The influence of air ions on the particle concentration in indoor environments

1. Introduction

Today in Europe many deaths can be related to air pollution, which causes it to be seen as an important health risk [1]. To avoid the introduction of these particles into the indoor environment, mechanical ventilation with appropriate filters can be used. The use of filters leads to a higher pressure drop, resulting in a higher energy consumption.

Electrically charged particles in air are called air ions. They are continuously produced in nature, e.g. by cosmic radiation, alpha emitters (radioactivity) or through the Lenard-effect (splitting of water near waterfalls or during rainfall). Studies have shown that a higher concentration of ions in air is correlated to a lower concentration of particles. Ling et al. [2] measured the ion and the particle concentration in Brisbane, Australia near a major road in the city. The other measurement positions were distanced from other roads and at a clean space away from urban particle sources. The reduction in ion concentrationcorrelates with the increase in particle concentration. This trend is also supported by Wang et al. [3] for three natural spaces (open space, near a lake, in the forest). The ion concentration was measured to be higher in the forest than at the open space or at the lake.

The correlation between ion and particle concentration in outdoor air is one reason why air ions are currently discussed as a possible solution to clean air from particles, either in rooms or in the supply air of a ventilation system. Ion concentrations in cities are found to be much smaller than at locations far from large particle sources. To use air ions for cleaning indoor air, artificial ion sources have to be used. The methods that are commonly used in commercial ion generators are corona discharge and dielectric barrier discharge (DBD). For DBD two electrodes are separated by an insulating dielectric barrier and electric discharge occurs when a high AC voltage is applied.

For corona discharge, which should be focused on in this literature review, a peak (tip or brush) is electrically charged with a high voltage (in general above 1 kV) until an electric field appears. Particles flying through this electrical field get charged (see Fig. 1). Whereas from a peak with a negative electric field, electrons are emitted and particles got negatively charged, from a positive field electrons are attracted and particles get positively charged. If just one kind is generated, the process is called unipolar ion generation. Bipolar ionization is the case of a generation of positive as well as negative air ions from different peaks. Electrically charged particles aim to neutralize and will therefore either combine with controversially charged particles or flow towards grounded or oppositely charged surfaces. The applied voltage as well as the material and the length of the wire [4,5] influence the number of ions emitted. In addition, recombination of oppositely charged particle encounters between charged and uncharged particles or molecules may also occur (ion evolution).

In Fig. 1, the ions are displayed simply as shapes, but in general the charged particle as well as the uncharged molecules are integrated into the ion. A detail of a possible ion is shown in Fig. 1 as well.

The time a charged particle stays in the air depends on the particle concentration as well as the size of the charged particle. Horrak et al. [6,7] correlated the mobility of air ions with their sizes. The mobility is thereby defined as the velocity a charged particle has under the influence of a constant electric field (Israel [8,9]). Horrak et al. [6,7] define five different size classes, which are summarized in Table 1.

Table 1. Mobility classes regarding Horrak [7].

Type	mobility in cm2/(Vs)	diameter in nm
small cluster ions	1.3 … 3.2	0.36 ... 0.85
big cluster ions	0.5 … 1.3	0.85 ... 1.6
intermediate ions	0.034 … 0.5	1.6 ... 7.4
light large ions	0.0042 … 0.034	7.4 ... 22
heavy large ions	0.00041 … 0.0042	22 ... 79

The smaller a charged particle, the faster it will move in air. Smaller air ions can more easily collide with particles in the air and in this case they will not survive as small air ions for a long time. After collision with larger ions, coagulation as well as sedimentation may occur. In case of a high particle concentration in air the region surrounding the ion generation may therefore be cleaned faster than the surrounding. Ions generated by corona discharge can mainly be assigned to the class of small cluster ions, which have a high mobility and can easily move through air. After encounter with other particles or molecules (charged or uncharged) the size increases and the mobility decreases.

The production of ozone may be a critical point regarding the usage of air ions to clean indoor air from particles. In some studies ozone production rates ranging from 0.06 to 4.3 mg/h (Niu et al. [10], Zhao et al. [11], Waring et al. [12], Berry et al. [13], Nunayon et al. [14], Britigan et al. [15]) were measured, whereas in other studies no ozone was measured (Wu et al. [16], Park et al. [17], Zeng et al. [18], Jakober et al. [19]) or at least not smelled (Grabarczyk [20]). The decay of ozone in the room is influenced by the surface material as well as temperature and relative humidity (Britigan et al. [15], Mueller et al. [21]).

