Potassium Chlorate at High Altitude: Lost to History

Introduction

A new gas we now recognize as oxygen was discovered during the phlogiston theory era of the eighteenth century by Swedish chemist Carl Wilhelm Scheele (1742–1786) and independently by British clergyman-scientist Joseph Priestley (1733–1804). However, the name oxygen and its explanation as an element and its proper role in combustion and respiration are attributed to French chemist Antoine Lavoisier (1743–1794).^1,2 Lavoisier, the first to clearly understand the role of not only oxygen but also of carbon dioxide and nitrogen, sadly was guillotined at the height of his scientific powers by a mob during the French Revolution.

Certain chemicals release oxygen. Potassium chlorate is an inorganic compound containing potassium, chlorine, and oxygen with the molecular formula KClO₃ and was discovered in 1788 by French chemist Claude Louis Berthollet (1748–1822). ³ Names for the compound include Berthollet's salt, chlorate of potassium, and chlorate of potash. Berthollet discovered that potassium chlorate gave off more oxygen than potassium nitrate on heating and that when mixed with other substances the salt readily exploded. Berthollet also worked with Lavoisier to improve French gunpowder. Although explosives are an important aspect of the chemical history of potassium chlorate, there are many other uses, including production of matches, fireworks, textile dyes, pulp and paper , herbicides, bleach, and mild antiseptics and chemical analysis and chemical production of oxygen and more.^3,4–6 Chemically similar to potassium chlorate (KClO₃) is sodium chlorate (NaClO₃). Chlorate is the common name of the chlorate anion (ClO₃^–), and when it is combined with a cation such as potassium or sodium, the chemical compound also can be called a chlorate. With 3 oxygen atoms per molecule, chlorates give off prodigious amounts of oxygen when heated to high thermal decomposition temperatures. For example, 1 kg of KClO₃ produces about 274 L of oxygen at standard temperature and pressure. This paper focuses on some of the history and speculative uses of oral potassium chlorate at high altitude.

Potassium Chlorate Lozenges for Pharyngitis

Before discussing oral potassium chlorate as a possible source of supplemental oxygen at high altitude, throat lozenges must be mentioned in the first instance. It is unknown when potassium chlorate was first recognized as having a soothing effect on oral-pharyngeal-laryngeal mucous membranes, but its cooling and astringent saline taste by itself or in combination with other components made it a popular throat lozenge for many years.^4,7 In 1866, US patent 55292-A was awarded for an “improved bronchial troche” containing potassium chlorate and glycyrrhizic acid, a sweet extract of licorice root. ⁸

In the 19th and 20th centuries, potassium chlorate pastilles, lozenges, tabloids, tablets, powders, and pellets were marketed widely as a self-remedy for sore throats, ulcerative stomatitis, hoarseness, and raspy or husky voices. Such “treatment” was recommended for singers, clerics, and public speakers as well as anyone suffering from what might be recognized today as pharyngitis. Part of the widespread popularity in the United Kingdom was the result of the adoption of an American practice of what we recognize today as pharmaceutical representatives pushing patent and also legitimate medicines in the still-nascent days of medical science. This overseas surge of interest in the “product” was likely facilitated by Silas Burroughs (1846–1895) leaving the Wyeth pharmaceutical company of Philadelphia for London. Rather than using the usual term pill or tablet, Burroughs promoted the blended word tabloid, likely from “tablet ovoid,” and even trademarked it in 1878. Potassium chlorate tabloids, typically 5 grains (325 mg), were among the products Burroughs brought with him and advertised in 1879 in Lancet. The marketing methods of Burroughs Wellcome & Co, formed in 1880, led the way with this and other products. ⁹ Figure 1 shows an empty antique pill container of potassium chlorate tabloids made by Burroughs Wellcome. ¹⁰

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Figure 1.

Empty antique pill container of Burroughs Wellcome potassium chlorate tabloids. ¹⁰

The remedy was still popular into the 20th century. In 1907, it was noted that one of the problems of staying at high alpine resorts was that people were “subject to a troublesome sore throat,” and “chlorate of potash [potassium chlorate] tabloids [were] recommended.” ¹¹ On his 1907–1909 South Pole attempt, Ernest Shackleton took 7 lb (3 kg) of medical supplies, including potassium chlorate for topical or oral use for the ravages of cold, dry air. ¹² At the time, it was still erroneously believed that potassium chlorate at body temperature gave up oxygen to the tissues when used topically for skin ulcers or orally as a gargle for inflamed and spongy gums, aphthous ulcers, and tonsillitis. ¹³ Potassium chlorate continued as an ingredient in mouthwashes, gargles, dentifrices, and preparations for oral conditions with an apparent disregard for its limited efficacy and its toxicity. Toxic effects, with most fatalities occurring in the 19th century, included hemolytic anemia, methemoglobinemia, acute tubular necrosis, and death from as little as 1 g in children or small animals or cumulative doses >5 g in adults.^4,7,14–17 Despite occasional tragic incidents from overdose, various products with lower doses were tolerated well, but pharmacologic considerations slowly discredited potassium chlorate use, and it was dropped from the US National Formulary in 1960.^18,19 It no longer has any legitimate medical indication. ¹⁶ However, as recently as 2023, potassium chlorate was still an ingredient in homeopathic remedies for hemorrhoids, canker sores, and other medical and veterinary complaints. ²⁰

