# Negative air ions

Negative air ions (NAI) are important air component, generally referring to the collections of negatively charged single gas molecules or ion clusters in the air. They play an essential role in maintaining the charge balance of the atmosphere. The main components of air are molecular nitrogen and oxygen. Due to the strong electronegativity of oxygen and oxygen-containing molecules, they can easily capture free electrons to form negatively charged air ions, most of which are superoxide radicals ·O$_2$$^−$, so NAI is mainly composed of negative oxygen ions, also called air negative oxygen ions.

## Research history

In 1889, German scientists Elster and Geitel first discovered the existence of negative oxygen ions.
At the end of the 19th century, German physicist Philipp Eduard Anton Lenard first explained the effects of negative oxygen ions on the human body in academic research. In 1902, scholars such as Ashkinas and Caspari further confirmed the biological significance of negative oxygen ions. In 1932, the world's first medical negative oxygen ions generator was invented in the United States.
In the middle of the 20th century, Professor Albert P. Krueger of the University of California, Berkeley, conducted pioneering research and experiments on the biological effects of ions at the microscopic level. Professor Krueger demonstrated the impact of negative oxygen ions on humans, animals, and plants from the aspects of biological endocrine, internal circulation, and the generation reactions of various enzymes through a large number of animal and plants experiments.
From the end of the 20th century to the beginning of the 21st century, many experts, scholars, and professional medical institutions applied negative ions (negative oxygen ions) technology to clinical practice. Through various explorations, new ways of treating diseases were opened up.
In 2011, the official website of the China Air Negative Ion (Negative Oxygen Ion) and Ozone Research Society was launched. This website is the first negative ions industry website in China, and its purpose is to rapidly promote the orderly development of the air negative ion (negative oxygen ion) industry. In 2020, the Tsinghua University successfully developed a medical-grade high-concentration negative oxygen ion generator. It only needs to be sprayed on the room's walls to form a uniform and dense layer of nanoparticles on the wall, allowing the indoor wall to stably and long-term release high-concentration small-particle negative oxygen ions.

## Generation mechanism

Common gases that produce negative air ions include single-component gases such as nitrogen, oxygen, carbon dioxide, water vapor, or multi-component gases obtained by mixing these single-component gases. Various negative air ions are formed by combining active neutral molecules and electrons in the gas through a series of ion-molecule reactions.
In the air, due to the presence of many water molecules, the negative air ions formed are easy to combine with water to form hydrated negative air ions, which are typical negative air ions, such as O$^−$·(H$_2$O)n, O$_2$$^−$·(H$_2$O)n, O$_3$$^−$·(H$_2$O)n, OH$^−$·(H$_2$O)n, CO$_3$$^−$·(H$_2$O)n, HCO$_3$$^−$·(H$_2$O)n, CO$_4$$^−$·(H$_2$O)n, NO$_2$$^−$·(H$_2$O)n, NO$_3$$^−$·(H$_2$O)n, etc. The ion clusters formed by the combination of small ions and water molecules have a longer survival period due to their large volume and the fact that the charge is protected by water molecules and is not easy to transfer. This is because in the molecular collision, the larger the molecular volume, the less energy is lost when encountering collisions with other molecules, thereby extending the survival time of negative air ions.

## Generation methods

Negative air ions can be produced by two methods: natural or artificial.The methods of producing negative air ions in nature include the waterfall effect, lightning ionization, plants tip discharge, etc. Natural methods can produce a large number of fresh negative air ions. The artificial means of producing negative air ions include corona discharge, water vapour，and other methods. Compared with the negative air ions produced in nature, although artificial methods can produce high levels of negative air ions, there are specific differences in the types and concentrations of negative air ions, which makes the negative air ions produced by artificial methods may not achieve the excellent environmental health effects of negative air ions produced in nature. Improving the artificial method to produce ecological-level negative ions is necessary.

