A Comparative Assessment of the Some Commercially Available Portable Bipolar Air Ionizers Particulate Pollutants (PM_2.5, PM₁₀) Removal Efficacies and Potential Byproduct Ozone Emission

Abstract

Indoor air cleaning interventions such as bipolar air ionizers have increased lately due to rampant air pollution and the COVID-19 pandemic. Hitherto, the bipolar air ionizer efficacy against particulate pollutants and byproduct ozone emission has not been fully understood and remained a critical concern. Currently, available diverse and complex methods are insufficient to determine commercially available bipolar air ionizer reliability. The National and International market of bipolar air ionizers is proliferating, while safety standards and information are comparatively limited, in such cases, any misleading information by manufacturers could be detrimental to consumers. To focus on those gaps, the present study comprised five different types of commercially available bipolar air ionizers labeled as BAI 1, BAI2, BAI3, BAI4, and BAI5, which were examined against the most concerned indoor particulate pollutants and potential byproduct ozone. Seven days of consecutive experiments were performed in five acrylic boxes, each box assembled with a testing bipolar ionizer model, calibrated air quality monitor, and particulate pollutant source (incense sticks). Two runs/day for each individual bipolar ionizer were performed for up to seven consecutive days. Overall performance was procured from the daily cumulative arithmetic average. All tested bipolar air ionizers models showed notable, up to 80% particulate matter (PM_2.5 and PM₁₀) removal efficiencies. The highest particulate matter removal was associated with bipolar air ionizers model 4 (PM₁₀ 79.7%, PM_2.5 80.4%) and the minimum with BAI model 5 (PM₁₀ 72.2%, PM_2.5 72.3%). Abnormal ozone emission was not observed with any bipolar air ionizer conduction in this study. Almost negligible elevation in background temperature (0.4 °C) and relative humidity (0.6%) were also observed. In conclusion, bipolar air ionizers could be byproduct ozone-free, indoor particulate matter removal, and low maintenance indoor air cleaning option.

Introduction

Air pollution has been a challenging global environmental concern. But air quality-associated repercussions are more immense in low- and middle-income countries (WHO 2021; IQAir 2022). Especially, air pollutant particulate matter, size 2.5 microns (PM_2.5) has been a leading cause of mortality in highly polluted countries, such as China, India, and Pakistan (Anwar et al. 2021). Growing evidence shows that indoor air cleaning intervention may reduce indoor air pollutant exposure and associated health risks. Studies also suggested that indoor air pollutants are comparatively, easy to manage, and can be controlled by mechanical interventions, such as HEPA filters and air ionizers (Küpper et al. 2019).

Clean indoor air could be the prerequisite for the minimization and prevention of air pollution associated with non-communicable and communicable diseases including COVID-19-aggravated cases (San Juan-Reyes et al. 2021).

In the last past few years, especially since the coronavirus disease 2019, indoor air cleaning products such as air ionizers, have beguiled substantial worldwide attention, due to their emerging potential against several indoor air pollutants and microbial contaminants (Zeng et al. 2021). Hyun and colleagues demonstrated that air ionizer antiviral effects remarkably increased when positive and negative ions are employed together. Over the conventional unipolar air ionizer (release only negative air ions), bipolar air ionizers ions are considered 1.7 times more effective (Hyun et al. 2017).

A bipolar air ionizer is an electric device that generates electrically charged ions positive (H +) and negative (O2−) ions into the air when air–water molecules are exposed to high-voltage electrodes. Those ions react with a variety of air pollutants suspended in the air and cause the agglomeration-oriented abatement. In the case of microbial air contaminants, it has been purposed that ions clustering occurred around the microbes which leads to the formation of OH radicals and removes hydrogen from microbial cells to form water vapor (Kanesaka et al. 2022).

Recently, bipolar air ionizers have been proposed to decrease particulate matter, pathogens, and volatile organic compounds in the air. Bipolar ionization devices are being regulated by the U.S. Environmental Protection Agency (EPA) under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), so misleading claims about those devices' efficacy or safety are usually not made but the local vendor's performance claims are not routinely reviewed by the EPA as part of a registration process.