These controversial results were somewhat explained by Awad and Castle [22] as well as Boelter and Davidson [23]. Awad and Castle [22] investigated the influence of wire length and material as well as power consumption on the ozone emission rate. They concluded that the ozone emission rate is higher for higher electrical power and for negative corona discharge. This was also supported by Boelter and Davidson [23]. In addition, Boelter and Davidson [23] investigated the influence of different temperatures and relative humidities and set up an equation for the correlation between the temperature and relative humidity as well as current and ozone production.

In conclusion, in some cases a production of ozone was found while in others it was not. This may be influenced by the boundary conditions (temperature and humidity) as well as the specific device (wire material and length as well as applied current). For currently available fiber-brush discharge ionizers, it can be assumed that no ozone was produced accidently, but in some cases it was produced intentionally. However, this phenomenon will not be considered in more detail within this review.

In this study, the current knowledge regarding the air cleaning efficiency of air ions is summed up and research questions are raised.

2. Material and methods

Google Scholar, Science Direct and Pubmed were searched with the keywords “ion” or “ions”, “air”, “room” or “indoor environment”, “channel” or “duct” and “ionizer” or “ioniser”. The search was repeated at different times. Studies describing experimental investigations were studied in detail. Solely studies which measured the ion concentration and at least one further value (distance, particle concentration, …) in either a room or a duct were included in the literature review. The focus was thereby on studies considering corona discharge. Studies referenced in the selected studies were considered as well, if they met the inclusion criteria. Furthermore, the studies were separated into studies in laboratories or empty rooms, field studies in occupied rooms, measurements in air ducts or ventilation plants.

In this literature review, the studies were rated regarding a given matrix for nine different parameters (see Table 2). Three criteria (printed in bold and italic in Table 2) are claimed to be especially important and were therefore rated double.

Table 2. Evaluation criteria for the quality of the selected studies.

Empty Cell	Empty Cell	0	1	2	3
1	Completeness of boundary conditions (room volume, temperature, relative humidity)	Not described	Incompletely mentioned	Minor details missing	Complete
2	Measurement of ion concentration and emission rate	Not described	Incompletely mentioned	Minor details missing	Complete
3	Measurement of further parameters	Not described	Incompletely mentioned	Minor details missing	Complete
4	Measurement accuracy	Not described	Used device mentioned	Average accuracy	High accuracy
5	Extent of measurements	Not described	One situation/measurement point	Field study at different places	Variation of different parameters
6	Measurement set-up	Not described	Reasonable set-up, but with unknown boundary conditions	Partially controlled boundary conditions or measurement of some boundary conditions in field studies	Control of boundary conditions (e.g. temperature, charge of surfaces) or measurement of all boundary conditions in field studies
7	Display of measurement results	Not described	Quantitative description	Graphical visualization	Displayed in a table
8	Consideration of errors (measurement devices as well as measurement inaccuracy)	Not described	Qualitative description	Description of measurement errors or measurement inaccuracies	Descriptions of both
9	Discussion/critical review	Not described	Comparison of different own measurements	Comparison with one other study or statistical analysis	Comparison with multiple other studies and statistical analysis
Parameter is rated double.

It has to be mentioned that measurement accuracy might be easy to consider for some measurements (e.g. temperature and velocity), but more difficult for particles or ions, where the accuracy of the measurement devices cannot be compared to a normative value easily and which are more susceptible for example to high concentrations. Furthermore, local differences in concentration can occur, which will result in deviations for single measurement points. Still, it is helpful for other researchers to mention at least the available information. In addition, repetition of measurements may decrease random measurement errors.

In total up to 36 points can be given for each study. 85–100% (31–36 points) are rated as very good, 70–85% (25–30 points) as good, 60–70% (22–24 points) as fair, 50–60% (18–21 points) as acceptable and studies with less than 50% (<18 points) are rated as bad and excluded from further evaluation. In case one parameter is not applicable, this parameter was not evaluated, and the final score was upscaled. This occurred e.g., if the influence of the distance was evaluated, but a second measured parameter was not measured.

