Ozone (O3)

Ozone (O3) forms when ultraviolet light breaks oxygen molecules (O2) in the stratosphere, creating O atoms that combine with O2 — a natural cycle that has existed for billions of years and filters out harmful UV radiation [1]. Ground-level ozone is an entirely different story: it is a secondary pollutant, meaning it is not directly emitted but forms in the atmosphere when nitrogen oxides (NOx from vehicle exhaust, power plants, and industry) react with volatile organic compounds (VOCs from gasoline, solvents, and vegetation) in sunlight [2]. Ground-level ozone is a defining characteristic of photochemical smog, first identified in Los Angeles in the 1940s. It worsens on hot, sunny, stagnant air days and in summer afternoons, particularly in urban areas and downwind of NOx sources [1]. EPA sets National Ambient Air Quality Standards (NAAQS) for ozone; despite progress, roughly one-third of Americans live in areas that regularly exceed standards [2].

Breathing outdoor air is the primary exposure route — ozone is absorbed in the respiratory tract during normal breathing [1]. Because ozone reacts rapidly with surfaces and tissues, it penetrates poorly into buildings, so indoor concentrations are typically 20-50% of outdoor levels [2]. Exposure is highest for people who exercise outdoors on high-ozone afternoons in cities and downwind suburban areas; heavy breathing during exertion increases the dose delivered deep into lungs [1]. Rural areas downwind of urban NOx sources can have high ozone in summer. Very high indoor ozone can be generated by ozone-producing air purifiers marketed (problematically) as air cleaners [2].

Ozone is the most oxidatively reactive gas in ambient air: it attacks the lung's antioxidant defenses and reacts with polyunsaturated fatty acids in cell membranes, generating inflammatory mediators [1]. Even single ozone exposure events during exercise cause measurable airway inflammation, reduced lung function, and increased airway responsiveness in healthy adults. At repeated exposures, ozone causes airways to remodel — explaining why children who grow up in high-ozone areas have reduced maximal lung function in adulthood [2]. Each 10 µg/m3 increase in ozone concentration is associated with a 2-4% increase in respiratory mortality in epidemiological studies. Ozone also makes people more susceptible to respiratory infections by impairing mucociliary clearance [1].

Children's lungs are disproportionately affected because they breathe more air per unit body weight, spend more time outdoors, and are in the critical growth window when ozone impairs lung development [1]. People with asthma and COPD experience dramatically increased symptom frequency and severity on high-ozone days [2]. Outdoor workers — agricultural laborers, landscapers, construction workers — receive the largest occupational ozone doses because they exercise outdoors during the afternoon hours when ozone peaks [1]. Athletes who train outdoors in urban or suburban areas, and anyone who exercises intensely outdoors in summer, have elevated exposure. The elderly with cardiovascular disease have higher ozone-related mortality risk [2].

1. Check AirNow.gov or your weather app's air quality index daily — on Orange (Unhealthy for Sensitive Groups) or Red (Unhealthy) days, move outdoor exercise to morning hours before ozone builds, or exercise indoors [1]. 2. Do not use ozone-generating air purifiers marketed as 'ionic' or 'plasma' cleaners — they worsen indoor air quality rather than improving it [2]. 3. If you have asthma, work with your healthcare provider on an Ozone Action Day plan that includes medication adjustments and activity modifications. 4. Plant trees and vegetation in your neighborhood — urban trees absorb some ozone and reduce heat that accelerates ozone formation [1]. 5. Reduce your vehicle NOx emissions by maintaining proper engine tune-ups, not idling unnecessarily, and supporting policies that accelerate electric vehicle adoption. 6. Refuel your car in the evening when ozone formation is lowest [2].