That sharp, clean, slightly metallic smell after a thunderstorm or near a sparking appliance is almost certainly ozone, a gas your nose can pick up at remarkably low concentrations. And yes, it is dangerous. Even at levels well below what most people assume is safe, ozone irritates your airways, inflames lung tissue, and with chronic exposure raises your risk of dying from heart and lung disease. The good news is that once you understand where the smell comes from, you can usually reduce your exposure with straightforward changes.
What Ozone Smells Like and Why You Can Detect So Little of It
Ozone has a distinctive odor often described as sharp, pungent, or “electric.” Some people compare it to chlorine or the air right after a lightning strike. The word “ozone” itself comes from the Greek word for “to smell,” which hints at how recognizable the scent is. Unlike many pollutants that build up silently, ozone announces itself.
Your nose is extraordinarily sensitive to it. In controlled experiments where young adults breathed filtered, low-ozone air before testing, subjects could detect ozone at concentrations as low as 7 parts per billion (ppb), which is far below most regulatory thresholds.1Indoor Air. Olfactory detection of ozone and d-limonene: Reactants in indoor spaces Recognition of the odor (not just detecting something but identifying it as ozone) required slightly higher concentrations, roughly 15 to 20 ppb. Individual sensitivity varied, but the spread was narrower than researchers typically see for other smells, suggesting humans are almost universally tuned in to this gas.
So if you catch a whiff of ozone, there is likely very little of it in the air. That sounds reassuring until you learn that even these low concentrations are biologically active. The question is where it is coming from.
Common Reasons You Might Smell Ozone Indoors
Most people first notice ozone inside their home or office, not outdoors. Electrical equipment is the most frequent culprit. Any device that creates a strong electrical discharge can split oxygen molecules and produce ozone as a byproduct.
- Laser printers and copiers: Older models that use corona-wire technology to transfer toner produce measurable ozone. Testing found that laser printers with traditional corona discharge emitted significant amounts of ozone (along with formaldehyde), and older printers were the worst offenders.2PubMed. Emission of ozone and organic volatiles from a selection of laser printers and photocopiers3Centers for Disease Control and Prevention. Final report: methods for characterizing emissions from laser printers Newer printers have largely switched to charge-roller technology, which produces far less ozone, but plenty of older units are still in service.
- Air purifiers and ionizers: This is the source that catches people off guard. Some air purifiers deliberately generate ozone to “clean” the air, while ionizer-type purifiers produce it as a side effect. Testing of several types found that ozone concentrations frequently exceeded safety limits set by the EPA, the California Air Resources Board, and OSHA.4PubMed. Quantification of ozone levels in indoor environments generated by ionization and ozonolysis air purifiers Even ionizer air cleaners that passed safety certification testing could pose an ozone exposure hazard to users, because the amount of ozone that accumulates depends heavily on room size and ventilation.5Journal of Electrostatics. Quantification of dust removal and ozone emission of ionizer air-cleaners by chamber testing
- Other household sources: Electric motors (especially brush-type motors in vacuum cleaners or power tools), arc welding equipment, UV germicidal lamps, and even some hot tubs with ozone sanitizers can produce detectable amounts. If you smell that distinctive tang near any device that sparks, arcs, or emits UV light, ozone is the likely explanation.
Ozone Outdoors and Photochemical Smog
If the smell hits you outside on a warm afternoon, you are probably encountering ground-level ozone formed by sunlight-driven reactions. Vehicle exhaust and industrial emissions release nitrogen oxides and volatile organic compounds (VOCs) into the air. Sunlight breaks nitrogen dioxide apart, freeing an oxygen atom that quickly bonds with an oxygen molecule to form ozone.6Environmental Technology & Innovation. Tropospheric ozone and NOx: A review of worldwide variation and meteorological influences Meanwhile, VOCs act as fuel that keeps the cycle going, regenerating nitrogen dioxide and producing more ozone.7Atmospheric Chemistry and Physics. Vertical changes in volatile organic compounds (VOCs) and impacts on photochemical ozone formation
This process is nonlinear, meaning it does not respond in a simple, proportional way to changes in emissions.8Copernicus Publications. Global patterns and trends in ground-level ozone chemical formation regimes from 1996 to 2022 In some cities, cutting nitrogen oxide emissions without also cutting VOCs can actually increase ozone levels in the short term. That is one reason ground-level ozone has been stubbornly hard to regulate. Ozone concentrations tend to peak on hot, sunny, still afternoons, which is why “ozone action day” warnings are issued in summer.
People sometimes confuse ground-level ozone with the stratospheric ozone layer. They are the same molecule, but in the stratosphere it absorbs harmful UV radiation and is protective. At ground level, it is a pollutant with no redeeming health effects for anyone breathing it.
