Clean air is essential for overall health (Rai and Tripathi, 2022). Howbeit, rampant air pollution, and associated repercussion have increased for the past few years, almost 99% of the world’s population, particularly in countries with low or middle incomes, is routinely exposed to polluted air (IQ Air, 2022). Studies indicated indoor air pollution could be more harmful due to the high exposure as people spend more than 65% of their time in indoor activities (Yadav et al., 2022b). It is important to understand and manage indoor air quality to prevent airborne health risks (Cooper et al., 2021; Nair et al., 2022). There is a great chance of indoor air is prone to be polluted with several combustible sources, responsible for substantial respiratory morbidity and ailments burden (Rice et al., 2020). Indoor smoke is one of the main indoor air pollution concerns. The indoor smoke source can be secondhand smoke, incense burning, and biomass smoke (Pathak et al., 2020; Yeh et al., 2022). Recent experiments showed incense sticks tend to generate persistent aerosol containing a greater amount of particulate matter. Incense sticks can be a hazardous combustive source of indoor pollution, mainly particulate matter (PM_2.5, PM₁₀), HCHO, CO₂, and volatile organic compounds (VOCs) (Yadav et al., 2020a; Yadav et al., 2020b; Yadav et al., 2021; Yadav et al., 2022a). A single incense stick burning may generate 45 mg/g of average particulate matter, more than a cigarette, which is 10 mg/g (Rauniyar et al., 2020; Thuy et al., 2022).

Such indoor smoke particles including secondhand smoke can accumulate in an indoor environment, mix with indoor dust, and be embedded in building materials, furniture, carpet, and upholstery, these deposited harmful residues are also known as thirdhand smoke (Matt et al., 2019; Matt et al., 2020). Therefore, incense smoke is a great source of indoor particulate matter, especially fine PM_2.5 is one of the most hazardous air pollutants and is associated with major cases of mortality and aggravating COVID-19 cases globally (Yadav et al., 2022b). In recent years, indoor air quality awareness has led to a surge of air cleaning products such as portable high-efficiency particulate air (HEPA) air filters, which are able to reduce residential PM_2.5 concentrations by 52%–67% (Wheeler et al., 2021). But despite effectiveness, HEPA-based multilayer filtration may be prohibitively expensive for some families due to the high maintenance cost (Allen and Barn, 2020). Low and middle-income countries, where novel technologies are limited, more effective and alternative high-performance household air cleaning options, may provide low-cost health hazard protection (Jumlongkul, 2022).

Bipolar air ionizers seem low-maintenance air cleaning options that draw considerable attention and interest during the COVID-19 pandemic. Bipolar ionizer devices can produce both positive and negative ions in a special ratio, negative ions composition usually high. Bipolar air ionization air cleaning potential is reported against a variety of air pollutants such as particulate matter, and airborne pathogens including viruses compare to unipolar (Jiang et al., 2018; Curtis, 2021). The evidence of bipolar air ionizers against particulate matter, i.e., PM_2.5 and PM₁₀ is also growing, a recent comparative analysis of several bipolar air ionizers showed, bipolar air ionizers can reduce particulate matter effectively in controlled laboratory conditions. Bipolar air ionizers are simple electrical devices that produce electrically charged positive (H⁺) and negative (O₂ ⁻) ions into the air, water molecules present in air breakdown into positive and negative ions due to the exposure to high voltage discharge generated by bipolar air ionizers. Those highly reactive ions react with air pollutants and cause agglomeration which lead settlement of several air pollutants, in case of microbial airborne contaminates OH⁻ ions attract H⁺ from microbial cell to form a water molecule, which causes the inactivation of airborne pathogens (Gupta et al., 2023).

Techniques such as atmospheric-pressure non-thermal plasma (NTP), which can be two types: multipin corona discharge and a dielectric barrier discharge generator, used to create an electrical field by air ionization devices and produce ions and free radicals may react with atmospheric water molecules and form reactive oxygen species (ROS), hydroxyl radical (OH−), superoxides (O₂−) and singlet oxygen [(O₂ (1Dg)] and ozone (O₃). The byproduct of ozone emission is mostly observed with negative ions production in unipolar ionizers (Hernández-Díaz et al., 2021). Incense smoke represents a group of indoor air pollutants including particulate matter, but portable bipolar air ionizers’ effect on incense smoke is not fully evaluated and limited so far. In addition, byproduct ozone remained a serious concern associated with ionization air cleaning techniques. The main objective of this research work is to investigate a portable bipolar air ionizer’s Incense smoke removal efficiency along with the potential byproduct (Ozone) in unventilated conditions.