Of 37 studies which met the inclusion criteria for content, five had to be excluded because their descriptions of the considered aspects were inadequate. In most cases this happened due to missing information on the measurement set-up or the measurement equipment.

3. Results

3.1. Overview

The results section is split up into the considered values (particle concentration, particle decay, CADR, filter efficiency and other aspects) as well as the place where measurements occurred (laboratory, field or duct). The boundary conditions for each section are summed up in Table 3, Table 4, Table 5, Table 6, Table 7, together with the evaluation of each study.

Table 3. Boundary conditions and evaluation of studies considering particle concentration under the influence of air ions.

Name	Year	Room size in m³	Volume flow in m³/h	Temper-ature in °C	Relative humidity in %	Particle dia-meter in μm	Particle concentration in 1/m³	Ion charge	Ion concen-tration in 1/cm³	Signifi-cance	Evaluation*
Grabarczyk [20]	2001	50	1.7	23.2–24.5	35.8–37.4	0.3–2.5	0.3–0.4 μm: $5 \cdot 10^{6} - 10^{8}$	negative	$5 \cdot 10^{3} -$ $12 \cdot 10^{3}$	partially	20/36 (3/2/3/3/1/1/3/1/0) Rating: acceptable
Grabarczyk [20]	2001	50	1.7	23.2–24.5	35.8–37.4	0.3–2.5	0.3–0.4 μm: $3 \cdot 10^{6} - 5 \cdot 10^{6}$	negative	$3.6 \cdot 10^{4}$ $- 1.8 \cdot 10^{6}$	partially	20/36 (3/2/3/3/1/1/3/1/0) Rating: acceptable
Černecký et al. [25]	2015	73.89	Inlet velocity 0.6 m/s	20 ± 1	37; 39	PM10	$190 - 360 \frac{μ g}{m ³}$	negative	$50 - 400$	yes	28/36 (3/3/3/3/3/2/3/0/3) Rating: good
Grinshpun et al. [[27], [28], [29]]	2001, 2004, 2005	0.026; 2; 2.6; 24.3	none	22	28 ± 5	0.3–3.0	Smoke generation until $1 \cdot 10^{9}$	positive and negative	$\sim 3.5 \cdot 10^{3}$	yes	28/36 (3/3/3/1/3/2/2/2/2) Rating: good
Offermann et al. [30]	1984	35.1	without	?	40–65	0.01–3	$1 - 2 \cdot 10^{11}$	negative	?	no	18/36 (2/0/3/1/1/1/3/2/1) Rating: acceptable
Qian [31]	2021	?	?	?	?	18.1–289; PM2.5	?	negative	?	x	12/36 (0/0/2/0/1/2/3/0/1) Rating: exclusion
Fletcher et al. [32]	2008	32	32–478.8	?	?	0.3->5	$1.5 \cdot 10^{6}$ $- 5.5 \cdot 10^{6}$	negative and bipolar	450–700	yes	28/36 (1/3/3/1/3/3/2/21/2) Rating: good
Ardkapan [33]	2013	30; 47.5; ?	inlet 0.3 m/s; duct: 216-360	?	?	0.007–1	$2.8 \cdot 10^{3} - 1 \cdot 10^{5}$	?	?	x	17/36 (2/0/3/2/2/1/3/0/1) Rating: exclusion
Berry et al. [13]	2007	151.3	43.2	?	?	0.3–3	Ratio indoor/outdoor	negative	$5.5 \cdot 10^{4}$	partially	25/36 (1/3/2/1/2/2/3/2/3) Rating: good
Pushpawela et al. [36]	2017	1; 20; 32; 45; 132	None, ?	20–23	67–78	0.01–0.4	$5 \cdot 10^{9}$ $- 7 \cdot 10^{10}$	negative	emission rate $10^{6} \frac{ions}{s}$	partially	22/36 (2/1/3/1/2/3/2/0/3) Rating: fair
Bohgard and Eklund [37]	1998	19.3	1.5; 3	20	40	0.017–0.9	$1 \cdot 10^{11}$	negative	?	x	15/36 (2/0/3/1/1/1/2/1/1) Rating: exclusion
Davidović et al. [38]	2020	?	?	20.1–32.6	20–49	0.1–10	$30 \cdot 10^{6}$ $- 1.2 \cdot 10^{11}$	negative	$643 - 871$	partially	28/36 (1/3/3/1/2/3/3/21/3) Rating: good
* Evaluation: Format: Total Score/maximum points (criteria 1/ … /criteria 9) Rating: 85–100% (31–36 points) … very good, 70–85% (25–30 points) … good, 60–70% (22–24 points) … fair, 50–60% (18–21 points) … acceptable, less than 50% (<18 points) exclusion ? … missing x … not applicable 1 … rated as description of one kind of error, because the results were compared with theoretical considerations
Values printed in bold are counted twice

