Ozone

• 1. Introduction

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  Ozone, a gas made of three oxygen atoms, announced itself by smell long before anyone knew what it was. In 1785, the Dutch chemist Martinus van Marum was running electrical sparks above water when an unfamiliar odour reached him. He credited the electrical reactions. He had in fact just made ozone.

  The same molecule has since been praised as healthy air, tested on wounds, and condemned as an acutely toxic gas. Yet high in the atmosphere, it guards living things.

  How can one molecule be both a shield and a hazard? Why did generations of people believe it was good for them? And how do you manufacture a gas that falls apart too quickly to be stored?

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• 2. Smell Before Substance

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  Christian Friedrich Schönbein recognized the pungent odour as the smell that often follows a bolt of lightning. In 1839 he isolated the gas and named it ozone, after the Greek word ozein, meaning to smell. That naming is why he is generally credited with the discovery.

  In 1844, Schönbein proved that white phosphorus reacting with air yields the very same substance, whose smell he had already compared to phosphorus. When others suggested calling ozone "electrified oxygen", he mocked the idea. He proposed calling the phosphorus-made version "phosphorized oxygen" instead.

  The formula O3 was not worked out until 1865, when Jacques-Louis Soret determined it. Schönbein confirmed the result in 1867.

  A rival idea survived for decades afterward. Until the 1920s, chemists could not rule out small amounts of a four-atom form called oxozone, written O4. Concentrated ozone is explosive, which made careful analysis very difficult.

  In 1923, Georg-Maria Schwab, working on his doctoral thesis under Ernst Hermann Riesenfeld, became the first to solidify ozone and analyse it accurately. His measurements conclusively refuted the oxozone hypothesis. Riesenfeld's group went on to record other properties of the pure, concentrated gas that nobody had measured before.

  Chemists still prefer the name Schönbein coined to the systematic alternatives, which include the unwieldy 2λ4-trioxidiene.

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• 3. A Fragile Molecule

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  The three atoms of ozone sit in a bent arrangement, with an angle of 116.78 degrees and oxygen-oxygen distances of 127.2 picometres. Microwave spectroscopy supplied the evidence, and the shape resembles that of a water molecule. Chemists describe it as a hybrid of two forms, each with a single bond on one side and a double bond on the other. Averaged together, the bonds have an order of 1.5. The molecule is also polar, with a dipole moment of 0.53 D.

  At 161 K, ozone gas condenses into a dark blue liquid. Below 80 K it freezes into a violet-black solid. The gas itself is colourless or pale blue and only slightly soluble in water. It dissolves far better in solvents such as carbon tetrachloride, where it forms a blue solution.

  Letting the liquid warm to its boiling point is dangerous, because concentrated ozone can detonate. Heat, physical shock or fast warming can all set off an explosive breakdown. That is why industry handles ozone only in low concentrations. A spark can trigger deflagration at concentrations of 10 percent by weight or higher.

  Under laboratory conditions, in still, dry air at 24 degrees Celsius, ozone has a half-life averaging about 1500 minutes, or 25 hours. Warmth speeds the decay. Humidity and air movement change it too.

  Ozone is among the strongest oxidising agents known. It oxidises most metals into oxides in their highest oxidation state, sparing only gold, platinum and iridium. It turns carbon into carbon dioxide even at room temperature. It converts nitric oxide into nitrogen dioxide, and that reaction gives off a glow called chemiluminescence.

  Organic molecules fare no better. In ozonolysis, ozone cleaves the double bonds of alkenes, usually in a solution of dichloromethane at minus 78 degrees Celsius. The first product is an organic ozonide.

  Reducing ozone itself gives the ozonide anion. Its compounds are explosive and must be stored at cryogenic temperatures, yet ozonides of every alkali metal are known.

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• 4. Fresh Air Folklore

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  Beaumont, California, once adopted the official slogan "Beaumont: Zone of Ozone", which appeared on postcards and Chamber of Commerce letterhead. For much of the late 19th century and well into the 20th, naturalists and health-seekers counted ozone as a healthy part of the environment. The naturalist Henry Henshaw, working in Hawaii, wrote of higher elevations: "There is quite a different atmosphere at higher elevation with enough ozone to sustain the necessary energy to work".

  Seaside air carried the same reputation, and it rested on a mistake. In Victorian Britain, a folk myth blamed ozone for the smell of the sea, which people found bracing. The real source is a mix of halogenated seaweed metabolites and dimethyl sulphide, a chemical generated by phytoplankton. Ozone's appeal seems to have come mostly from its fresh smell, which suggested purity.

  Scientists noticed the harm early. In 1873, James Dewar and John Gray McKendrick reported that frogs grew sluggish, birds gasped for breath, and rabbit blood carried less oxygen after exposure to "ozonized air". Schönbein himself had recorded chest pains, irritated membranes and breathing trouble, and small mammals died after inhaling it.

  In 1911, Leonard Hill and Martin Flack wrote in the Proceedings of the Royal Society B that ozone's healthful reputation had become common belief "by mere iteration". They found almost no exact physiological evidence for any benefit. The one well-established effect, they said, was irritation and oedema of the lungs, and death at strong concentrations.

