A variety of gaseous pollutants in indoor air, emitted from indoor sources or brought into the building with outdoor air, pose risks to health or may affect perceived air quality. Examples include various volatile organic compounds, ozone, nitrogen dioxide, carbon monoxide, and radon. Among the volatile organic compounds, formaldehyde is predicted to pose the largest health risks [22, 23]. Technologies used for gaseous pollutant air cleaning are described in Table 2 in the supporting information. Air cleaning systems can reduce indoor pollutant concentrations of volatile organic compounds, nitrogen dioxide, and ozone; however, their practicality remains uncertain . Air cleaning for carbon monoxide and radon is challenging because these compounds have a low reactivity. Most of the studies of gas-phase air cleaning systems have been laboratory studies .There have been few studies with measurements that document the long term performance of gas phase air cleaning [24, 25] in non-industrial buildings. It is reasonable to expect that gas phase air cleaning systems, when effective in reducing indoor concentrations of harmful gaseous pollutants, will reduce the adverse health effects associated with these pollutants, but confirmatory research from use of gas phase air cleaners in actual buildings was not identified.
Laboratory studies have investigated the effects of gas phase air cleaning on perception of air quality, which is not a health outcome, but an indicator of an effect on people. Each of these studies assessed initial perceptions of air quality, e.g., perceptions people have immediately upon entering a laboratory or perceptions of people smelling air drawn from a test system, and not the perceptions of occupants of a space, with and without gas-phase air cleaning. Key findings are provided in the following paragraphs.
- Two laboratory studies [26, 27] found that incorporation of activated carbon in particle filters can improve the sensory acceptability of air exiting the particle filters after they have been used and particulate matter has accumulated in the filters. Activated carbon is a material that removes some volatile organic compounds and ozone. It is known that air exiting used filters can be less acceptable from a sensory perspective than air that does not pass through a particle filter because of pollutants released from the particles deposited on used filters.
- Another study  had panels of five subjects rate the acceptability of air drawn from a chamber facility with and without different air cleaners operating in the chamber. At the start of the tests, chamber air was contaminated with tobacco smoke. Twelve different air cleaners were employed. Only the air cleaner incorporating 8.2 kg of activated carbon significantly improved sensory ratings of the air drawn from the chamber. The small size of the sensory panels, five persons, represents a study weakness.
- One study  showed that satisfaction with air quality and odor intensities were substantially improved in a simulated office environment when a rotary desiccant dehumidifier was operated. The desiccant dehumidifier removed both water vapor and several volatile organic compounds from the indoor air.
- A pair of studies [30, 31] investigated satisfaction with air quality, perceived air freshness, and perceived odor intensity with and without operation of a photocatalytic oxidation air cleaner that removes some volatile organic compounds. When the laboratories contained only building materials and building products as sources of pollutants, air cleaner operation significantly improved the measures of perceived air quality; however, when occupants were a source of bio effluents air cleaner operation significantly worsened the measures of perceived air quality. The authors speculated that the air cleaner incompletely decomposed alcohols released into the indoor air by people and that the products of incomplete decomposition of these alcohols degraded the sensory quality of the air.
One additional study investigated  investigated the effects of two photocatalytic oxidation air cleaning systems on perceived air quality, sick building syndrome symptoms, and a set of objective (measured) outcomes (visual acuity, nasal peak flow, skin dryness, and a measure of tear film quality) in a simulated aircraft cabin. The photocatalytic air cleaner reduced indoor concentrations of some volatile organic compounds but increased indoor concentrations of other compounds including formaldehyde and acetaldehyde (both can be irritants at high concentrations, formaldehyde is a human carcinogen). Incomplete decomposition of some volatile organic compounds, particularly of alcohol released by hand wipes, was the cause of pollutant concentration increases. Operation of the air cleaners had both positive and negative effects on satisfaction with air quality and sick building syndrome symptoms. After six hours, most symptoms were unchanged by air cleaner use. There were no significant effects of air cleaner operation on the objective (measured) health outcomes, except for an improvement in skin dryness with one air cleaner. Since humidity was not affected by the air cleaners, the authors hypothesized that the change in skin dryness was related to chemical reactions between indoor pollutants and the skin.