Table 4. Boundary conditions and evaluation of studies considering particle decay under the influence of air ions.

Name	Year	Room size in m³	Volume flow in m³/h	Temper-ature in °C	Relative humidity in %	Particle dia-meter in μm	Particle concentration in 1/m³	Ion charge	Ion concen-tration in 1/cm³	Signifi-cance	Evaluation*
Wu and Lee [40]	2003	169.3	?	?	?	PM 2.5	$2 \cdot 10^{9}$ $to 12 \cdot 10^{9}$	negative	1400	x	15/36 (1/3/3/1/1/1/2/0/1) Rating: exclusion
Shiue and Hu [34,35]	2011	3.7	none, 14.8	24 ± 1	45 ± 2	0.1–0.5	$3 \cdot 10^{3}$	negative	$6 \cdot 10^{5}$	yes	22/36 (3/3/3/0/1/3/3/0/2) Rating: fair
Wu et al. [41]	2015	0.216	Velocity in the chamber: 0.56 m/s; 1.2 m/s; 2.0 m/s	?	$50 \pm 3$	0.03–0.3	?	negative	$1.65 - 1.8 \cdot 10^{6}$	yes	28/36 (2/3/2/1/3/2/3/2/3) Rating: good
Yu et al. [42]	2017	0.216	Velocity in the chamber: 0.56 m/s; 1.2 m/s; 2.0 m/s	?	$50 \pm 3$	0.03–0.3	$(5.0 \pm 2.3) \cdot 10^{11}; (6.6 \pm 1.5) \cdot 10^{10}$	negative	Up to $2 \cdot 10^{13}$	yes	29/36 (2/3/3/1/3/2/3/2/3) Rating: good
Shi et al. [43]	2016	0.46		?	?	0.0181–0.289; PM2.5	$1.5 \cdot 10^{6}$	negative	?	partially	19/36 (1/0/3/1/1/1/3/2/3) Rating: acceptable
Wu et al. [44]	2006	1	1	25	40–45	0.03 and 0.3	$1.6 \cdot 10^{10}$ $- 1.8 \cdot 10^{10}$	negative	$3 \cdot 10^{3}$ $- 5 \cdot 10^{3}$	yes	31/36 (2/3/3/1/3/3/3/2/3) Rating: very good
Zeng et al. [18,45]	2021, 2022	36.7	40–340	23.4–26.9;	26.2–39	0.01–10	$5 \cdot 10^{8}$ $- 1.6 \cdot 10^{11}$	bipolar	$1.4 \cdot 10^{3}$ $- 2 \cdot 10^{3}$	no	32/36 (3/3/3/1/2/3/3/3/3) Rating: very good
Zeng et al. [18,45]	2021, 2022	36.7	40–340	26.1–29.1	36–50	0.01–10	?	bipolar	$6.7 \cdot 10^{3}$ $- 1.4 \cdot 10^{4}$	no	32/36 (3/3/3/1/2/3/3/3/3) Rating: very good
Nadali et al. [39]	2020	20–40	different	?	?	PM1	6.5 ± 10.1 μg/m³	negative	$1 \cdot 10^{6}$	yes	24/36 (1/2/3/2/2/1/3/2/3) Rating: fair
						PM2.5	49.5 ± 18.2 μg/m³
						PM10	90.1 ± 33.5 μg/m³

* Evaluation: Format: Total Score/maximum points (criteria 1/ … /criteria 9).

Rating: 85–100% (31–36 points) … very good, 70–85% (25–30 points) … good, 60–70% (22–24 points) … fair, 50–60% (18–21 points) … acceptable, less than 50% (<18 points) exclusion.