### Natural environments

* Waterfall method : When people are in a water-rich environment such as a waterfall, fountain, or seaside, they usually feel relaxed and release stress, which is related to many negative air ions in the environment, as one of the most common methods for producing negative air ions in nature.
    The mechanism of producing negative air ions by the waterfall method was first discovered by German scientist Lenard in 1915. The Lenard effect is achieved through two methods: the rupture of the "ring-bag" structure and the local protrusion separation. The "ring-bag" structure rupture theory believes that during the collision between water and gas, the water droplets will form a "U"-shaped intermediate with a "ring-bag" structure when subjected to external impact. The intermediate will then break apart to form small droplets with negative charges and large droplets with positive charges. The local protrusion separation theory believes that when water droplets collide with each other or are subjected to external forces, the water droplets will automatically protrude locally and generate negative charge aggregation. When subjected to shear force, this part will form negative ions with crystal water and be released into the air.
* Lightning strike method : The atmosphere itself is a huge electric field. Positive and negative charges will accumulate above and below the clouds. When the droplets in the clouds continue to accumulate and gradually approach the ground due to gravity, a giant capacitor will be formed between the clouds and the ground. When the electric field strength between the two exceeds the dielectric strength of the air, discharge will occur and break through the air. During the lightning discharge process, charged particles bombard the surrounding air molecules, ionizing the molecules to generate negative air ions. At the moment of the lightning strike, hundreds of millions of negative air ions will be generated. This is why people feel the air is fresh and clean after rain. It is not only because the rain increases the humidity of the air, but more importantly, the concentration of negative ions in the air has increased significantly.
* Plants tip discharge : The tip of the leaves of vegetation will discharge under the action of the photoelectric effect. Like the corona discharge, the needle tip will continue to ionize and release negative air ions. In addition, the reason why negative air ions can maintain a high concentration for a long time in forests and some areas covered by green vegetation is that the oxygen concentration released by vegetation during photosynthesis is much higher than that in cities, and a large amount of water vapor is released through respiration and leaf transpiration. Oxygen and water vapor can produce free electrons under ionization. Due to their strong electronegativity, water molecules and oxygen molecules can easily capture free electrons to form negative air ions.

### Artificial ionization

* Corona discharge method : Currently, the most common artificial method is to use corona discharge to produce negative ions. The specific process of using corona discharge to produce gaseous negative ions is to connect the high-voltage negative electrode to a thin needle-shaped wire or a conductor with a very small radius of curvature, so that a strong electric field is generated near the electrode, releasing high-speed electrons. The speed is enough to drive the electrons to collide with gas molecules, further ionize, and produce new free electrons and positive ions. The newly generated free electrons will repeat the previous process, continue to collide and ionize, and this process will be repeated many times, so that the tip electrode will continuously release negative air ions.
* The water vapour method : The water vapour method refers to the use of artificial technology and modern instruments to simulate the principle of waterfall generation, using high-speed airflow to collide with water droplets, dispersing larger water droplets into a large number of microdroplets. As the water droplet dispersion process, the Leonard effect occurs, generating negative ions.

## Determination method

Detection of negative air ions is divided into measurement and identification. NAI measurement can be achieved by measuring the change in atmospheric conductivity when NAI passes through a conductive tube. NAI identification is generally achieved using mass spectrometry, which can effectively identify a variety of negative ions, including O$^−$,O$_2$$^−$,O$_3$$^−$,CO$_3$$^−$,HCO$_3$$^−$,NO$_3$$^−$, etc.

## Application of negative air ions
### Health Promotion

The effects of NAI on human/animal health are mainly concentrated on the cardiovascular and respiratory systems and mental health. The impacts of NAI on the cardiovascular system include improving red blood cell deformability and aerobic metabolism and lowering blood pressure. In terms of mental health, a experiments have shown that after exposure to NAI, performance on all the experimenters test tasks (mirror drawing, rotation tracking, visual reaction time and hearing) was significantly improved, and symptoms of seasonal affective disorder (SAD) were alleviated. The effects of NAI in relieving mood disorder symptoms are similar to those of antidepressant non-drug treatment trials, and NAI have also shown effective treatment for chronic depression.

### Environmental Improvement

Negative air ions can be effectively used to remove dust and settle harmful pollutants such as PM. In particular, they can significantly degrade indoor pollutants, improve people's indoor living environment, and purify air quality. Some experts and scholars have used a corona-negative ions generator to conduct experiments on particles sedimentation through three steps: charging, migration, and sedimentation. They found that charged PM will settle faster or sink faster under the action of gravity so that PM will settle/precipitate faster than uncharged PM. In addition, experimental studies have shown that negative air ions have a specific degradation effect on chloroform, toluene, and 1,5-Hexadiene and produce carbon dioxide and water as final products through the reaction.