Indoor plants produce negative air ions: a comparison across species, temperature and humidity

Negative air ions (NAI) are negatively charged gas ions, commonly released within the natural environment via plant metabolic activity. Exposure to NAI-enriched environments has been shown to provide substantial health benefits, and the ability to charge particulate matter (PM) and precipitate it from the airstream, leading it to be proposed as potential contributors to indoor botanical air purification systems. Current literature indicates low levels of NAI production by indoor plants, while there is a general consensus that reasonable levels of NAI can be produced by plants under correct physiological and microclimate conditions, the mechanisms of plant NAI generation remain insufficiently studied. Here we assess the ability of 45 plant species and their physiological structures to produce NAI's within a simulated indoor environment, to determine whether specific characteristics are correlated with higher NAI release. Spathiphyllum wallisii was revealed as the most effective NAI producer over 15-min. Further environmental factor assessments, revealed humidity and temperature to have the strongest correlations with NAI production, followed by increased plant density. Following high-temperature exposure, an adaptive "rapid NAI surge" was observed, resulting in NAI production at significantly greater levels than before high-temperature exposure. These results provide significant novel insights into the factors affecting plant NAI production, although future research is needed in combination with current indoor green infrastructure systems to provide a more comprehensive understanding of the optimal capabilities of indoor plant NAI generation.