? … missing.

x … not applicable.

1 rated as description of one kind of error, because the results were compared with theoretical considerations.

Values printed in bold are counted twice.

Table 5. Boundary conditions and evaluation of studies considering CADR under the influence of air ions.

Name	Year	Room size in m³	Volume flow in m³/h	Temper-ature in °C	Relative humidity in %	Particle dia-meter in μm	Particle concentration in 1/m³	Ion charge	Ion concen-tration in 1/cm³	Signifi-cance	Evaluation*
Niu et al. [10]	2001	6.4	fan for mixing	24	60	PM10	400–600 μg/m³	?	?	no	21/36 (3/0/3/1/3/2/3/0/1) Rating: acceptable
Zhao et al. [11]	2005	11, 14.5	fans for mixing	?	?	0.1–1; 0.5–20	?	?	?	no	19/36 (1/0/1/1/3/2/3/0/3) Rating: acceptable
Waring et al. [12]	2008	14.75	?	?	?	0.0126–0.514	?	?	?	no	21/36 (1/0/1/1/3/1/3/2/3) Rating: acceptable
Chan and Cheng [46]	2006	6.4	fan for mixing	22–24	63–68	PM2.5; PM10	?	?	?	no	25/36 (3/0/3/3/3/2/3/1/1) Rating: good
Ongwandee and Kruewan [47]	2012	8	fan for mixing, v = 0.1 m/s	23–25	64–75	PM2.5; PM10	Up to $0.8 \frac{m g}{m ³}$	negative	?	no	21/36 (3/0/3/2/1/2/3/2a/0) Rating: fair
Sung et al. [48]	2019	30.4 112.9	fans for mixing	?	?	0.2–1	$1 \cdot 10^{8}$ $- 1 \cdot 10^{9}$	?	$0.2 \cdot 10^{6}$ $- 9.4 \cdot 10^{6}$	yes	21/36 (2/2/3/1/3/1/2/0/3) Rating: acceptable

* Evaluation: Format: Total Score/maximum points (criteria 1/ … /criteria 9).

? … missing.

x … not applicable.

Values printed in bold are counted twice.

a: Rated as description of one kind of error, because the results were compared with theoretical considerations.

Table 6. Boundary conditions and evaluation of studies considering filter efficiency under the influence of air ions.

Name	Year	Duct area in m2	Velocity at the filter in m/s	Temper-ature in °C	Relative humidity in %	Particle dia-meter in μm	Particle con-centration in 1/m³	Ion charge	Ion emission rate in 1/s	Signifi-cance	Evaluation*
Agranovski et al. [49]	2006	?	1.1	22–24	50–55	0.5; 0.8; 1.0; 1.5	$1 \cdot 10^{7}$	negative	$1 \cdot 10^{12}$	yes	21/36 (1/1/3/1/3/1/2/2a/1) Rating: acceptable
Tian et al. [50]	2018	0.14 × 0.24	0.5–1	24.1–30.2; 14.4	absolute humidity: 3.9–7 g/kg	0.01–5	$P M_{2.5} = 153.1 \frac{μ g}{m ³}$ , $P M_{2.5} = 114.6 \frac{μ g}{m ³}$	positive	Ion concentration: $0 - 33200 \frac{1}{c m ³}$	yes	31/36 (3/3/3/1/3/3/3/2/2) Rating: very good
Blondeau et al. [51]	2021	0.61 × 0.61	0.9; 1.2; 2.7	19	45	0.2–5	?	negative	?	x	15/36 (3/0/1/2/3/0/0/1) Rating: exclusion
Blondeau et al. [51]	2021	0.61 × 0.61	0.9; 1.2; 2.7	24	70	0.2–5	?	negative	?	x	15/36 (3/0/1/2/3/0/0/1) Rating: exclusion
Shi and Ekberg [52]	2015	0.6 × 0.6	0.67–6.4	16–24	45–85	0.3–25	$1 - 1.5 \cdot 10^{7}$	negative and positive	Ion concentration: $5 \cdot 10^{5} \frac{1}{c m ³}$	partially	30/36 (3/3/3/1/3/2/3/2/3) Rating: good
Park et al. [17]	2009	0.04 × 0.04	0.5	22.5 ± 0.3	10 ± 5	0.05–0.6	$5 \cdot 10^{9} \pm 1 \cdot 10^{8}$	Positive	$1.6 \cdot 10^{11},$ $6.4 \cdot 10^{12}$	yes	19/36 (1/3/3/1/1/1/2/2a/1) Rating: acceptable
Yang et al. [53]	2007	?	0.1–0.2	?	30–70	0.05–0.5	?	Negative	$2 \cdot 10^{4} \frac{1}{cm ³}$ , $1 \cdot 10^{5} \frac{1}{cm ³}$ , $2 \cdot 10^{5} \frac{1}{cm ³}$	yes	20/36 (1/3/2/1/3/2/2/0/1) Rating: acceptable
Zeng et al. [18]	2021	534	1700	?	?	0.01–10	$2 \cdot 10^{9}$	Bipolar	?	no	32/36 (3/3/3/1/2/3/3/3/3) Rating: very good**
Nunayon et al. [14]	2019	0.2 × 0.2	3.0–6.5	17.5, 21.0, 25.0	50, 70, 90	0.4–0.8, 0.96, 1.0	?	negative and positive	?	partially	27/36 (3/0/3/2/3/2/2/2/3) Rating: good
Baldelli et al. [54]	2022	0.6 × 0.6	0.77–2.31	18–33	43–63	Bacteria and fungus	$1 \cdot 10^{3} - 7.5 \cdot 10^{3}$	?	?	no	25/36 (3/0/3/1/3/1/3/2/3) Rating: good