  During World War I, the Queen Alexandra Military Hospital in London tested ozone as a wound disinfectant. The gas was applied directly to wounds for as long as 15 minutes. It damaged bacterial cells and human tissue alike, and rinsing with antiseptics proved the better choice.

  The London Underground tried a wider version. In 1908, artificial ozonisation of the Central Line began as a way to disinfect the air. It was found worthwhile but was phased out by 1956.

  The old reputation survives in marketing. Ozone generators sold as air cleaners use phrases like "energized oxygen" and "pure air". The US Environmental Protection Agency states that when concentrations stay within public health standards, ozone applied to indoor air "does not effectively remove viruses, bacteria, mold, or other biological pollutants". Another report found that ozone levels far above safety standards are possible even when users follow the instructions.

  California answers with certification. Every portable indoor air cleaner sold there must be tested by the California Air Resources Board and meet an ozone emission limit of 0.050 parts per million.

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• 5. The Stratospheric Shield

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  Between about 10 and 50 kilometres above the surface, ozone makes up only two to eight parts per million of the air. Ordinary oxygen in the same region stands near 210,000 parts per million. Totals for a whole column of sky are expressed in Dobson units, and systematic study of atmospheric ozone began in the 1920s.

  The process that builds the layer is called the Chapman cycle. It begins when ultraviolet light shorter than 240 nanometres splits an oxygen molecule in two. Each freed atom joins another oxygen molecule, with a third body carrying off the surplus energy, and ozone results.

  Ozone in turn absorbs ultraviolet-C light and splits apart again. The spare energy becomes motion, which warms the stratosphere. The freed oxygen atoms mostly go on to rebuild ozone. The cycle is cut short when an atom meets ozone and both become ordinary oxygen, a step that hydroxyl, nitric oxide, chlorine and bromine all speed up.

  The layer filters sunlight from about 200 to 315 nanometres, with its strongest absorption near 250. Consider UV-B light at 290 nanometres. It is 350 million times more intense at the top of the atmosphere than at the ground. The small remainder still causes sunburn and direct DNA damage in plants and animals. Those same wavelengths help humans produce vitamin D.

  UV-A, from 315 to 400 nanometres, passes through largely untouched. It causes neither sunburn nor direct DNA damage, though it probably harms the skin of some people over the long term.

  In the second half of the 20th century, researchers found that stratospheric ozone was declining. The main cause was a rise in chlorofluorocarbons and similar chlorinated and brominated molecules. Chlorine radicals, freed when ultraviolet light strikes these compounds or sea salt, catalyse ozone's destruction.

  Concern over the health effects led to the 1987 Montreal Protocol, which banned production of many ozone-depleting chemicals. Stratospheric ozone began to recover in the first and second decades of the 21st century.

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• 6. Smog and Heat

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  Mexico City records ground-level ozone near 125 parts per billion, while Houston sits around 41. This ozone is not emitted by car engines or factories. Sunlight acting on hydrocarbons and nitrogen oxides forms it at the pollution source or many kilometres downwind.

  Ozone starts removing some hydrocarbons, such as aldehydes, from the air, but the products are key ingredients of smog. Its breakdown by ultraviolet light also yields the hydroxyl radical, the first step toward peroxyacyl nitrates, which are powerful eye irritants. Ground-level ozone lasts about 22 days, removed by settling onto surfaces and by reactions with radicals.

  Heat makes cities worse. During urban heat waves, ground-level ozone can run 20 percent above its usual level. Readings peak in summer and autumn. An investigation of the 2003 European heat waves concluded that the deaths from ozone and from heat appeared to add together.

  The American Lung Association ranks the Denver-Aurora area as the 14th most ozone-polluted in the United States. In 2004, the EPA designated the Denver Metro and North Front Range as a non-attainment area under the 1997 eight-hour ozone standard, then deferred that status until 2007. The Colorado Ozone Action Plan, dated 2008, expanded car emission testing into counties such as Larimer and Weld.

  Oil and natural gas operations in the Denver-Julesburg Basin are a major contributor. University of Colorado Boulder researchers linked elevated readings to air arriving from the north to east-southeast, upwind sectors that include the Wattenberg Field. A 2011 Regional Haze Plan targeted nitrogen oxides more specifically. A growing population and mountains that trap emissions keep levels stubbornly high.

  Farms pay too. A 2022 study estimated that East Asia loses 63 billion dollars of crops each year to ozone pollution. China loses about one-third of its potential wheat and one-fourth of its rice. Ozone interferes with photosynthesis and stunts growth, and pepper plants in studies produced less fruit.

  The Intergovernmental Panel on Climate Change Third Assessment Report suggests that tropospheric ozone has a radiative forcing about 25 percent that of carbon dioxide. Per molecule it is roughly 1,000 times as strong, but it decays quickly. Over 20 years its warming potential is only 62 to 69 tons of carbon dioxide equivalent per ton. Some regions see forcing up to 150 percent of carbon dioxide. Ozone increases are blamed for about 30 percent of the warming of the upper Southern Ocean interior between 1955 and 2000.

