Negative air ions (NAIs) have been discovered for more than 100 years and are widely used for air cleaning. Here, we have carried out a comprehensive reviewing on the effects of NAIs on humans/animals, and microorganisms, and plant development. The presence of NAIs is credited for increasing psychological health, productivity, and overall well-being but without consistent or reliable evidence in therapeutic effects and with controversy in anti-microorganisms. Reports also showed that NAIs could help people in relieving symptoms of allergies to dust, mold spores, and other allergens. Particulate matter (PM) is a major air pollutant that affects human health. Experimental data showed that NAIs could be used to high-efficiently remove PM. Finally, we have reviewed the plant-based NAI release system under the pulsed electric field (PEF) stimulation. This is a new NAI generation system which releases a huge amount of NAIs under the PEF treatment. The system may be used to freshen indoor air and reduce PM concentration in addition to enriching oxygen content and indoor decoration at home, school, hospital, airport, and other indoor areas.
Negative air ions (NAIs) have been discovered for more than 100 years . Now, NAI generators are widely available for home or industrial uses. In the meantime, various new technologies were developed and used to further improve NAI generation and reduce the release of its byproduct ozone. However, some controversial results or comments have been reported for the beneficial effect on humans/animals or on reductions in bacterial densities. Here, we have carried out a comprehensive reviewing on NAIs. On the other hand, strong evidence had shown the roles of NAIs in high-efficiently reducing particulate matter (PM) concentration. Thus, more work should be done to further improve NAI release by new methods or devices so that NAIs could be more widely used for air cleaning. Here, we review the generation of NAIs and their effects on humans, animals, and microorganisms. We then discussed the involvement of superoxide ions in biological effects of NAIs. Subsequently, we focused on plant-based NAI generation systems as these are relatively new NAI generation systems with some advantages on traditional corona discharge NAI generators. We also reviewed the air cleaning ability of NAIs, especially in removing PM with diameters less than 10 micrometers (PM10).
2. Systematic Review of Literatures
The systematic review of studies on NAIs initiated by literature searches. Three databases were selected including PubMed database (available online: https://www.ncbi.nlm.nih.gov/pubmed/advanced), ScienceDirect database (available online: https://www.sciencedirect.com/search/advanced), and IEEE Xplore Digital Library (available online: https://ieeexplore.ieee.org/search/advsearch.jsp). The coverage period of the database search is around 100 years from 1918 to 2018 (the updated search date is 10 September 2018). Three keywords “negative”, “air”, and “ion” were used to search all collected articles in these databases. The database searches harvested a total of 335, 681, and 221 articles that contain all of the three keywords in either titles or abstracts when the PubMed database, ScienceDirect, and IEEE Xplore databases were employed, respectively. Only peer-reviewed and English-language articles were considered. We then screened these articles by manually reviewing titles, which led to 170, 279, and 117 studies selected (duplicated references were excluded); the remaining were excluded due to no relationship to the NAI topic. We further carried out abstract screening to exclude more unrelated articles. Based on the abstract reviewing, 93, 113, and 57 references were selected, which were subjected to full-length reviewing. During full-length article reviewing, some cited references, which were not included in the full-length review list, were also selected for additional reviewing.
3. Negative Air Ions and Their Generation
Air ions are electrically charged molecules or atoms in the atmosphere . An air ion is formed when a gaseous molecule or atom receives sufficiently high energy to eject an electron . NAIs are those that gain an electron, while positive air ions lose an electron. The natural and artificial energy sources include (1) radiant or cosmic rays in the atmosphere; (2) sunlight including ultraviolet; (3) natural and artificial corona discharge including thunder and lightning; (4) the shearing forces of water (Lenard effect); (5) plant-based sources of energy.
3.1. Radiant or Cosmic Rays in the Atmosphere
The radioactive elements such as uranium, radium, actinium, and thorium widely exist in our planet. They decay in the atmosphere and emit α, β, and/or γ rays, which ionize the air. Thus, radiant and cosmic ray ionization is ubiquitous in the Earth’s atmosphere. The cosmic ray ionization accounts for around 20% of the total ionization over land surfaces . They are also the principal energy sources that generate NAIs over the oceans . The concentration of NAIs produced by these rays might reach from 500 ions per cm3 in land surface  to more than 1000 ions per cm3 at 15 km away from the land surface .
3.2. Sunlight Including Ultraviolet
The photoelectric effect is the emission of electrons when a certain wavelength of light is shone onto a metallic surface. NAIs are generated by accepting these emitted electrons. The photoelectric effect may contribute less to the NAI generation as only some wavelength of lights shows the ability to emit electrons by lighting. One of the examples is the negative ion generator using an ultraviolet source to irradiate electrically conductive material, which was patented as early as 1964 (Patent No. US 3128378 A). In this patent, an ultraviolet lamp was used to irradiate metal materials, which photo-electrically eject electrons. The electrons then collide with air molecules and generate NAIs.