Currently, there are no international standardized test methods for bipolar air treatment technology except the Association of Home Appliance Manufacturers (AHAM)’s AHAM AC-5–2022, Method. Yet, comparing diverse methodologies and results across different studies and technology is difficult. Most testing has been restricted to a small-scale laboratory setting under specific conditions that are more often commissioned by the device manufacturers. On the other hand, some of these air ionization technologies may be liable to emit harmful byproducts, such as ozone, carbon monoxide, and formaldehyde (Ratliff et al. 2023).

Bipolar air ions can be generated by several methods, such as dielectric barrier discharge, needle point, corona discharge, plasma cluster, etc. In corona discharge, a tip or brush is electrically charged with a high voltage until a desired electric field occurs, while dielectric barrier discharge is based on two electrodes. Such electrically produced air ions could be associated with the potential byproduct ozone which may also determine by ionizer material, wire length, and power consumption. Yet, the byproduct ozone emission association with bipolar air ionizer is not fully understood and under growing investigations as some studies observed it while others did not (Hartmann and Kriegel 2022).

The objective of the present study is to assess different brand’s bipolar ionizer (with different specification) particulate pollutants reducing efficiency and potential byproduct ozone emission.

A comparative side by side assessment of bipolar ionizers could enhance the knowledge about bipolar air ionizer indoor cleaning options, especially regarding safety. Since the efficacy of bipolar air ionizers and byproducts, ozone emissions have been subjected to several complex methods but remain critical and vague so far. The present study focuses on bipolar air ionizers' particulate matter (PM_2.5 and PM₁₀) removal efficacy and potential byproduct ozone emission evaluation by a simple method. The study included a series of experiments (for 7 days), in which five different commercially available indigenous bipolar ionizers were tested in (L1.5 × W1.5 × H1.5 feet) unventilated acrylic boxes. The results from the present study may enhance the understanding of emerging bipolar air ionizers' safe uses.

Materials and Methods

Five different types of indigenous household portable bipolar air ionizers were brought from the e-commerce website. Bipolar air ionizers’ selection criteria included diverse bipolar ion concentrations, distinguish Clean Air Delivery Rate (CADR), locally manufactured, and public rating (Fig. 1). Bipolar ionizers’ fans, outer covers, and brand names were removed. The main specifications of bipolar air ionizer models are given in Table 1.

Fig. 1.

Selected Bipolar Air Ionizer (BAI) models with different specification and CADR values (a); air Quality monitor (b)

Table 1.

Bipolar Air Ionizer (BAI) models Specification

Specifications	Model 1	Model 2	Model 3	Model 4	Model 5
Positive ions	> 2.9 million pcs/cc (200 mm away in the direction of air flow)	> 2.7 million pcs/cc	> 2.8 million pcs/cc	> 3 million pcs/cc	> 2.6 million pcs/cc
Negative ions	> 3 million pcs/cc	> 2.8 million pcs/cc	> 2.8 million pcs/cc	> 3.5 million pcs/cc	> 2.8 million pcs/cc
Temperature range	Up to + 60 °C	+ 65 °C	+ 65 °C	+ 70 °C	+ 60 °C
Power supply	230 V, 50 Hz	230 V, 50 Hz	230 V, 50 Hz	250 V, 50 Hz	230 V, 50 Hz
CADR for PMs	110 m3/hr or 64CFM	120 m3/hr or 70 CFM	130 m3/hr or 76 CFM	153 m3/hr or 90 CFM	125 m3/hr or 73 CFM

CFM Cubic foot/Minute, PMs Particulate matter PM_2.5, PM₁₀

To provide individual and homogenous vacuumed test chambers for each bipolar air ionizer, five Acrylic boxes (L 1.5 × W 1.5 × H 1.5 feet) were assembled in the laboratory.

