by Rosie Coates

Here at science made simple we are celebrating Chemistry Week with a week’s worth of ‘Halogens and Health’.  Today is Tuesday, so it’s the turn of chlorine (Catch up with Monday’s edition on fluorine if you missed it).  They will be resources, experiments and we’re going to be taking part in the global experiment, why don’t you get involved too?


Chlorine gas. Photo: W. Oelen, CC-BY-SA

Clean Chlorine Chemistry

One of the best ways of keeping healthy is to keep clean, preventing the spread of disease. Chlorine has been used in cleaning and disinfection since at least 1773, when Louis-Bernard Guyton de Morveau used hydrochloric acid vapour to neutralise the ‘putrid exhalations’ found in the cathedral at Dijon.  There followed a rocky road in its use, partly because of the dangers of dealing with chlorine (not only could it kill off disease, but also people who came into contact with it, if incorrectly dosed), but perhaps more significantly because their was no agreed upon theory of contagion at the time.  It was hard to decide upon a weapon with which to fight disease when the enemy was not clearly understood.  Once a germ theory of disease was developed in the 1880s and 1890s chlorine really came into its own, and it hasn’t looked back!  We still use chlorine today, in cleaning products in our homes as well as in swimming pools.

If you want to find out more The Royal Society of Chemistry have put together some great resources on the use of chlorine in cleaning.


Chlorine Cures Colds?

Chlorine had been used in the first world war to devastating effect so it might come as a surprise that in 1924 the Washington Post bore the headline ‘Chlorine gas, war annihilator, aids President’s cold’  President Calvin Coolidge did indeed receive chlorine gas treatment over three days in May 1924.  What on earth was he thinking?


President Calvin Coolidge (1923-1929). Photo: Public Domain

In fact, Coolidge’s reaction to getting a cold wasn’t quite as extreme as it first sounds.  At the time influenza was a serious problem, an epidemic of the flu in 1918-1919 had killed at least 20 million people world-wide and the president had taken over the presidency from Warren G Harding who had died suddenly after a short illness with cold-like symptoms, Coolidge couldn’t take any chances and the nation needed to know he was looking after himself.  Still, a chlorine chamber?

It’s not actually as far-fetched as it sounds.  Chlorine was known to be a powerful disinfectant by this time and it was thought that this could enable it to kill many of the bacteria in the nose and throat.  It was also thought that it’s oxidising properties might break down toxins and that the fact that it caused production of mucus (that’s snot and phlegm to you and me) was actually a good thing- it meant that the germs causing the cold or flu should be being eradicated from the body. Not only were these plausible ideas but a paper published by Vedder and Sawyer in The Journal of the American Medical Association seemed to support this.  Their results showed that patients with a variety of respiratory diseases showed a great improvement following chlorine treatment.

So why aren’t we all sniffing this gorgeous green gas as the days get shorter, colder and more full of sneezes?

For one thing, even a small change in atmospheric conditions could turn a slight sniff of chlorine from something relatively harmless to a very dangerous activity, forming hydrochloric acid in the lungs.  Pretty unpleasant.  The main reason we don’t use it though is that the evidence wasn’t nearly compelling as it first appears.

Randomised Controlled Trials

In 1924 a control group of patients were not generally used when testing medicines.  This is what we now call a randomised controlled trial and it is recognised that this is essential to ensure that the medicine being tested is more effective than not giving the patient any treatment.  It was around the time of the chlorine trials that this idea really took hold, with Harold S Diehl, physician at the University of Minnesota criticising the previous study for failing to use any control patients and carrying out his own research which showed little difference between the chlorine and control groups. As he put it the ‘large percentage of recoveries within….seven days under such a variety of medical treatments is evidence only of the self limited character and short duration of most colds’.

A double-edged sword

Chlorine can be both helpful and harmful when it comes to health.  Inhaling the gas can cause severe irritation and death but when chlorine reacts with water it forms a powerful disinfectant, helping us to kill bacteria which could transmit diseases.  Chlorine is perfectly placed in the halogens to do this- not so mind-blowingly oxidising as fluorine but still oxidising enough to be a very useful tool in the fight against diseases.  Chlorine’s history in health shows us both the importance of understanding the causes of disease in order to fight them and how properly designed medical tests are essential when investigating new medicines.

You can test out the oxidising ability of chlorine in comparison with the other halogens with this neat experiment from the Royal Society of chemistry.

Need more chlorine in your life?

I hope you’ve enjoyed this dip into the history and chemistry of chlorine, I’ve certainly enjoyed indulging my interest in chlorine having been fortunate enough to be involved in a project in which undergraduate students over five ‘generations’ carried out original research into the history of chlorine.  If a blogpost on chlorine just isn’t enough for you our work was published as ‘An element of controversy: The Life of Chlorine in Science, Medicine, Technology and War’, Hasok Chang and Catherine Jackon Eds. (British Society for the History of Science, 2007).  I have used the excellent chapters by Anna Lewcock, Fiona Scott-Kerr, Elinor Mathieson, David Nader and Spasjole Marčinko as the basis for this blogpost.

an element of controversyAn element of controversy


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