Oral Potassium Chlorate for Mountain Sickness

Possibly the first reported use of potassium chlorate for high altitude mountaineering was by the president of the Royal Meteorological Society, physician William Marcet (1828–1900) on Mont Blanc in 1868 or on a subsequent climb. During a later address to the Alpine Club, Marcet recalled that “I have known the substance, much used in medicine, called potassium chlorate, or chlorate of potash, taken in ten or fifteen grain doses [649–972 mg] dissolved in water prove very useful as a preventive or cure of mountain sickness [today referred to as acute mountain sickness], and would advise those who may be subject to that kind of illness to take some with them.” ²¹ How, why, and when this specious idea came about are not known to the authors. Marcet’s “much used in medicine” likely meant the commonplace panacea for sore throats and related symptoms that would be further aggravated in the cold, dry thin air of high altitude.

Nonetheless, others also began using potassium chlorate as a prophylaxis or treatment for mountain sickness. In 1870, on the first T. D. Forsythe (1827–1886) expedition to Central Asia, the medical officer was Calcutta Botanical Gardens superintendent George Henderson, MD. The team had crossed a pass of >18,800 ft (5730 m) height and suffered headache, prostration of mind and body, nausea, and blue lips. The local explanation blamed the strong-scented Artemisia plant, but Henderson correctly described that rapid ascent produced these signs and symptoms and descent alleviated them. Among his medical supplies, he wrote that he had, “a quantity of chlorate of potash [potassium chlorate], and gave a strong solution of it more as a placebo than with any belief that it would relieve the symptoms. However, it seemed to have a very good effect, but on what principle it acted I will not venture to conjecture.” ²² He was correct about the lack of principle. He did not state how he knew of this chemical, its possible use at high altitude, or what symptoms potentially might be relieved, but at least he admitted that he did not know how it might have worked.

In 1873–74, on a second Forsythe expedition to Central Asia, the Indian-born British medical officer-surgeon H. W. Bellew (1834–1892) wrote about disagreeable symptoms at 15,150 ft (4617 m). He listed headache, nausea, sense of suffocation, and what was likely periodic breathing of high altitude and felt that these were “produced by a continued deprivation of the natural quantum of oxygen in the atmosphere.” ²³ It was not a lesser quantity of oxygen but reduced oxygen pressure, a concept not well understood until later in Paul Bert's classic studies on the partial pressure of oxygen.^24,25 Bellew availed himself of “Dr G. Henderson's experience on his journey across this region in 1870” and wrote that “I had provided myself with a large supply of the salt he found so useful, and with very satisfactory results, as our further progress proved. I distributed little bottles of this chlorate of potash.” He mistakenly declared that the chemical mitigated the headache and nausea because a “large proportion of oxygen contained in the salt probably supplies to the blood what in these regions it fails to derive from the air and thus restores through the stomach what the lungs lose.” This is not chemically or physiologically correct, but he concluded that “no traveller ought to venture across these passes without a supply of this simple remedy in his pocket.” At the same time, he wisely recommended that “rest of mind and body were the best remedies.” ²³

British mountaineer Edward Whymper (1840–1911) also used oral potassium chlorate. Whymper was known for the 1865 first ascent of the Matterhorn when 4 members of his party died on the descent. Exploring in the Andes in 1879 and 1880, he had prolonged stays in the mountains with descriptions of symptoms that are now recognized as high altitude deterioration. ²⁶ As for acute mountain sickness, he reported headaches, shortness of breath, open-mouth breathing, altered respiratory patterns, and thirst. Whymper noted that these symptoms “lasted all night, and all the next day, and I then managed to pluck up spirit enough to get out some chlorate of potash, which by the advice of Dr Marcet had been brought in case of need.” He attributed the first use to Henderson rather than Marcet who had advised him: “Before my departure, Dr W. Marcet (with whom I had been in communication) urged me to experiment, with a view of confirming these experiences. Ten grains [648 mg] to a wine glass of water was the proportion he recommended, the dose to be repeated every two or three hours, if necessary. It appeared to me to operate beneficially, though it must be admitted that it was not easy to determine, as one might have recovered just as well without taking it at all. At all events, after taking it, the intensity of the symptoms diminished, there were fewer gaspings, and in some degree a feeling of relief.”^27,28 This response may have been due in large part to the placebo effect, rest and recovery, or even improved acclimatization.

