Summer’s Invisible Respiratory Threat
The Chemistry of Ground-Level Ozone
On a July morning in 1943, Los Angeles residents looked out on a city reduced to three blocks of visibility. Eyes burned. Lungs hurt. Some people feared that the city had suffered a chemical attack. Officials blamed a synthetic-rubber plant, but closing the suspected facility did not clear the air. The haze returned, especially when sunshine settled over the Los Angeles basin.
The answer emerged several years later in a laboratory at the California Institute of Technology. Arie Haagen-Smit, a Dutch-born biochemist known for studying the compounds responsible for the flavor of pineapples and wine, exposed automobile exhaust and industrial emissions to sunlight. The resulting mixture produced ozone and the same eye-stinging smog that had covered the city. Los Angeles itself was acting as a photochemical reactor.
Ozone is a molecule composed of three oxygen atoms. Its effects depend on where it forms. In the stratosphere, roughly 10 to 50 km above Earth, naturally occurring ozone absorbs much of the sun’s harmful ultraviolet radiation. In the troposphere, the lowest layer of the atmosphere, elevated ozone concentrations damage living tissue. The molecule is chemically identical in both places; altitude changes whether it serves as a shield or an inhaled pollutant.
Most ground-level ozone is not released directly from a tailpipe or smokestack. It forms in the surrounding air through reactions involving nitrogen oxides, volatile organic compounds and sunlight. Motor vehicles, power plants, industrial boilers and other forms of high-temperature combustion emit nitrogen oxides. Fuels, solvents, paints, refineries, vegetation and many industrial processes release volatile organic compounds. Carbon monoxide and methane can also participate in ozone chemistry, particularly over larger geographic and temporal scales.
Sunlight starts the central reaction by breaking nitrogen dioxide into nitric oxide and a free oxygen atom. That oxygen atom rapidly combines with an oxygen molecule in the air to form ozone. The process does not automatically produce a lasting accumulation. Nitric oxide can react with ozone and consume it, regenerating nitrogen dioxide.
Volatile organic compounds change that balance. Their atmospheric oxidation produces reactive radicals that convert nitric oxide back into nitrogen dioxide without using up an ozone molecule. Sunlight can then split the regenerated nitrogen dioxide, producing another oxygen atom and another molecule of ozone. As the cycle repeats, ozone accumulates.
This chemistry explains why cutting one precursor does not always produce a proportional reduction in ozone. In some places, ozone formation is especially sensitive to nitrogen oxides; in others, it responds more strongly to volatile organic compounds. Dense urban centers with abundant nitrogen oxide emissions can even record lower ozone immediately beside busy roads because freshly emitted nitric oxide consumes it. Farther downwind, after the emissions have mixed, reacted and moved through sunlight, ozone may reach higher concentrations in suburbs or rural communities.
The Los Angeles basin offered unusually favorable conditions for this atmospheric reactor. Mountains restricted air movement, sea breezes carried emissions inland, and intense sunshine powered hours of chemical reactions. Many regions have their own versions of that chamber. Ozone and its precursors can travel hundreds of kilometers, carrying pollution away from the traffic corridors and industrial sites where the process began. A clear rural horizon can hold ozone produced from emissions released somewhere else.
Warm, sunny and stagnant weather often favors ozone formation, which is why many regions experience their worst episodes during summer. Heat itself is not one of the required chemical ingredients, and high ozone is not confined to hot cities. Cold-weather episodes occur in some high-elevation oil and gas regions when strong sunlight reflects from snow and precursor emissions collect beneath temperature inversions.
The hour of greatest exposure also varies. Ozone concentrations often rise through the day and peak during the afternoon or early evening, after sunlight has driven the chemistry for several hours. Wind can shift the peak later or carry it downwind. Local monitoring and forecasts provide a firmer guide than a universal rule such as “ozone is highest between 2 p.m. and 7 p.m.”
Once inhaled, ozone encounters the fluid lining the nose, airways and lungs. Its high reactivity means that little of it needs to enter the bloodstream intact. It reacts at the respiratory surface with lipids, proteins and antioxidants, generating secondary oxidation products and biological signals. These reactions can injure epithelial cells, provoke inflammation and activate sensory nerves.
The comparison sometimes made to a sunburn inside the lungs conveys irritation but distorts the mechanism. Sunburn begins when ultraviolet radiation damages cells. Ozone injury begins through chemical oxidation at the airway surface. Both can produce inflammation, although they reach tissue by different routes.
Controlled human-exposure studies have shown that ozone can temporarily reduce lung function and produce coughing, throat irritation, chest discomfort and pain during deep breathing. The response differs markedly among people. Some healthy adults experience measurable effects at concentrations and exertion levels that produce little obvious reaction in others. A person’s response on one occasion does not reliably predict another person’s.
Ozone can also increase airway responsiveness and inflammation. In people with asthma, exposure may aggravate symptoms and contribute to attacks. Children, older adults, people with respiratory disease and those who work or exercise outdoors face greater risks, although healthy people are not immune. Epidemiological studies associate higher ozone levels with increases in respiratory emergency visits and hospital admissions; research also links short-term exposure with premature mortality.
Several claims about the exact sequence of injury remain less settled than simplified health diagrams suggest. Ozone exposure can affect epithelial-barrier function and mucociliary clearance, but it does not follow that a single summer exposure leaves every airway broadly open to infection, allergens and PM2.5. The evidence depends on dose, exposure duration, individual susceptibility and the outcome being measured. Ozone can alter mucus and airway immune responses, while the clinical consequences vary.
Exercise raises the dose delivered to the lungs. Breathing becomes faster and deeper, and people shift toward mouth breathing, which provides less removal of ozone than the nose. An outdoor runner can therefore inhale more ozone during an hour than someone resting in the same air. Duration, concentration and ventilation rate work together; the number displayed by an air-quality monitor tells only part of the exposure.
Public-health agencies advise using local air-quality forecasts to adjust the timing, location or intensity of prolonged outdoor exertion when ozone is elevated, particularly for people with asthma or other lung conditions. Morning activity often reduces exposure in places where ozone rises later in the day, though local patterns should decide the schedule. Moving indoors may help because ozone generally reacts with surfaces and concentrations are often lower inside, but open windows and outdoor-air ventilation can bring it indoors. Air conditioning changes temperature; its effect on ozone depends on whether the system recirculates and filters indoor air or draws contaminated air from outside.
More than eight decades after Los Angeles residents mistook smog for an attack, the city’s atmospheric reactor is better controlled. Vehicle standards, fuel rules and industrial-emission limits have sharply reduced many of the precursors that once accumulated over the basin. The chemistry Haagen-Smit exposed has not changed. On a hot afternoon, sunlight still falls on exhaust and solvent vapors, sea breezes still push the reacting mixture inland, and an oxygen molecule in the air can acquire a third atom before it reaches someone’s lungs.