![]() ![]() During full-length article reviewing, some cited references, which were not included in the full-length review list, were also selected for additional reviewing. Based on the abstract reviewing, 93, 113, and 57 references were selected, which were subjected to full-length reviewing. We further carried out abstract screening to exclude more unrelated articles. 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. Only peer-reviewed and English-language articles were considered. 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. Three keywords “negative”, “air”, and “ion” were used to search all collected articles in these databases. The coverage period of the database search is around 100 years from 1918 to 2018 (the updated search date is 10 September 2018). ![]() Three databases were selected including PubMed database (available online: ), ScienceDirect database (available online: ), and IEEE Xplore Digital Library (available online: ). The systematic review of studies on NAIs initiated by literature searches. We also reviewed the air cleaning ability of NAIs, especially in removing PM with diameters less than 10 micrometers (PM 10). 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 then discussed the involvement of superoxide ions in biological effects of NAIs. Here, we review the generation of NAIs and their effects on humans, animals, and microorganisms. 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. ![]() On the other hand, strong evidence had shown the roles of NAIs in high-efficiently reducing particulate matter (PM) concentration. Here, we have carried out a comprehensive reviewing on NAIs. However, some controversial results or comments have been reported for the beneficial effect on humans/animals or on reductions in bacterial densities. In the meantime, various new technologies were developed and used to further improve NAI generation and reduce the release of its byproduct ozone. Now, NAI generators are widely available for home or industrial uses. Negative air ions (NAIs) have been discovered for more than 100 years. 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. This is a new NAI generation system which releases a huge amount of NAIs under the PEF treatment. Finally, we have reviewed the plant-based NAI release system under the pulsed electric field (PEF) stimulation. Experimental data showed that NAIs could be used to high-efficiently remove PM. Particulate matter (PM) is a major air pollutant that affects human health. Reports also showed that NAIs could help people in relieving symptoms of allergies to dust, mold spores, and other allergens. 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. Here, we have carried out a comprehensive reviewing on the effects of NAIs on humans/animals, and microorganisms, and plant development. Negative air ions (NAIs) have been discovered for more than 100 years and are widely used for air cleaning. ![]()
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