Oral Potassium Chlorate as Supplemental Oxygen

The concept of using potassium chlorate orally for supplemental oxygen is only plausible to the modern reader when one considers the state of knowledge with regard to medical science in the 19th century. There was limited understanding of how oxygen was assimilated into the body. Granted, by the late 1870s, the great experimental physiologist Paul Bert had, for instance, produced the first oxygen and carbon dioxide dissociation curves and published other groundbreaking findings on the respiratory gases.^24,25 However, as is often the case with breakthroughs in medical science, there was considerable criticism of Bert's discoveries, and entrenched ideas from an earlier era persisted for decades. ²⁹

It would have been a substantial leap of speculation to suggest supplying oxygen by orally ingesting dry powders, granules, or tablets or dissolving them in liquids rather than heating potassium chlorate to 400°C. The convenience of a hypothetical oral method was highlighted later during the early 20th century controversy about using any supplemental oxygen on high mountains. Australian physical chemist and Everest oxygen pioneer George Ingle French (1888–1970) stated that there would be no objection by mountaineers if science could “produce oxygen in easily digestible form.” ³⁰ This quote suggests that the convenience of easily ingested oral “oxygen tablets” may not have been seen as an unsporting advantage in the 1920s. Perhaps this was the case then in much the same way that a modern mountaineer may readily use, for example, the drug dexamethasone to enhance performance at altitude while simultaneously decrying the use of supplemental oxygen as an unfair benefit.

Potassium Chlorate Used for the Chemical Production of Oxygen

As for the usual production of oxygen, nonindustrial users since the 1860s heated potassium chlorate with manganese dioxide as a catalyst in a retort (a device used for the distillation or decomposition of substances) to about 400°C. ³¹ During the latter 19th century and early 20th century, this heated chemical method in stationary devices was used to fill copper, cast iron, and steel cylinders or thick rubber bags. ³² The latter method was taken on the Annie Peck expedition to the Bolivian Andes in 1903. ³³ For decades, this was the preferred source of supplemental oxygen for minor users such as clinics or oxygen parlors. There, gaseous oxygen—sometimes with other ingredients such as nitrous oxide—was injected under the skin or infused by tubing into the rectum, vagina, urethra, nose, or mouth.

Other Chemical Processes Used to Generate Oxygen

Around the same time, other chemical processes were used to generate oxygen. For example, the industrial-grade barium oxide Brin process was used for large commercial purposes. A modification of a small superoxide Austro-Hungarian Nachfolger Pneumatogen mining and rescue device was briefly tried by British mountaineer Arnold Mumm (1859–1927) in the Himalaya in 1907.^33–35 During a 1920 Himalayan expedition, Scottish chemistry lecturer Alexander Kellas (1868–1921) tested a simple Leonard Hill closed-circuit breathing bag device. This was accomplished using the 1903 Oxylithe (sodium peroxide) of French chemical engineer George Jaubert (1870–1959) to produce oxygen and absorb carbon dioxide.^36–38

Contemporaneously, commercial-scale volumes of oxygen were produced using the refrigeration and cryogenic innovations of German mechanical engineer Carl von Linde (1842–1934). ³⁹ The fractional distillation of liquid oxygen from liquid air became the preferred production source of gaseous oxygen to compress into cylinders. During the 20th and 21st centuries, compressed oxygen containers made of progressively stronger, lighter alloys and filament-wound composites yielded much longer run times. As outlined later, the chlorates of the 19th century had applications for supplemental oxygen that grew in future years into unimaginable situations and heights.

Potassium Chlorate and the Everest Story

The 1922 British Everest expedition was the first to make serious use of supplemental oxygen on a big Himalayan peak. ⁴⁰ George Ingle Finch, the main proponent of supplemental oxygen on the 1922 British Everest climb, suggested that oxygen compressed into cylinders was the superior method for facilitating supplemental oxygen delivery for purposes of mountaineering. After the 1922 Everest climb, Finch recognized oral potassium chlorate tablets but forcefully dispelled the allure: “The oxygen of potassium chlorate is chemically very stable, and it is not absorbed by the blood, and for all the oxygen you would obtain by this means you might just as well take sodium chloride [table salt].” ⁴⁰ Major oxygen production options in the early 20th century included using heated potassium chlorate, the heated barium oxide Brin process, and Jaubert's Oxylithe. Around 1905, commercial quantities of liquid oxygen from liquid air and then volatilized to a gaseous state and compressed into cylinders became the standard production method. A major use was to fill tanks for oxyacetylene welding, but it quickly found applications in many other settings. ³⁹