What Ozone Does to Your Body
Ozone is a powerful oxidant. When you inhale it, it reacts almost immediately with the thin liquid lining inside your airways, called the epithelial lining fluid. This fluid contains surfactant, a mixture of fats and proteins that keeps your lungs flexible. Ozone attacks the unsaturated fats in surfactant, breaking their chemical bonds and producing toxic byproducts that damage and kill the cells beneath.9PubMed. Oxidized phospholipids derived from ozone-treated lung surfactant extract reduce macrophage and epithelial cell viability10PubMed. Toxic oxidant species and their impact on the pulmonary surfactant system The damaged surfactant can no longer function properly, which may stiffen the airways and make breathing harder.11PubMed. Degradation and rearrangement of a lung surfactant lipid at the air-water interface during exposure to the pollutant gas ozone
This is not a theoretical risk requiring high industrial exposures. A study exposed healthy young adults to just 0.06 parts per million of ozone (60 ppb) for about six and a half hours while they exercised intermittently. Even at this low concentration, lung function dropped and airway inflammation increased compared to clean air.12American Journal of Respiratory and Critical Care Medicine. Lung Function and Inflammatory Responses in Healthy Young Adults Exposed to 0.06 ppm Ozone for 6.6 Hours The current U.S. National Ambient Air Quality Standard for ozone is 70 ppb averaged over eight hours, which means biologically meaningful inflammation occurs at levels below the legal limit. The subjects in that study were young and healthy; people with asthma or lung disease would likely respond more severely.
Short-term spikes in ozone are tied to immediate increases in hospital admissions and deaths from both heart and lung problems.13PubMed Central. Health Effects of Ozone on Respiratory Diseases Symptoms during acute exposure typically include chest tightness, coughing, shortness of breath, and throat irritation. Some people also report headache and fatigue.
Long-Term Exposure Raises Mortality Risk
The danger is not limited to bad air days. Chronic, lower-level ozone exposure over years is associated with increased risk of dying from cardiovascular disease, respiratory disease, and especially chronic obstructive pulmonary disease (COPD). A large U.S. study found that for every 10 ppb increase in long-term average ozone exposure, the risk of death from COPD rose by about 9 percent, from ischemic heart disease by about 6 percent, and from respiratory disease overall by about 4 percent.14PubMed Central. Long-Term Exposure to Ozone and Cause-Specific Mortality Risk in the United States These associations held up even after accounting for fine particulate matter and nitrogen dioxide, though there was some evidence that high temperatures amplified the respiratory risk.
A separate analysis of the U.S. Medicare population confirmed a similar pattern. After adjusting for particulate matter, the strongest ozone-mortality links were for COPD and congestive heart failure, with roughly 7 percent and 6 percent increases in risk per 10 ppb, respectively.15Journal of Exposure Science & Environmental Epidemiology. Long-term ozone exposures and cause-specific mortality in a US Medicare cohort In other words, even modest, sustained differences in the ozone levels where you live and breathe appear to shift the odds of premature death.
Who Is Most Vulnerable
Children and older adults bear the brunt. A study in Vancouver found that ozone levels in the days before hospital admission were strongly linked to respiratory admissions in both groups, with the strongest association appearing about four days after exposure.16PubMed. Association between ozone and respiratory admissions among children and the elderly in Vancouver, Canada The lag makes sense biologically: ozone-triggered inflammation builds over days before it becomes severe enough to send someone to the hospital.
Children face a particular risk because their lungs are still developing. Early-life ozone exposure, even modest increases of a couple of ppb, has been linked to higher odds of developing asthma and wheezing by the time children reach ages four to six.17PubMed Central. Early-Life Ozone Exposure and Asthma and Wheeze in Children People who exercise outdoors during high-ozone periods also inhale more of it simply because they breathe more deeply and rapidly. Outdoor workers, joggers, and cyclists on hot summer afternoons are getting substantially higher doses than someone sitting indoors.
The Hidden Problem of Ozone Reacting with Indoor Products
One of the less obvious hazards is what ozone does when it mixes with everyday household products. Many cleaning sprays, air fresheners, essential oil diffusers, and wood finishes release terpenes, a broad family of fragrant compounds found naturally in citrus peels, pine, and lavender. Terpenes themselves are generally not a health concern at indoor concentrations. But when they meet ozone, they react to produce formaldehyde and ultrafine particles, both of which are harmful to inhale.18PubMed. Indoor air chemistry: Terpene reaction products and airway effects
Testing with commercial cleaning products and air fresheners confirmed that using terpene-containing products in the presence of ozone produced formaldehyde and fine particulate matter at levels that could exceed health-based guidelines.19Atmospheric Environment. Indoor secondary pollutants from cleaning product and air freshener use in the presence of ozone Essential oil diffusers are a growing concern in this area. When essential oils are evaporated indoors, they release high levels of terpenes like linalool, eucalyptol, and d-limonene, all of which can react with any ozone present to form secondary pollutants.20Atmospheric Environment. The effects of evaporating essential oils on indoor air quality
The practical takeaway: running an ionizer air purifier while also diffusing essential oils is a particularly bad combination. Each product individually might seem harmless, but together they generate pollutants that neither would produce alone.
