Ozone is a widespread outdoor air pollutant. Increased concentrations of ozone in outdoor air are associated with irritation of the respiratory system, exacerbation of asthma, respiratory hospital admissions, and premature death [33-36]. Because the adverse health effects of ozone are well established and substantial, the United States and many other countries have made concerted efforts over several decades to reduce outdoor air ozone concentrations. If there are no indoor sources of ozone, indoor ozone concentrations are generally substantially less than outdoor air ozone concentrations because ozone is removed from the indoor air by chemical reactions with indoor materials and with some types of airborne volatile organic compounds [36, 37]. Although these chemical reactions reduce indoor ozone concentrations, they can produce other indoor air pollutants that may pose health risks, including formaldehyde and ultrafine particles .
Despite the known health effects of ozone, devices often called ozone generators (but sometimes with other names) that intentionally release ozone into indoor air are sold as air cleaners [38-43]. The manufacturers of these devices may claim that the ozone destroys indoor air volatile organic compounds and kills bacteria and mold, or they may make only general claims about purifying indoor air. Other types of electronic air cleaners (see Tables 1 and 2 in the supporting information), such as electrostatic precipitators, intended as particle air cleaners sometimes produce ozone, but their ozone production rates are often smaller  than the ozone production rates of ozone generators. This section focuses on ozone generators that intentionally release ozone with the objective of using ozone to clean the indoor air, although the rates of ozone release by other air cleaners can also be high enough to pose health risks.
No measured data on the health effects of ozone generators were identified. Research has shown that ozone generators are generally not effective in reducing indoor air concentrations of most volatile organic compounds [37, 39, 45]. Indoor air concentrations of selected volatile organic compounds are reduced by ozone generators, but concentrations of most volatile organic compounds are not significantly reduced [37, 39]. In tests by Chen et al. , the concentration of only one of 16 volatile organic compounds was substantially decreased by operation of three air cleaners emitting substantial ozone. Also, the chemical reactions driven by the increased ozone concentrations are a source of potentially harmful pollutants [37, 41]. Other research indicates that the indoor ozone levels attained with ozone generators are not effective as a biocide for bacteria and mold on indoor surfaces. The concentrations of ozone required to kill bacteria and mold on surfaces are far too high and at these concentrations the ozone poses immediate risks to health [40, 43]. Finally, many ozone generators can increase indoor ozone concentrations to well above health-based ozone standards [38, 42, 45, 46]. In the United States, the eight-hour ambient air quality standard for ozone, established in 2008, is 75 parts per billion (ppb) . In 2005, California established an eight-hour standard of 70 ppb and a one-hour standard of 90 ppb . In studies by Britigan et al. , the increases in indoor ozone concentrations with ozone generators operating usually exceeded 100 parts per billion (ppb) and were as high as 650 ppb. In studies by Phillips and Jakober , maximum time-average concentrations from operation of ozone generators in a test room for 60 minutes were 88 to 435 ppb, except for one test result with a concentration of 1 ppb. In studies by Chen et al. , peak ozone concentrations ranged from approximately 100 to 450 ppb. Thus, research data clearly show that ozone generators can increase indoor ozone concentrations to above the levels in health-based standards.
In summary, ozone generators do not effectively remove the pollutants known or suspected to cause adverse health effects. Their operation can often produce high indoor air ozone concentrations that pose risks to health. The ozone produced by ozone generators can also drive chemical reactions that result in increased concentrations of formaldehyde, ultrafine particles, and other pollutants that pose risks to health.