* Evaluation: Format: Total Score/maximum points (criteria 1/ … /criteria 9).

**same evaluation as in Table 4.

? … missing.

x … not applicable.

Values printed in bold are counted twice.

a: Rated as description of one kind of error, because the results were compared with theoretical considerations.

Table 7. Boundary conditions and evaluation of studies considering other particle related aspects under the influence of air ions.

Name	Year	Room size in m³	Volume flow in m³/h	Temper-ature in °C	Relative humidity in %	Particle dia-meter in μm	Particle concen-tration in 1/m³	Ion charge	Ion concen-tration in 1/cm³	Signifi-cance	Evaluation*
Alonso and Alguacil [55]	2008	0.004	?	?	?	?	Up to $6.8 \cdot 10^{13}$	unipolar + bipolar	?	partially	22/36 (1/0/2/1/3/2/3/2a/1) Rating: fair
Park et al. [56]	2014	?	?	?	?	x	$1.4 \cdot 10^{7} (Fig 2)$ $6.4 \cdot 10^{6} (Text)$	negative and positive	$1.7 \cdot 10^{7} \frac{1}{cm ³}$	yes	22/36 (1/1/3/1/3/1/3/2a/1) Rating: fair
Wu et al. [16]	2006	169.3	none	25.2 ± 1.4	38.1–73.6	x	x	negative	$2.2 \cdot 10^{6}$	partially	29/33 (3/3/x/1/2/3/3/3/3) Rating: very good
Duan et al. [57]	2022	43.75	4500	25 ± 1	60 ± 10	0.5–2.5	$1 \cdot 10^{9}$	negative	$1.5 \cdot 10^{5}, 3 \cdot 10^{5},$ $7 \cdot 10^{5}$	yes	27/36 (3/3/3/1/3/2/2/2/1) Rating: good

* Evaluation: Format: Total Score/maximum points (criteria 1/ … /criteria 9).

? … missing.

x … not applicable.

Values printed in bold are counted twice.

a: Rated as description of one kind of error, because the results were compared with theoretical considerations.

A separation into laboratory and field measurements is necessary, because a transfer between the two situations may be difficult [24]. This is due to significantly different boundary conditions (e.g. fluctuating sources or a wide range of surface materials).