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• 7. Bodies and Limits

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  Neutrophils, white blood cells that fight bacteria, appear in the airways within 6 hours of exposure to high ozone. Yet their ability to clear bacteria seems impaired. Macrophages change the inflammatory signals they release, either overreacting or weakening the lung's defences. Lymphocytes produce more cytokines, which may worsen airway reactivity and asthma.

  The cilia that sweep pathogens from the airway are damaged by ozone, and the epithelial barrier weakens. Germs can then cross into deeper tissue. Inhaled ozone reaches the heart as well. It upsets the autonomic nervous system, lowering heart rate variability, and an hour of high exposure caused an abnormal rhythm called supraventricular arrhythmia in the elderly.

  The American Lung Association names five groups as especially vulnerable. They are children and teens, people aged 65 and older, people who work or exercise outdoors, people with lung diseases such as asthma and COPD, and people with cardiovascular disease.

  Chronic exposure, meaning more than eight hours at a time over weeks, months or years, carries its own toll. One study found risk of all-cause, circulatory and respiratory death rising by 2, 3 and 12 percent per 10 parts per billion. Children with asthma face a higher risk of hospitalization, and younger and low-income children fare worse. A study of 450,000 people in US cities followed for 18 years found more than a 30 percent higher risk of dying from lung disease in high-ozone cities such as Houston and Los Angeles.

  The body also makes reactive oxygen species itself. White blood cells, and even the roots of marigolds, produce ozone alongside other reactive forms of oxygen to destroy foreign bodies. When inhaled ozone meets cholesterol in the lungs, it forms metabolites called atheronals, which are thought to promote the plaque behind heart disease.

  In May 2008, under a court order, the EPA lowered its ozone standard from 80 to 75 parts per billion. The move was controversial, because the agency's own scientists and advisory board had urged 60. The World Health Organization recommends 100 micrograms per cubic metre, about 51 parts per billion. The EU target is 120 micrograms per cubic metre, which is about 60 parts per billion.

  On the 26th of October 2015, the EPA published a final rule, effective the 28th of December 2015, lowering the eight-hour primary standard from 0.075 to 0.070 parts per million. Workers have their own limits. The US Occupational Safety and Health Administration sets a permissible exposure limit of 0.1 parts per million over eight hours, and the immediately dangerous level is 5 parts per million.

  Passenger aircraft are a special case. Ozone levels in cabins depend on altitude and turbulence. The Federal Aviation Administration caps exposure at 250 parts per billion, with a four-hour average of no more than 100. Some planes carry ozone converters in the ventilation system.

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• 8. Made on Demand

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  A 9 volt battery, a pencil graphite rod, a platinum wire and a 3 molar sulphuric acid solution are enough to make ozone in a laboratory. Oxygen forms as a competing product. A neon-sign transformer connected to two close metal rods can do it too, because the arc converts oxygen to ozone. The classic containing apparatus is the Siemens ozoniser, built from two concentric glass tubes.

  Industry cannot store or ship ozone like other gases, since it quickly turns back into ordinary oxygen. Every plant produces it on site. Large facilities may install several megawatts of power, with peak voltages between 3,000 and 20,000 volts. Almost 90 percent of the effective power ends up as heat, so cooling water is essential. Corona discharge is the most common method. It works from ordinary air and yields concentrations of 3 to 6 percent, but it also produces nitrogen oxides as a by-product.

  Ultraviolet generators reach about 0.5 percent or lower, and they suit swimming pools and spas holding millions of gallons of water. Cold plasma machines fed pure oxygen reach about 24 percent. Electrolytic generation splits water directly and can reach 20 to 30 percent. It needs catalysts such as lead dioxide or boron-doped diamond, and cooling the electrolyte near 0 degrees Celsius improves the yield.

  The largest use of ozone is in preparing pharmaceuticals, synthetic lubricants and other organic compounds, where it severs carbon-carbon bonds. Many municipal drinking water systems kill bacteria with ozone instead of chlorine. It forms no organochlorine compounds and leaves no taste or odour behind. The drawback is bromate, a suspected carcinogen that can form when source water contains high bromide. The US Safe Drinking Water Act also requires a residual of 0.2 parts per million of free chlorine in the pipes. Ozone also removes iron and manganese from well water and converts cyanides into cyanates.

  Rubber suffers. Ozone attacks natural rubber, nitrile rubber and styrene-butadiene rubber, opening cracks that grow longer and deeper. Car tyres once suffered badly, but modern ones do not. O-rings, gaskets and fuel lines can still be attacked, and a DC electric motor near stored rubber can speed up the damage. Waxes added as antiozonants protect the material.

  Devices make ozone by accident, too. Laser printers, photocopiers, tasers, arc welders and ionic air purifiers all do. So do brushed electric motors, and large ones in elevators or hydraulic pumps make more.

  Nature runs the largest single natural generator of ozone, the Catatumbo lightning storms above the Catatumbo River in Venezuela. Because ozone is so unstable, it probably has little effect on the ozone layer. The phenomenon has prompted calls for it to be designated a UNESCO World Heritage Site.

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