# Air Purifiers That Make Your Air Worse: Ionizers, Ozone, and What Actually Works > Ionizers, plasma purifiers, and hydroxyl generators all have plausible-sounding science behind them. Some also generate ozone as a byproduct. This is what the research actually shows, what CARB certifies, and what CADR tells you about real-world performance. - Type: explainer - Published: 2026-08-09 - Updated: 2026-08-09 - Author: Lucas Gruber - Canonical: https://nontoxicnook.com/articles/air-purifiers-ionizers-ozone-what-actually-works Prices are deliberately omitted from this mirror — they go stale. Current prices, stock, and where to buy live on the product pages linked below. --- ## The appeal is real. So is the problem. The marketing pitch for ionizer air purifiers is compelling: no filter to replace, a "natural" cleaning mechanism inspired by the way lightning clears the air after a storm, and a price tag that undercuts true HEPA machines by 60 percent or more. A reasonable person could buy one. The problem is that a meaningful subset of ionizers, electrostatic precipitators, "plasma" purifiers, and hydroxyl generators produce ozone as a byproduct. Ozone is effective at oxidizing odors and some pathogens. It is also a respiratory irritant at concentrations high enough to do that job. This is not a fringe concern. The California Air Resources Board (CARB) bans the sale of ozone-generating air cleaners in California. The EPA has published guidance on the same issue. And "ozone generator" is a specific, well-documented product category, not a smear applied to ionizers broadly. Understanding the difference matters because the terminology is deliberately blurry. ## What ionizers actually do A traditional ionizer emits charged particles (negative ions) that attach to airborne particulates. Those charged particles then stick to walls, floors, furniture, and collection plates inside the device. The room's particle count drops. On that narrow metric, ionizers work. The side effect: the corona discharge that generates ions can also split oxygen molecules and recombine them as ozone (O₃). Some designs do this incidentally; others do it intentionally, marketing ozone as a cleaning agent. Electrostatic precipitators work on a related principle: particles pass through a charged field and stick to oppositely charged collection plates. These also produce ozone, typically at lower rates than dedicated ozone generators but still at measurable levels. The quantity of ozone produced varies enormously by device. A well-designed ionizer may stay below 0.05 parts per million (ppm), the CARB indoor standard. A purpose-built ozone generator marketed as an "air purifier" can exceed that by a factor of ten or more. ## What ozone does to your body Ozone irritates the airways. At low concentrations, that means coughing and mild shortness of breath. At higher concentrations, it aggravates asthma and can cause lasting lung damage with repeated exposure. Children, elderly people, and anyone with a respiratory condition are more sensitive. The EPA notes that it is impossible to predict what ozone concentration a device will produce in any given room, because the level depends on the size of the space, ventilation, humidity, and what else is in the room for the ozone to react with. Ozone that reacts with VOCs from cleaning products or furnishings can produce formaldehyde and ultrafine particles as secondary pollutants. That reaction piece matters: you can buy a low-ozone ionizer, use it in a room with new paint or a synthetic rug, and end up with worse air chemistry than you started with. ## CARB's position: ozone generators are not air cleaners California's Air Resources Board has the most detailed regulatory framework on this in the United States. Since 2010, CARB has maintained a list of certified air cleaning devices that meet its indoor ozone standard of 0.050 ppm. Devices on that list can be sold in California. Devices that exceed that limit cannot. Ozone generators sold as air cleaners are banned outright in California under Title 17 of the California Code of Regulations. CARB also publishes a separate warning list of specific models that tested above the limit, including many devices marketed under terms like "plasma," "ionizer," and "air purifier." The certification database and warning list are both searchable at arb.ca.gov. CARB certification is a useful proxy for the rest of the country: if a device is CARB-certified, it has been independently tested to produce ozone below 0.050 ppm at its highest setting. If a device is on the warning list, that test went the other way. Not every device has been tested by CARB. But CARB certification is the best independent ozone-output signal available to a consumer without access to a laboratory. ## The vocabulary of confusion Several marketing terms appear on air purifiers that produce ozone. Understanding what they mean: **Plasma air purifiers.** "Plasma" technology typically refers to a bipolar ionization or dielectric barrier discharge system. Some produce little ozone. Others produce significant amounts. The term itself tells you nothing about ozone output. Check for CARB certification. **Hydroxyl generators.** Hydroxyl radical generators use UV light to produce OH radicals that oxidize pollutants. These devices also produce ozone, sometimes in significant quantity. The same CARB check applies. **PCO (photocatalytic oxidation).** PCO systems use UV light on a catalyst (usually titanium dioxide) to generate reactive oxidants that break down VOCs. Studies have found that some PCO devices produce formaldehyde and acetaldehyde as byproducts of incomplete oxidation, and ozone as a secondary product. The EPA has raised specific concerns about this class. **Ultraviolet germicidal irradiation (UVGI).** UV-C light used inside a sealed purifier housing to inactivate microorganisms is a legitimate technology. The UV source needs to be fully enclosed: UV-C light is itself harmful to skin and eyes, and some UV wavelengths generate ozone from ambient oxygen. A UV lamp enclosed inside the air path of a HEPA purifier, where no UV exits the device, is different from a bare UV-C lamp or an open PCO reactor. **Negative ion generators.