3.2. Measurement of particle concentrations

3.2.1. Measurements of particle concentrations in test chambers or unoccupied rooms

Grabarczyk [20] found a 20 (0.4–2.5 μm, 1–2.5 μm 106 to 5 × 104 Particle/m³) to 30-fold (0.3–0.4 μm, from 108 to 3 × 106 Particles/m³) decrease of the particle concentration after 1 h in an unoccupied room (V = 50 m³, ϑ = 23.2–24.5 °C, φ = 35.8–37.4%) if a large number of ions is introduced (70 electrodes distributed within the room, ion concentration in the center of the room between $c_{n e g a t i v e i o n s} = 5 \cdot 10^{3} t o 12 \cdot 10^{3} \frac{1}{c m ³}$ ). If a portable air cleaning device (ion concentration depending on the distance from the ion generator (0.15 < d < 1.5 m) between $c_{n e g a t i v e i o n s} = 3.6 \cdot 10^{4} t o 1.8 \cdot 10^{6} \frac{1}{c m ³}$ ) with a lower ion emission rate and an uneven distribution of air ions in the room was used, no significant decrease in the particle concentration after t = 4 h was measured. The quality of the paper was rated as “acceptable”, because the measurements considered just one situation and the boundary conditions (temperature, velocity, …) were uncontrolled. In addition, the results were not considered regarding the measurement errors as well as the measurement inaccuracies. No comparison to results from other studies were implemented.

The distance between the measurement position and the ion inlet into the room was considered by Černecký et al. [25]. Cigarette smoke was introduced into the test chamber and the maximum concentration as well as the decay curve were evaluated as values for the efficiency of the ions to clean the room air from PM10. It has to be mentioned, that the majority of particles generated by cigarette smoke is a lot smaller than 10 μm [26]. Furthermore, the relative humidity (37% and 39%) was studied, but it has to be mentioned that the difference between the two considered values is quite small. The concentration of PM10 with and without ionization was used as an indicator for the efficiency of the cleaning device. The authors concluded that with increasing distance from the ion inlet, the efficiency of the ions decreased. The study is considered as “good”.

Another aspect that must be considered is the material of the particles, which shall be discharged from the air. Grinshpun et al. [[27], [28], [29]] used four different materials (NaCl (0.3–3.0 μm), PSL spheres (Polystyrene Latex Spheres) (0.3–3.0 μm), smoke aerosols and bacteria (0.8 μm)) in chambers of different sizes (V1 = 0.026 m³, V2 = 2 m³, V3 = 2.6 m³ and V4 = 24.3 m³). A collision nebulizer introduced the particles. In all cases, the air cleaning was improved with ions compared to without ions. Therefore, the efficiency was defined as the ratio of the difference of the particle concentration with and without ions compared to the concentration without ions. The distance from the ion source, the room volume as well as the air movement influenced the efficiency of the ions, whereas the ion polarity, the particle size, the aerosol properties as well as the temperature and breathing of the manikins did not significantly influence the efficiency. Furthermore, the authors suggested to use the ion generator in an intermittent way to avoid electric charging of surfaces. In general, the measurement set-up and the documentation of this series of studies is reasonable and well-documented. However, because of some minor missing information (e.g. accuracy of the used devices), the study was rated as “good”.

In 1984 Offermann et al. [30] compared the efficiency of eleven air cleaning devices. With cigarette smoke as the particle source, the decay of particles was considered for four panel-filter units, two extended-surface filter units, two electrostatic precipitators, two negative air ion generators and two recirculation fans. The ion generators performed in the lower end of the measured devices. The study is evaluated as “acceptable”.

Qian [31] investigated the efficiency of negative air ions in reducing the concentration of nanoparticles as well as PM2.5 in air. He therefore selected four devices and measured the particle decay. The CADR for nanoparticles ranged from 1.09 to 19.7 m³/h and for PM2.5 from 0.7 to 2.47 m³/h. Two of the devices emitted ozone, whereas the other two did not. The measurement is described incompletely, which is why this study is excluded from further consideration.

To evaluate the accuracy of their model, Fletcher et al. [32] measured the concentration of particles in a test chamber (V = 32 m³, p = −1 Pa) under the influence of different volume flows (32.0–478.8 m³/h). The concentration of particles was measured every 5 min with an optical particle counter (Kanomax-Geo-α Optical Sciences, UK). The particle concentration ranged from 107 1/m³ for particles <0.3 μm to 103 1/m³ for particles >5 μm. Negative as well as bipolar ionization was investigated in the study. The ion concentration, measured at the opposite site of the room, was between $450 \cdot 10^{6} t o 700 \cdot 10^{6} \frac{1}{m ³}$ . The steady state concentration of particles as well as the decay were measured. From their model, the authors concluded that for high ion concentrations (>1010 1/m³) the ventilation rate does not affect the effectiveness of air ions in cleaning the air from particles. The study considered just one situation, but the authors compared the measurement results with theoretical values, causing the study quality to be rated as “good”.

