Ion generators charge particles with a corona prior to their removal on collector plates or indoor surfaces and also emit ozone, which can react with terpenes to yield secondary organic aerosol, carbonyls, carboxylic acids, and free radicals. This study characterized the indoor air quality implications of operating an ion generator in a 27 m3 residential room, with four different test room configurations. Two room configurations had carpet overlaying the original flooring of stained/sealed concrete, and for one configuration with and without carpet, a plug-in air freshener was used as a terpene source.
Measurements included airborne sampling of particulate matter (0.015–20 μm), terpenes and C1–C4 and C6–C10 aldehydes, ozone concentrations, and air exchange rates. When the heating, ventilating, and air-conditioning system was not operating (room air exchange rate = 0.5/h), the use of the ion generator in the presence of the air freshener led to a net increase in ultrafine particles (<0.1 μm). Also, increased concentrations of ozone were observed regardless of air freshener presence, as well as increases in formaldehyde and nonanal, albeit within measurement uncertainty in some cases. Thus, it may be prudent to limit ion generator use indoors until evidence of safety can be ascertained.
Practical Implications // Portable ion generators arc intended to clean the air of particles, but they may emit ozone as a byproduct of their operation, which has the potential to degrade indoor air quality. This study showed that under certain conditions in a residential room, the use of a portable ion generator can increase concentrations of ozone and, to a lesser degree, potentially aldehydes. Also, if operated in the presence of a plug-in air freshener that emits terpenes, its use can increase concentrations of secondary organic aerosol in the ultrafine size range.
Exposures to ultrafine ( < 0.1 μm) and fine (0.1 – 2.5 μm) particles have been associated with adverse health effects. Portable air cleaners are designed to remove particles from the indoor air, where a significant portion of exposure may occur because the average Canadian spends 18 hours indoors for every hour outdoors. Portable ion generators are marketed as air cleaners, and their intended purpose is to clean the air of particles by charging them with a corona before removal to oppositely charged collector plates or indoor surfaces. Portable ion generators can be set on a floor or table-top and are meant to clean a room-sized space. The particle removal capability of an ion generator can be quantified with the clean air delivery rate (CADR), which is the effective volumetric flow rate of particle-free air delivered by an air cleaner (m3 /h). CADRs for portable ion generators range from 0- 90 m3/h, at least an order of magnitude less than high efficiency particulate air (HEPA) cleaners.
The use of a corona causes ion generators to emit ozone as a byproduct of operation, at measured rates of 0.056 – 13.4 mg/h, and these rates are lower than dedicated ozone generators but still increase indoor ozone concentrations. Ozone is harmful to human health, and it is also a primary driver of indoor chemistry. The largest loss mechanism of ozone indoors is by surface reactions, which can lead to secondary emissions of carbonyls; for instance, ozone reacts with unsaturated fatty acids in carpets to form nonanal. Ozone can also react in the gas-phase with terpenes and other unsaturated organics to form secondary organic aerosol (SOA) in the ultrafine and fine particle size ranges, as well as carbonyls, carboxylic acids, and free radicals. Terpenes are common indoors and are emitted indoors from wood and consumer product such as air fresheners, surface cleaners, and perfumes. Since ion generators are generally not very effective at removing particles and emit ozone during operation, they can operate as net producers of particles and gaseous pollutants in the presence of terpenes. A 2007 study that operated an ion generator in an office, injected d-limonene into the air, and observed transient elevations of ultrafine particles. Other studies report the particle forming effects of dedicated ozone generators, which often emit more than 30 mg/h of ozone, in real environments with terpenes. Also, we observed steady-state net particle and formaldehyde formation when ion generators were operated in a 14.75 m3 stainless steel chamber with terpene-emitting air fresheners. This work extends the chamber investigation to a residential space, since real indoor spaces have larger volumes and surface-to-volume ratios, as well as other sources and sinks of particles, ozone, and carbonyls. The goal of this study was to determine the impact of using a portable ion generator on indoor air quality in a room with varying ozone sinks and terpene concentrations.
The room air was sampled in four different Room Configurations (RC):
• RC I: Original flooring of sealed/stained concrete, without air freshener
• RC 2: i nstalled flooring of carpet with padding, without air freshener
• RC 3: Original flooring of sealed/stained concrete, with air freshener
• RC 4: i nstalled flooring of carpet with padding, with air freshener

Impact of ion generators on indoor air quality

The ion generator used in this investigation increased concentrations of ultrafine particles, ozone, and, to a lesser extent, formaldehyde and nonanal. It also slightly decreased concentrations of fine particles. Portable ion generators are common in the US and Canada, and other brands and models may lead to different results. For instance, IG 2 in our previous work had a CADR (±s.d.) of 35 (13) m3/h and an ozone emission rate of 4.3 ± 0.2 mg/h. The lower CADR and higher ozone emission rate would likely lead to increased concentrations of particles, ozone, formaldehyde, and nonanal over what we observed here. However, the electrostatic precipitator (ESP) in our previous work had a much higher CADR of 284 (62) m3/h and a slightly higher ozone emission rate of 3.8 ± 0.2 mg/h. Thus, the ESP would likely lead to decreased particle concentrations in all RC because the CADR is approximately an order of magnitude greater than the other loss mechanisms. The ESP would, however, still likely increase concentrations of ozone, formaldehyde, and nonanal. Also, our study was conducted in an unoccupied room, but ion generators are often used in occupied spaces. Ozone can react with human skin and hair, and produce considerable levels of acetone, nonanal, decanal, 6-MHO and 4-0PA.
The pollutants increased by the ion generator have consequences for human exposure and health. An outdoor ozone increase of 10 ppb in the previous week’s ambient concentration has been associated with a 0.52% increase in mortality (Bell et al., 2004). Reaction products of ozone and d-limonene have been shown to increase respiratory burden in mice. However, newer research has shown that not the SOA from ozone/terpene reactions but gasphase products, such as formaldehyde, more complex aldehydes, carboxylic acids, and free radicals, may be responsible for acute effects. We know of no investigation into the chronic effects of SOA exposure. Formaldehyde is a human carcinogen, having been associated with nasopharyngeal cancer, and nonanal is an odorous and irritating compound. Thus, given the results in this investigation as well as previous research on ion generators, the operation of ion generators have the potential to degrade indoor air quality, and it may be prudent to limit their use indoors until evidence of safety can be ascertained.