Automatic, rechargeable air quality monitors (Model PM2510CVTH, Fig. 1b) were obtained from the certified air monitoring device manufacturer and customized with the help of the manufacturer according to the required detector, such as PM_2.5, PM₁₀, CO₂, O₃, temperature, and relative humidity. Customization criteria were mainly worldwide accepted calibration standards for sensors and real-time detection of concerned pollutants and byproducts. The monitors also enabled WiFi and online data transferring facility.

The concerned sensors are well calibrated, for instance, the particulate matter laser-based sensor was calibrated according to the most accepted beta attenuation pollutant sensor technology (Gobeli et al. 2008). Comparison data with beta attenuation monitor (BAM) show a correlation of greater than 90%.

In the regression analysis of the air sensors, the regression coefficient (R2) was measured between 0 and 1, 0 being no correlation and 1 being 100% correlation and found a correlation of 0.9 + , which is considered appropriate. BAM and used LASER scattering senor comparative calibration details provided by the manufacturer Fig. 2, 3.

Fig. 2.

Regression coefficient and calibration against BAM of used particulate matter sensor (Images retrieved with permission of air monitor manufacturer, https://www.airveda.com/)

Fig. 3.

Outline of the experiment plan

Similarly, the ozone sensor is based on electrochemical ozone detection (module ZE25-O3). The sensor utilizes electrochemical principle to detect O₃ in air which considered as significantly sensitive and stable toward target the gas ozone detection.

More calibration, inbuilt sensors, and other details of air quality monitors are given in Table 2.

Table 2.

Air quality monitor specification and calibration details

Monitoring sensors	Measuring range	Relative error/accuracy	Working principle	Calibrated against
PM2.5	Range of PM2.5: 0–999 μg/m3	Maximum of ± 10% and ± 10 μg/m3	LASER scattering principle	Beta Attenuation Monitor
PM10	0–1999 μg/m3	Maximum of ± 10% and ± 10 μg/m3	LASER scattering principle	Beta Attenuation Monitor
CO2	0 – 5000 ppm (part per million)	50 ppm 3% ± 3%	The CO2 sensor is dual wavelength NDIR-based (Nondispersive infrared sensor)	Can be self-calibrated to 400 ppm outdoors
Ozone	0 ~ 10,000 ppb	Maximum of ± 10% and 10 ppb	Electrochemical Ozone Detection	Can be self-calibrated
Temperature	10 to 60 0C	Accuracy: 1 0C Resolution: 1 0C	CMOS Sensor	Master traceable to NABL
Relative Humidity	0 to 90% RH	Accuracy: 3% Resolution: 1%	Capacitive sensor	Master traceable to NABL*

NABL National Accreditation Board for Testing and Calibration Laboratories, India

Experiment Plan

A series of experiments were conducted in 5 airtight transparent acrylic boxes for seven consecutive days in the laboratory-controlled environment. During the experiments, laboratory humidity and temperature were kept under control (temperature 25 °C and RH 43%). All testing boxes were assembled with air quality monitors, BAI models (outer shell and fans were removed to avoid possible interference), and incense sticks (Fig. 3). Each individual testing box with BAI models was labeled as model 1, model 2, model 3, and so on. Two runs/day for each model were performed to get daily average efficacy against concerned PM_2.5, PM₁₀, and potential O₃ emission.

Boxes were cleaned and dried thoroughly before the next run. Air-tightness in boxes was confirmed by the tracer gas method.

To generate the desired particulate pollutant in the test boxes, an incense sticks (10 X 26 mm) were lit. Incense sticks are simple and easily available source of particulate pollutants. Incense sticks are known to release a variety of air pollutants, including particulate matter and others, such as CO, CO₂, NO₂, SO₂, and polycyclic aromatic hydrocarbons (Lin et al. 2008).