** These vary by design. Some produce negligible ozone. Some produce meaningful amounts. CARB certification is the signal to look for. ## What true HEPA actually does A true HEPA filter (High Efficiency Particulate Air) captures at least 99.97 percent of particles 0.3 microns in diameter. It works by three physical mechanisms: interception, impaction, and diffusion. Particles do not "get through" a HEPA filter via a chemical reaction; they are physically trapped in the filter medium. HEPA filters do not produce ozone. They do not react with VOCs. They remove particles: dust, pollen, pet dander, mold spores, smoke particles, and fine particulate matter (PM2.5 and smaller). They do not remove gases or odors on their own. Activated carbon handles the gas phase. Carbon has an enormous surface area at the molecular level; VOCs and odor molecules adsorb onto that surface and are removed from the air stream. The capacity is finite: an activated carbon filter becomes saturated and needs replacement. Thin "carbon layer" designs saturate quickly. Substantial activated carbon bed depth (typically half an inch or more of granular carbon, not a sprayed coating) is what does meaningful gas-phase work. A purifier with a true HEPA filter plus a substantial activated carbon bed covers both the particle and gas-phase problems. Neither component produces secondary pollutants. ## CADR: the number that tells you what you actually get CADR stands for Clean Air Delivery Rate. It is a standardized measurement certified by AHAM (the Association of Home Appliance Manufacturers) that tells you how quickly a purifier delivers clean air for three specific pollutants: tobacco smoke, dust, and pollen. The unit is cubic feet per minute (CFM). A CADR of 200 for smoke means the purifier delivers 200 cubic feet per minute of air that is free of smoke particles. Higher is better. The number is measured at the highest fan speed. The AHAM rule of thumb: a purifier's smoke CADR should be at least two-thirds of the room's square footage (assuming 8-foot ceilings). A 150-square-foot bedroom needs a smoke CADR of at least 100. A 300-square-foot living room needs at least 200. Why smoke CADR specifically: smoke particles include fine PM2.5, which is the hardest category to filter. A device that scores well on smoke CADR generally handles the full particle spectrum well. CADR is a volume rate, not an efficiency percentage. A purifier with 99.99 percent filter efficiency but a tiny fan produces a low CADR. Real-world performance is the product of filtration efficiency and airflow, and CADR captures both in a single number. Ionizers typically have low or unverifiable CADR because the particle-capture mechanism (electrostatic attraction to surfaces) is hard to standardize. Most ionizer manufacturers do not submit for AHAM certification. ## When a purifier helps, and when it does not An air purifier addresses airborne particles and some gases already in the room's air. It does not address the sources of those particles and gases. **Where purifiers help:** * Seasonal allergens (pollen, outdoor mold spores) that enter through windows and doors * Pet dander and hair circulating in the air * Fine particulate from cooking (a range hood exhausted outside is better, but a purifier handles the remainder) * Wildfire smoke intrusion, especially during acute smoke events * Dust in high-turnover environments **Where purifiers do not fix the underlying problem:** * Mold. An air purifier captures mold spores that become airborne, but it cannot address mold growth inside a wall or under flooring. The source must be remediated. * High-VOC sources. A purifier with activated carbon will reduce VOC concentration in the air, but a new synthetic carpet off-gassing continuously will outpace the purifier's capacity. Source removal or ventilation is the primary intervention. * Carbon monoxide, radon, or other non-particulate hazards. These require source control and appropriate detection equipment, not an air purifier. * Humidity-driven problems. An air purifier is not a dehumidifier. Relative humidity above 60 percent drives mold growth and dust mite populations regardless of filtration. Ventilation is the intervention most often underestimated. A home that is well-ventilated with filtered outdoor air has lower indoor pollutant concentrations than a sealed home running an air purifier. Where outdoor air quality is acceptable, opening windows is not a step backward. ## What to look for The combination that works: true HEPA, substantial activated carbon, CADR certified by AHAM for the room size you need, and CARB certification (or at minimum, no mention of ions, plasma, or ozone generation in the product description). For a full breakdown of specific purifiers that meet those criteria, including eight vetted models from $80 to $600 across room sizes, see [our air purifiers buying guide](/articles/best-air-purifiers-non-toxic-home-2026). **Sources:** 1. [Ozone Generators that are Sold as Air Cleaners (US EPA)](https://www.epa.gov/indoor-air-quality-iaq/ozone-generators-are-sold-air-cleaners) 2. [Indoor Air Cleaning Devices (California Air Resources Board)](https://ww2.arb.ca.gov/our-work/programs/air-cleaners-ozone-and-health/indoor-air-cleaning-devices) 3. [CARB Certified Air Cleaning Devices Search](https://ww2.arb.ca.gov/our-work/programs/air-cleaners-ozone-and-health/certified-air-cleaning-devices) 4. [AHAM CADR Ratings (Association of Home Appliance Manufacturers)](https://ahamverifide.org/ahams-air-filtration-standards/) 5. [Photocatalytic Oxidation Air Cleaners: A Need for Stringent Performance Evaluation (Environmental Health Perspectives, 2006)](https://ehp.niehs.nih.gov/doi/10.1289/ehp.961S) 6. [California Code of Regulations, Title 17, Section 94800: Ozone Standard for Electrical Air Cleaners](https://govt.westlaw.com/calregs/Document/I21B66BB01B4411E4B9CB9B8AA3B5D82D) --- Nontoxic Nook is an independent guide: every listed product cleared the published criteria at https://nontoxicnook.com/our-promise. This mirror links only to our own pages; affiliate disclosure: https://nontoxicnook.com/affiliate-disclosure.