On the other hand, NAIs can be generated by a certain wavelength of lights through directly ionizing air molecules. For example, ultraviolet (UV) can be used to directly ionize air molecules to generate NAIs [5,8]. Actually, UV-mediated ionization is the dominant NAI sources in the above 60 km altitude of atmosphere . These highly concentrated NAIs from UV in the upper layers of the atmosphere are diffused to the ground surface at low speeds. Ionization by UV radiation is not a major contributor of NAIs in the lower atmosphere due to the low dose of UV rays available in this layer . Although reports showed that the UV rays significantly mediated the air ionization, little systematical study was carried out on the effect of artificial UV light on NAI generation. We carried out an experiment to investigate the contribution of UV light to the generation of NAIs (Figure 1; Table S1). The experiment was carried out in a growth chamber with 80 cm length × 80 cm width × 80 cm height and detailed description was provided in the Supplementary Experiment. NAIs were measured under UV light conditions with the normal light condition as a control. The data from three replicates of experiments showed that UV lights indeed promoted the NAI generation. In the chamber, the average NAI concentration was 344 ions/cm3 within one hour. The NAI concentration was increased to 825 ions/cm3 under UV light condition, significantly higher than the control (Figure 1A). Further observation showed that there were peaks of NAI generation within 8 min after UV lighting (Figure 1B–D; Table S1). After the peaks, NAI concentration was kept at a relatively stable value but was still higher than the control. For all three replicates or each replicate, a nonparametric two-tailed Mann–Whitney U test was carried out as described in the Supplementary Experiment and the statistical analysis showed that NAI concentrations under UV lighting conditions in all three replicates were significantly higher than those under normal lighting conditions (CK) with p < 0.00001. The analysis further confirmed the promoting effect of UV lighting on NAI generation. Thus, our experiment showed that UV lighting could be used to generate NAIs. However, only low concentrations of NAIs were generated under our UV light conditions.
3.3. Natural and Artificial Corona Discharge Including Thunderstorms and Lightning
The atmosphere surrounding the earth is subjected to a natural electric field and its intensity is continuously fluctuating under both local and global influences . The local influences include geographical location and weather conditions such as thunderstorms, rain, fog, mist, and so on; the global facts refer to classical daily electric field variations . When leaf points or branches of trees have a high potential difference from their surroundings in their electric fields, corona discharge (also called point discharge) occurs and NAIs may be released [5,10]. Generally, corona discharge occurs at the atmosphere conditions under high average electric fields . For example, in a mountain area, high electric fields and low atmospheric pressure promote the onset of corona discharge . Thunderstorms and lightning will generate very high electric field conditions and corona discharge subsequently occurs. Therefore, NAIs will be released at a huge amount after thunderstorms and lightning. However, released NAIs will be gradually decayed with the discontinuous thunderstorms. In addition to thunderstorms and lightning, mist may also contribute to NAI generation. In a forest, electric field variations were observed during the mist formation and dissipation, which may trigger corona discharge and NAI generation .
Artificial corona discharge is an efficient way to generate NAIs. When a high negative voltage is applied to a conductor/electrode and generated electric field is high enough, corona discharge occurred [12,13]. If a charged conductor/electrode has a needle-type with a sharp point, the electric field around the tip will be significantly higher than other parts and air near the electrode can become ionized and NAIs are generated . Intensity of corona discharge depends on the shape and size of the conductors as well as applied voltage. Irregular conductor, especially with a sharp point, gives rise to more corona than a smooth conductor and large-diameter conductors produce lower corona than small-diameter conductors; the higher the voltage applied, more NAIs are generated [14,15]. The closer the distance to corona point, the higher NAI concentration is detected as continuous generation of NAIs by corona discharge is related to a chain reaction process called an electron avalanche . The application of artificial electric field and corona discharge on plants was carried out as early as the 1960s [17,18]. Bachman and Hademenos (1971) showed that under high voltage, artificially applied electrical fields near the pointed barley leaf tips were intensified  and as a result, corona discharge occurred and air ions and ozone were generated. Studies mainly focused on biological effects such as growth response, evaporation, and plant damage as well as the effects of generated ozone and NAIs on plant growth [11,18,19,20,21,22].
3.4. The Shearing Forces of Water (Lenard Effect)
The considerable numbers of NAIs are found under waterfalls or in the seashores. These NAIs are generated by Lenard effect. Lenard effect was also called spray electrification or waterfall effect and was first systematically studied by Philipp Lenard , who won the Nobel Prize for Physics in 1905 for his research on cathode rays and the discovery of many of their properties. The study showed that NAIs were generated from the surrounding air molecules by charging themselves negatively when water droplets collide with each other or with a wetted solid to form fine spray of drops. The study also showed that several factors may affect the degree of charge separation in spray processes and, therefore, may affect the generation and concentration of NAIs. These factors include water drop temperature, dissolved impurities, speed of the impinging air blast, and foreign impinging surfaces of droplets. Based on the “Lenard effect”, water shearing appliance has been designed to generate NAIs . Water shearing produced only superoxide ions (O2−) which was bound to clusters of water molecules to form the structure O2−(H2O)n , and was essentially regarded as a natural source of NAIs . NAIs generated by the “Lenard effect” might improve erythrocyte deformability, thereby aerobic metabolism .
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