Incense sticks' decaying time was 10 to 15 min in test chambers during this the levels of particulate matter and CO₂ reached the highest limits given in the air quality monitor (PM_2.5 999 µg/m³, PM₁₀ 1999 µg/m³, CO₂ 1967 ppm). The average natural decay time of particulate pollutants with other factors was calculated when bipolar ionizers were kept off, which took about 8 h to settle down the PMs naturally. Each BAI model switched on when particulate pollutant levels reached the highest level and the incense stick decayed completely. Automatic air quality monitors recorded changes per second inside the boxes simultaneously. However, significant particulate matter reduction was observed at only 10-min intervals. All test were conducted over the a span of 200 min and sampling period was 0, 20, 40, 60,80, 100, 120, 140, 160, 180, and 200 min. The real-time data of the internal environment of testing boxes were transferred and saved to the laboratory computer. Time lapse images of the experimental setup are given in Fig. 4.

Fig. 4.

An overview of the experiment plan and time lapse, before and after bipolar ionizer models switched on

Data Recording and Processing

The received data were processed with Microsoft Excel 2010 and SPSS Statistics. Matplotlib software was used to graphical representation. Arithmetic averages of PMs (PM_2.5, PM₁₀), CO₂, O₃, temperatures, and relative humidity were obtained from two daily runs, which were calculated separately, for each concerned factor by the following simple arithmetic average formula:

$$\mathrm{BAI}m=\frac{1}{n{\sum }_{i=0}^{n}ai}=\frac{a1+a2+a3\cdots \cdots an}{n}$$

where BAIm is the arithmetic average for different BAI models, ‘m’ represents associated factors, 'n' is the number of values, and 'ai' different values of concern factor obtained from runs/day. The overall average performance of examined BAI models was estimated from the cumulative daily arithmetic average data (BAIm).

Results

The outcomes from 14 experimental run in 7 days (two runs for each BAI model conducted per day) including particulate pollutant removal efficiencies and potential byproduct ozone generation observed as follows:

Particulate Matter Removal Efficacies and Potential Byproduct Emission

In Table 3, tested BAI models, consecutive daily performance against particulate matter was observed from 70% to 81.7%. Individually, the highest particulate matter removal efficacy was shown by BAI model 4 (PM₁₀ 79.7%, PM_2.5 80.4%) and minimum by BAI 5 (PM₁₀ 72.2%, PM_2.5 72.3%). The second highest efficacy was shown by BAI Model 2 (PM₁₀ 76.4%, PM_2.5 77.04%) followed by BAI Model 1 (PM₁₀ 75.05%, PM_2.5 75.6%) and BAI Model 3 (PM₁₀ 74.9%, PM_2.5 75.5%). A comparative particulate removing efficiencies of each tested BAI model is given in Fig. 5.

Table 3.

Daily average percentage of particulate matter reduction and potential byproduct O₃ emission linked with different bipolar air ionizer models

BAI Models	Day 1%			Day 2%			Day 3%			Day 4%			Day 5%			Day 6%			Day 7%
	PM10	PM2.5	O3	PM10	PM2.5	O3	PM10	PM2.5	O3	PM10	PM2.5	O3	PM10	PM2.5	O3	PM10	PM2.5	O3	PM10	PM2.5	O3
BAI 1	75	75.5	0	74.9	75.3	0	76	76.8	0	75	75	0	74	75	0	75.5	76	0	75	76	0
BAI 2	77	77	0	76	77	0	77.2	78	0	76.4	77	0	75.9	76.3	0	76.5	77	0	76	77	0
BAI 3	75	75	0	75.2	76.4	0	75	75.5	0	74	75	0	74.5	75	0	76	76.4	0	75	75.2	0
BAI 4	78	79.5	0.3	78.5	79	0.2	80	81	0.5	80	81	1	79.8	80	0.7	80.6	81	0.7	81	81.7	1
BAI 5	78	74.6	0	72	72.6	0	72	70.5	0	71	72	0	71	72	0	70	71	0	72	73	0

Bold  Reduction%, Italic  Insignificant%, bold italic   Elevation%

Fig. 5.

Particulate Matter (PM_2.5, PM₁₀) removal comparative efficacies of BAI models

During the daily observations, ozone emission was not traced with BAI 1, BAI 2, and BAI 3 models conduction Fig. 6. However, model BAI 4 showed a slight fluctuated elevation in ozone emissions from 0.2 to 1% (average 0.62%) (Fig. 7, 8).

