – it’s simply water – whilst we cover our chips in sodium chloride and dilute acetic acid – salt and vinegar. And no matter what manufacturers would like you to think, a chemical is no safer if it comes from a “natural” source. Once it is purified, a chemical does not remember where it came from, and whether a certain chemical is made in a lab or extracted from a plant, it will have an identical effect on you. Sense About Science has produced a great pamphlet that goes into further detail.
So, why do we hear more and more about chemicals being found in places they supposedly shouldn’t be? Quite simply, science has found better and better ways to analyse smaller and smaller amounts of material.
Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) can detect heavy metals down to the parts-per-trillion (ppt) level. To put that in perspective, if I dissolved a pinch of sodium chloride – table salt – into an Olympic-sized swimming pool full of pure water, the water would have sodium levels of approximately 100 ppt. The UK Government advises (PDF) that drinking water must have lead levels of less than 10,000 ppt to be safe, so we can easily detect lead to 0.01% of these safety limits. Not bad.
We’ve all heard the stories about cocaine being found on the majority of our banknotes. Well, high resolution liquid chromatography mass spectrometry (LC-MS) can detect as little as 4 picograms (0.000000000004 grams) of cocaine on one tenner. At that rate, you would have to be as rich as Bill Gates before you got a buzz!
Now that we are so efficient at detecting chemicals, should we be panicking when they are found where we don’t expect them? Famed Renaissance all-rounder Paracelsus said “the dose makes the poison”, and this is a perfect principle to fall back on. Essentially any substance on the planet can kill you if you ingest enough of it. But how much is enough?
To quantify the dangers of a substance, the LD50, or median lethal dose is used. This equates to the amount of the substance needed to kill, on average, 50% of the (animal rather than human) subjects of a toxicity test, and is usually expressed in mass of substance per body mass. Even benign chemicals like water have an LD50 value – water poisoning is a known killer.
So we have the LD50 of our rogue chemical – what does it mean? As an example, our old friend table salt has an LD50 for oral ingestion in rats of 3000 mg/kg. Extrapolating to an average human adult weighing 70 kg, this means eating 210 g (roughly ½ lb for imperialists) of salt would be deadly to 50% of humans. LD50 values can usually be found by searching for the Materials Safety Data Sheet (MSDS) for a substance, but remember that the data come from animal, rather than human testing.
Overall, the advice is not to panic. Chemicals are everywhere and can be detected in ever more minute amounts, so look for quantities detected and compare them to available toxicity data. Sensationalist news stories will never go away, so be ready to do your own research from reliable sources, and, if in doubt, get in touch with us!