There May Be No Truly Safe Level
A striking finding from epidemiological work is how far down the concentration scale ozone’s harms extend. A large study across 98 U.S. communities found that daily increases in ozone were significantly associated with daily increases in deaths, even when the analysis was restricted to days that met all existing U.S. ozone regulations. The researchers estimated that a truly “safe” daily ozone level, if one exists at all, would be roughly 10 ppb or less, far below any current standard.21PubMed Central. The Exposure–Response Curve for Ozone and Risk of Mortality and the Adequacy of Current Ozone Regulations Even California’s more stringent standards did not eliminate the mortality risk. Statistical models that allowed for a threshold below which ozone stops being harmful fit the data no better than models that assumed any amount of ozone increases risk.
This does not mean a single breath of ozone will hurt you. It means the relationship between ozone and health harm appears to be continuous, without a clean cutoff below which everyone is safe. That is a fundamentally different picture from what many people assume when they see an air quality index report that says “good.”
How to Reduce Your Exposure
If you can smell ozone regularly in your home or workspace, you have enough of it to take action. Some steps are obvious; others less so.
- Check your air purifier: If you own an ionizer or “ozone generator” air cleaner, consider replacing it with a HEPA-based filter that removes particles mechanically without producing ozone. The irony of an air purifier that makes indoor air quality worse is well documented.
- Ventilate laser printer areas: If you still use an older laser printer, place it in a well-ventilated room away from where people sit for long periods. Newer models with charge rollers produce far less ozone.
- Use activated carbon filtration: Carbon-based filters are effective at removing ozone from indoor air. Testing of commercial filters found initial removal efficiencies ranging from about 5 to over 98 percent, though performance declines over time as chemical reactions permanently alter the carbon.22PubMed. Evaluation of activated carbon filters for removal of ozone at the PPB level Activated carbon fiber filters can achieve over 99 percent ozone removal when fresh.23Building and Environment. Carbon nanotubes / activated carbon fiber based air filter media for simultaneous removal of particulate matter and ozone The key limitation is that these filters wear out, so building engineers recommend tracking their service life and replacing them before breakthrough occurs.24Building and Environment. Removal of indoor air ozone using carbon-based filters: Systematic development and validation of a predictive model
- Avoid combining fragrance products with ozone sources: Do not run essential oil diffusers, pine-scented cleaners, or citrus air fresheners in a room that also contains an ionizer or that has high outdoor ozone infiltration.
- Time outdoor activity: On high-ozone days (usually hot, sunny, still afternoons in summer), shift vigorous exercise to morning or evening hours when ozone levels are lower.
Ozone Can Impair Your Ability to Smell It
There is a cruel twist to ozone exposure. While your nose is quite sensitive to the gas at first, acute exposure to higher concentrations can temporarily dull your sense of smell. Research found that people exposed to 0.2 ppm of ozone had significantly elevated olfactory thresholds afterward, meaning it took a stronger smell for them to detect any odor at all. In effect, the pollutant partially disables the very sense that warns you about it. So if you noticed an ozone smell earlier in the day and it seems to have faded, one possibility is that the ozone is still there but your nose has stopped alerting you.
Ozone’s Toll on Agriculture
Beyond human health, ground-level ozone is the most damaging air pollutant for crops. It enters plant leaves through the same tiny pores the plant uses to breathe and photosynthesize, then attacks cells from the inside. Visible damage includes leaf yellowing and brown spots (chlorosis and necrosis). Over a growing season, ozone disrupts photosynthesis, water uptake, and nutrient transport, leading to reduced yields and lower crop quality.25PubMed Central. Elevated tropospheric ozone and crop production: potential negative effects and plant defense mechanisms Multi-model simulations combining satellite data and ground measurements indicate that present and projected future ozone levels threaten food security, particularly for staple grains.26PubMed. Effect of ozone stress on crop productivity: A threat to food security
This is worth knowing because it reframes ozone as more than a personal health annoyance. The same gas that makes your chest feel tight on a smoggy day is quietly reducing wheat, rice, and soybean harvests across the globe. Unlike other air pollutants whose agricultural impacts are debated, the evidence on ozone and crop damage is extensive and consistent. It is the rare case where human health and food supply interests are perfectly aligned: reducing ground-level ozone benefits both.
Industrial Uses of Ozone
Given everything above, it might seem strange that ozone is deliberately manufactured for industrial use. But its aggressive chemistry is exactly why it is valued. Ozone is a powerful disinfectant used widely in water treatment, food processing, pharmaceuticals, textile manufacturing, and healthcare settings.27PubMed Central. Ozone application in different industries: A review of recent developments Municipal water systems use it to kill bacteria and break down organic contaminants without leaving the chemical residues that chlorine does. Hospitals and food processors use it to sterilize surfaces and equipment.
The difference between therapeutic and harmful exposure comes down to containment and dosing. In water treatment, ozone is dissolved in water inside sealed systems and decomposes rapidly, so workers and the public are not breathing it. In medical applications, ozone concentrations and exposure times are tightly controlled. The problem arises when ozone is generated carelessly in occupied spaces, such as a bedroom with an ionizer running all night, a poorly ventilated copy room, or a car interior with a wearable “personal air purifier” clipped to your collar. In those situations, the same molecule that is a useful industrial tool becomes an uncontrolled respiratory hazard.