Fig. 6.

Average effect of Bipolar Air Ionizer (BAI) models on PM_2.5, PM₁₀ (μg/m³), and byproduct Ozone (ppb)

Fig. 7.

Comparative fluctuation rates of background temperature, relative humidity, CO₂, and potential byproduct ozone during BAI models conduction

Fig. 8.

Maximum Particulate matter removal efficacies and corollary background temperature and Relative humidity of tested BAI models

Initial background concentrations of particulate matter and atmospheric factors in test boxes were kept identical and increased with incense sticks as PM_2.5 999 µg/m³, PM₁₀ 1999 µg/m³, CO₂ 1967 ppm, O₃ 49 ppb, temperature 25 °C, and relative humidity 48% as compared to the outside natural concentration of same factors (PM_2.5 68 µg/m³, PM₁₀ 70 µg/m³, CO₂ 550 ppm, O₃ 48 ppb, air change rate was ~ 1.2–1.6 per hour (1/h), temperature 25 ± 2 °C, and relative humidity 43%).

The BAI models real-time collective data showed PM_2.5 and PM₁₀ concentrations began to decrease within 10 min when BAI models switched on and gradually decreased up to 150 min. The maximum particulate matter removal, per-day typical values of BAI models were reported as BAI 1 PM_2.5 6 µg/m³, PM₁₀ 8 µg/m³, BAI 2 PM_2.5 8 µg/m³, PM₁₀ 10 µg/m³, BAI 3 PM_2.5 9 µg/m³, PM₁₀ 11 µg/m³, BAI 4 PM_2.5 4 µg/m³, PM₁₀ 4 µg/m³, BAI 5 PM_2.5 µg/m³ 10, and PM₁₀ 15 µg/m³.

Further, consecutive values of all BAI models almost follow the same pattern of PM_2.5 and PM₁₀ reduction. Overall average trends of PM_2.5, PM₁₀, and potential byproduct ozone association of tested BAI models are plotted in figure 6.

Fluctuations in Background CO₂, Temperature, and Relative Humidity

Daily average values of temperature, relative humidity, and CO₂ fluctuations during BAI conduction (Table 2) showed the trend pattern in background factors was almost similar and marginal for each BAI model.

Slight elevation in temperature and relative humidity (RH) was observed as BAI model 1: Temp 1.2% RH 1.1%, BAI model 2: Temp 1.1% RH 1.1%, BAI model 3: Temp 1.1% RH 1.2%, BAI model 4: Temp 1.3% RH 1.6%, BAI model 5: Temp 1.1% RH 1.2%. The temperature and the RH are comparatively higher in BAI model 4, slight elevation in ozone (0.6%) is also observed with model 4 operation. The reduction in CO₂ level was almost negligible (Figs. 7, 8).

Discussion

Results of the present study showed varied but significant particular matter removal efficacies of the tested bipolar air ionizers. The observed efficacies almost corresponded to the given CADR values (110 m³/hr to 153 m³/hr or 64 to 90 Cubic feet/Minute), despite unventilated test boxes. However, it was reported that ventilation influences the CADR values (Noh and Yook 2016). It has been reported that PM_2.5 deposition velocity usually increased with natural ventilation (Liu et al. 2018). Therefore, significant natural or managed ventilation is required for proper indoor air cleaning. WHO also recommends a 288 m³/h per person ventilation rate to control possible opportunistic airborne transmission (Zhao et al. 2020).

The present study added bipolar ionizer’s particulate matter removal behavior in an unventilated atmosphere. This could be additional data over the conventional CADR values and other previously performed experiments in different ventilated laboratory conditions. Whether inappropriate ventilation influences bipolar air ionization was not confirmed by the present study as observed particulate matter removing efficacies almost corresponded to the given CADR values of BAI models though the results may be helpful to understand bipolar air ionizer efficacy in low ventilation conditions.