A concatenation of experiments conducted repeatedly up to 7 days, to examine the incense smoke (including particulate matter PM_2.5 and PM₁₀) reduction capacity of a locally manufactured bipolar air ionizer. The experiment setup comprised two glass boxes (54.8 cm × 54.8 cm × 54.8 cm or 0.16 m³ size each), incense sticks (16 cm long and 1.5 mm thick), 2 customized air quality monitors (Airveda, Model PM2510CVTH) 2 bipolar air ionizers (needlepoint, made by PetriMed CA, model AI 100), specification given in Table 1; air quality monitor and bipolar air ionizer were purchased from the manufacturer, specifications given in Table 1; concentration of negative and positive ions of bipolar air ionizer was measured before each test, by an air ion counter, model NKMH-103 [measurement range 10–19,990,000 (pcs/cc)], Figures 1.

Air quality monitor selection criteria certification and validation report of inbuilt sensors; particulate matter sensors working principle is LASER scattering which is validated/calibrated by widely accepted Beta Attenuation Monitor (BAM) method, specification and details of other sensors given in Table 2. The calibration report was provided by the manufacturer and can be accessed online (https://www.airveda.com/).

The air quality monitor is rechargeable and can transfer real-time data by Wireless Fidelity (WiFi) to the laboratory computer. The air quality monitor is customized with the required sensors (PM_2.5, PM₁₀, CO₂, O₃, Temp., RH) which have been employed in our previous study (Gupta et al., 2023). The experiment was conducted at River Engineering Private Limited’s research and development laboratory in Greater Noida, India, near the national capital Delhi (28.4744° N, 77.5040° E).

During experiments, ambient Particulate matter (PM) concentration in the study area was derived from the nearby regional air quality monitoring station (Knowledge Park—V, Greater Noida-UPPCB) operated by the central pollution control board of India (https://app.cpcbccr.com/AQI_India/).

Experiment boxes A (with BAI ON) and B (BAI Off) assembled with bipolar air ionizers, air quality monitor, and incense sticks as represented in Figure 2. Burning incense sticks were placed in each box and then the boxes were sealed. The authors have observed that a single incense stick (16 cm long and 1.5 mm thick) decayed in about 15 min and was able to increase particulate matter and CO₂ concentration up to the highest limit given in the air quality monitor, increased concentration persisted for several hours in unventilated text boxes (PM_2.5 999 μg/m³, PM₁₀ 1999 μg/m³, CO₂ 1967 ppm). The airtightness of the boxes has been tested using a tracer gas technique, and the test indicated that air leakage is negligible. A run of tests with the above setup was conducted each day (up to 190 min, as the variables graph flattened in <120 min) with box A and box B. The same test was repeated for up to 7 days to get the average values of cumulative results. Laboratory temperature and relative humidity (RH) were maintained during the experiment. Temperature and RH were measured by an additional air quality monitor kept in the laboratory. CMOS sensor with an accuracy of 1°C and RH capacitive sensor with an accuracy of 3% were used. The sensors were calibrated as per master traceable to National Accreditation Board for Testing and Calibration Laboratories, India (NABL).

There are certain limitations of our study such as controlled temperature and humidity settings in the laboratory, authors did not access different temperatures, and RH settings, which could be useful to elaborate correlation with bipolar air ionization. The authors have assessed only the main particulate matter proportion of incense smoke, over the other many air pollutants. The study is confined to a small unventilated experimental box, which could be limited to predicting bipolar ionizer smoke removal efficacy in a full-size room; the test bipolar ionizer claimed Clean Air Delivery Rate (CADR) range was 154 m³/h which may show the results according to small boxes size 0.16 m³. Despite, no significant byproduct of ozone emission in our study, the current data is insufficient to predict biopolar air ionizers associated safety concerns. More trials and studies in larger cabins or buildings and other indoor places with occupants may be helpful to understand bipolar reliability and efficacy against other indoor air pollutants such as bioaerosol, allergens, and TVOC.

The present study contributes to understanding the bipolar air ionizer removal efficacy against the particulate proportion of incense smoke. Bipolar air ionizer-associated average reduction of PM_2.5 and PM₁₀ concentration was observed by 74%–75%. While compared to control, individual PM₁₀ removal efficacy (BAI eff.pm10) was observed 84% and PM_2.5 removal efficacy (BAI eff. pm 2.5) was 97%. The overall efficacy of tested BAI (BAI eff.) against incense smoke was 90% ± 10% in 0.16 m³ boxes. Byproduct ozone abnormal elevation was devoid. Tested bipolar air ionizers showed substantial incense smoke removal efficacy under limited experimental settings, which could be helpful to understand bipolar ionizers’ role against indoor smoke and particulate matter. However, other potential byproducts and background temperatures relative humidity correlation, and occupants’ presence relation with bipolar air ionizers requires larger cabin based investigations and trials.