In his PhD Ardkapan [33] investigated five different technologies to reduce the concentration of ultrafine particles in indoor environments. He considered Non-Thermal Plasma, Corona Discharge, a Portable Air Purifier, an electrostatic fibrous filter and a three-dimensional filter. The effectiveness in reducing the particle concentration ranged from 0.2 for the Non-Thermal Plasma to 0.42 for the electrostatic fibrous filter in an office room. In the clean room the lower boundary remained the same, but the efficiency of the electrostatic fibrous filter increased to 0.7. Finally, the efficiency was also measured in a ventilation duct. It was 0.09 for the Non-Thermal Plasma and 0.78 for the electrostatic fibrous filter. It was measured to be 0.4 for all settings with the corona discharge. Since the ion concentration was not measured by the authors and the extent of the investigation is limited to different places in a field study, the study is “excluded” from further analysis.

3.2.2. Measurements of particle concentrations in occupied rooms

Berry et al. [13] installed a commercially available air cleaner in a one-bedroom apartment (two adults, one dog). The measurements of the ratio of indoor to outdoor particles were performed both with and without occupants. With occupants, the ratio of indoor to outdoor particles was reduced from 1.03 to 0.73 if the air cleaner was used. Without occupants, the ratio at the beginning of the measurements (before the introduction of air ions) was between 0.9 and 1.4 and decreased to between 0.3 and 0.4 within 8 h after switching on the ion generator. Furthermore, a high ion concentration near the generator was measured, but the vertical distribution of ions was limited. The missing of vertical distribution was confirmed by Shiue and Hu [34,35]. The measurements were performed in one room, but with different research questions. Because of limited description of the used measurement devices the study quality was rated as “good”.

Similar results were also found by Pushpawela et al. [36] in rooms with five different sizes (V1 = 1 m³, V2 = 20 m³, V3 = 32 m³, V4 = 45 m³, V5 = 132 m³), where either ambient air or smoke was used as a particle source. The ion emission rate of the ion generator was about $10^{6} \frac{i o n s}{s}$ , but the concentration in the room was not measured. The particle concentration at the beginning of the measurements ranged from $5 \cdot 10^{3} \frac{p a r t i c l e s}{c m}$ in the 1 m³-chamber with ambient air to $7 \cdot 10^{4} \frac{p a r t i c l e s}{c m}$ with smoke. Particles of 0.02–1 μm were measured. The ratio of particles cleaned out from the air with ionization and with a HEPA filter was compared. The larger the chamber, the smaller the percentage of particles which were cleaned out after 15 min. It was measured to be 40% with ionization after 15 min in the 32 m³-chamber, but just 22% in the 132 m³ room. In all cases, the ratio with ionization was larger than with HEPA filter (32 m³: 40% with ionization, 25% with HEPA filter, 132 m³: 22% with ionization, 6% with HEPA filter). The authors concluded that the ionization was more effective in ventilated than in unventilated rooms. The quality of the study could have been improved if the measurement accuracy as well as measurement errors would have been discussed. Therefore, it was rated as “fair”.

In a test chamber as well as an office room, Bohgard and Eklund [37] applied a commercially available air cleaning device to consider the reduction of particles in the room. NaCl was therefore introduced into the test chamber and a significant reduction of particles is found. This was also proved in the office room during regular use with an air change rate of 3 1/h. Because of the short report and consequently many unclear boundary conditions, especially regarding the field study, the study is excluded from further evaluation.

The particle concentration was used by Davidović et al. [38] in an approach to forecast the small ion concentration in schools without the need to measure them. In two schools in Belgrade, Serbia, particle concentration (10 nm–10 μm), radon concentration as well as ion concentration were measured. The concentration of particles 0.3–1.0 μm was similar, but the concentration of smaller particles was larger in the school with the lower concentration of small air ions. Incomplete documentation of boundary conditions (e.g. air volume flow, measurement accuracy) led to an evaluation of “good” for the quality of this study.