Moreover, available studies are still growing and limited to determining bipolar air ionization efficacy against particulate pollutants and byproduct ozone.

Recent experiment on in-duct bipolar air, the ionizer in the cabin, and in-duct bipolar ionization technology can significantly augment the regular filter, such as MERV 8 efficiency (Shen et al. 2022). In a similar experiment tramp HVAC, Spain, a substantial impact of bipolar ionization was reported on indoor bioaerosols but low on surface contaminants (Baselga et al. 2023).

Another in-duct study observed that a bipolar air ionizer contributed only a slight loss rate of ultrafine particulate matter (< 0.15) while almost negligible in PM_2.5 loss rate. However, the byproduct ozone was not reported, probably due to ionization energies < 12.07 eV, which is below the ionization energy of molecular oxygen (Zeng et al. 2021).

A most recent study also similarly in another experiment bipolar ionization impact on particle concentrations, size distributions, deposition rates, and bacteriophage MS2 was not appreciable but byproduct ozone concentration was not raised above the background level (Ratliff et al. 2023). All tested BAI models in the present study did not show abnormal ozone emission. A slight elevation of about 0.6% (49 ppb to 49.29 ppb) during the BAI 4 model operation was reported, which is negligible as per WHO’s recommended limit of 51 ppb.

In addition, it could be noticeable compared to the rest model, BAI model 4 showed not only the highest particulate removal efficacy but also higher positive and negative (> 3 million pcs/cc and > 3.5 million pcs/cc viz) air ion generation and electricity supply (250 V) consumption which possibly associated with slight ozone elevation. Marginal elevation in background temperature and relative humidity may have no significance but associated with each BAI model conduction. Background CO₂ was negligible in each unventilated chamber and that was obvious since CO₂ removal is highly ventilation dependent, but still cannot be overlooked (Bartyzel et al. 2020).

The present study provided important comparative data of some commercially available bipolar air ionizer particulate pollutants removing potencies. The study also provides a simple and quick method for bipolar air ionizer efficacy and byproduct ozone safety evaluation.

The results of the present study concrete that bipolar air ionizers could be a safe and ozone-free indoor air cleaning option for highly polluted and less developed countries where other air filtration methods, such as induct HEPA and ULPA, are less frequent due to high-cost maintenance. Since indoor air cleaning has great significance as people spend more than 80% of their time indoors and the role of air purifiers to abate exposure of indoor PM_2.5 is not fully addressed in under developed countries. A study in China emphasized that a 35 or 25 μg/m³ reduction in the indoor PM_2.5 concentration using cost-effective air purifiers may reduce the PM_2.5 associated death risk (Liu et al., 2021).

The role of other background factors may also be considered during air cleaning. However, examined bipolar ionizers' effect on background CO₂, temperature and relative humidity were almost negligible (Fig. 7, 8).

But interrelationships between temperature, relative humidity, and bipolar ions have been complex and less understood (Hawkins 1981) so far.

Yet, the present study has some limitations, like limited bipolar ionizer models, confined small chambers, occupant presence, and controlled atmospheric conditions.

Conclusion

The present study has appended more experimental data for a simple and quick evaluation method of bipolar air ionizer efficacy and byproduct ozone safety determination. All examined BAI models showed promising PM_2.5 and PM₁₀ abatement effect within the unventilated testing boxes under air conditioned laboratory atmosphere. Visible effect on incense smoke was noticeable and expeditious, particulate matter removal range from 71 to 80% was achieved within 200 min experiment span. Abnormal emission of byproduct ozone was not associated with examined BAI models conduction. Negligible fluctuation in background temperature up to 1.6% in temperature (0.4 °C) and in RH up to 1.3% was reported during the experiment. Overall results from this study indicate that bipolar air ionizers could be a byproduct ozone-free indoor particulate pollutants cleaning option for highly polluted less developed countries.

Funding

Self-funded. No external funding was received.

Data Availability

Not applicable.

Declarations

Conflict of Interest

Authors declared no competing interests.