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At a glance – How substances in trace amounts can cause large effects

At a glance – How substances in trace amounts can cause large effects

Posted on 22 August 2023 by John Mason, BaerbelW

On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added “At a glance” section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a “bump” for our ask. This week features “How substances in trace amounts can cause large effects“. More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.

At a glance

When the first edition of this rebuttal was posted in August 2011, atmospheric CO2 was at around 390 parts per million (ppm). Now (August 2023) the number is 421 ppm – and rising. 

To a non-chemist, 424 ppm might sound like a vanishingly small amount, simply because you’re comparing a small number (424) with a very big one (1,000,000). It therefore comes as no surprise to find that this apparent contrast was seized upon by practitioners of misinformation. Claiming that things occuring in apparently tiny amounts must be harmless is such an easy talking-point to bandy about, since much of the intended audience is unlikely to deal with such numbers on an everyday basis. But it’s also one great big red herring. Why? 

Hundreds of parts per million may not sound like a lot, but in fact many substances have important properties when present at such levels. Here are a few examples:

  • He wasn’t driving drunk, he just had a trace of blood alcohol; 800 ppm (0.08%) is the limit in all 50 US states and limits are even lower in most other countries.

  • Don’t worry about your iron deficiency, iron is only 4.4 ppm of your body’s atoms.

  • That ibuprofen pill can’t do you any good; it’s only 3 ppm of your body weight (200 mg in 60 kg person).

  • The Earth is only 3 ppm of the mass of the solar system.

  • Your children can drink that water, it only contains a trace of arsenic (0.01 ppm is the WHO and US EPA limit).

  • Ozone is only a trace gas: 0.1 ppm is the exposure limit established by the US National Institute for Occupational Safety and Health. The World Health Organization (WHO) recommends an ozone limit of 0.051 ppm.
  • A few parts per million of ink can turn a bucket of water blue. The color is caused by the absorption of the yellow/red colors from sunlight, leaving the blue. Twice as much ink causes a much stronger color, even though the total amount is still only a trace relative to the water.
  • Only 500 ppm of hydrogen sulphide (bad egg gas) in air is a hazardous level, as any health and safety fact-sheet will tell you. It will make you seriously unwell at that kind of concentration. In fact, at above 100 ppm, you can no longer smell the gas because its toxicity has switched off your sense of smell. 

Just a trace-gas? Yeah, right.

It’s not the concentration of a substance that necessarily matters. Instead, it’s what any substance can do at a certain concentration and that property will of course vary from one substance to another. These are all useful things to recall when someone dismissively tells you that carbon dioxide is “only a trace gas”. It doesn’t matter.

Please use this form to provide feedback about this new “At a glance” section. Read a more technical version below or dig deeper via the tabs above

In case you’d like to explore more of our recently updated rebuttals, here are the links to all of them:

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