Despite Finch's considerable expertise and comments, potassium chlorate was still used on later British Everest expeditions. On the 1924 expedition, Irish physician R. W. G. Hingston's list of medical supplies included “Tabl. pot. chlorate gr 5, 100.” ⁴¹ It is not known if he intended to treat sore throats or mountain sickness, but it could have been for either or both purposes. Even in the next decade, oral potassium chlorate use on the mountain was still in discussion. F. S. (Frank) Smythe (1900–1949) was a well-known British author and mountaineer in the Alps and Himalayas. Smythe was on three British Everest expeditions in the 1930s, but the 1933 expedition was perhaps the one for which he is best known. ⁴² Smythe later recounted Whymper's account of using potassium chlorate in an attempt to treat mountain sickness and, almost as an aside, revealed that the chemical had “been experimented with by [the] Mount Everest Expeditions but without noticeable alleviation of the effects of altitude.” ⁴³ During the 1933 Everest expedition, physician C. Raymond Greene's list of medical supplies was from the pharmaceutical company Burroughs Wellcome. The list included “2 containers ‘Tabloid’ Potassium Chlorate gr.5 (N.B. This quantity should be quadrupled)” ⁴⁴ (“N.B.” is Latin nota bene for “note well.”) Again, it is not known if Greene intended to treat sore throats in 1933 or, as Smythe suggested, mountain sickness. Smythe also was on the 1937 expedition, where physician Charles Warren used a modified version of Greene's medical list including 500 potassium chlorate tabloids. ⁴⁵

Chlorates and Supplemental Oxygen in Aircraft and Spacecraft

Berthollet would never have envisioned where various chlorates and perchlorates would go in the far-off future. In the present day, sodium chlorate is used as a chemical oxygen generator (COG), also known as an oxygen candle, for emergency oxygen supply in mines, ship holds, other enclosed spaces, and aircraft. Although compressed oxygen tanks of aviator's breathing oxygen and supplies of liquid oxygen are occasionally used in nonmilitary passenger aircraft, COGs usually supply short-term supplemental oxygen for passengers during a cabin depressurization emergency. ⁴⁶ Overhead of seat rows are drop-down masks. A passenger pulling on a mask and its tubing simultaneously tugs a lanyard attached to an ignitor that thermally activates the COG. A single COG the size of a household thermos bottle contains about 1 kg of mostly sodium chlorate, an impressive potentially destructive amount. The chemical is stable and usually safe, but not without risk as 3 incidents suggest.

In 1966, ValuJet Flight 592 caught fire and crashed in the Florida Everglades with the loss of 110 lives. This was a chlorate incident. The causes were multiple egregious errors in the labeling, handling, storing, and protection of firing pins of pallets of COGs in the cargo hold. ⁴⁷ Apparently the lesson was not learned. Another chlorate fire incident with aircraft COGs occurred in 2006. An aircraft maintenance facility in Alabama failed to properly identify and discharge COGs that had reached their expiration date but were still functional. The facility shipped them to a hazardous waste facility in North Carolina, where a fire broke out in the area where the COGs were stored underneath solid chlorine pool products. Thousands of residents had to be evacuated, and the building was destroyed. ⁴⁸ On January 28, 2025, an overhead bin caught fire while South Korean Air Busan Flight BX391 was on the ground. The interior and metal top of the aircraft were incinerated from nose to tail. ⁴⁹ The official investigation conclusions are pending, but news services have suggested that the fire began in a personal lithium battery power bank in an overhead compartment. ⁵⁰ The authors of this work speculate that while a lithium battery fire alone can be intense, the spread of the conflagration suggests that oxygen accentuated the fire. We speculate that extreme heat from a lithium battery fire might have thermally activated a chlorate-based COG located in close proximity below the storage bin and spread to multiple additional units.

Finally, manned spacecraft systems use electrolysis as the primary source of crew oxygen, but some craft use thermally activated perchlorate COGs to provide backup supplemental oxygen. ⁵¹ That form of oxygen-containing chemical boldly goes where 19th century Berthollet could not have foreseen.

Conclusions

This paper reviews potassium chlorate from the 18th century until the present. The uses on mountaineering expeditions ranged from simple lozenges for sore throats, to the erroneous idea that oral ingestion could provide supplemental oxygen, to actual chemical production of supplemental oxygen by thermal decomposition. Today's aircraft and spacecraft still employ chlorates and perchlorates to produce emergency oxygen.