Wikipedia

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[1][2]. The main components of air are nitrogen and oxides. Due to the strong electronegativity of oxygen and other molecules, they can easily capture free electrons to form negatively charged air ions, most of which are superoxide radicals ·O2−,so NAI is mainly composed of negative oxygen ions, also called air negative oxygen ions[3].

Negative air ion


?? are mainly composed of oxygen-containing negative ions in the air combined with several water molecules to form atomic groups. According to the theory adopted by the Joint Atmospheric Commission of the International Union of Geophysics and Geodesy, NAI is O2−(H2O)n, or OH−(H2O)n, CO4(H2O)2, which is a general term for negatively charged single gas molecules and their light ion groups. Since oxygen molecules are more electrophilic than CO2, N2 and other molecules, oxygen molecules will preferentially obtain electrons to form negative ions, so NAI is mainly composed of negative oxygen ions, so it is often called air negative oxygen ions.

Research history

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In 1889, German scientists Elster and Geitel first discovered the existence of negative oxygen ions[4].

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, Hamsen of the American RCA company invented the world's first medical negative oxygen ions generator in the United States.[5]

In the middle of the 20th century, Professor Albert P. Krueger of the University of California, Berkeley,condu plantcted 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[6][7][8].

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. They opened up a new way to treat diseases by exploring various diseases[9][10][11][12][13].

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 Tianjin Advanced Manufacturing Research and Development Center of 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[14].

Elster and Geitel first discovered the existence of NAI.[15] At the end of the 19th century, German physicist Dr. Philip Leonard first proved the efficacy of negative oxygen ions on the human body in academic research. Scholars such as Aschkinass and Caspari further confirmed the biological significance of NAI in 1902. In 1932, the world's first medical NAI generator was born in the United States. In July 2020, the Advanced Manufacturing R&D Center of the Tianjin Institute of Advanced Equipment, Tsinghua University, successfully developed a medical and healthcare-grade high-concentration negative oxygen ion generator. It only needs to be sprayed on the walls of the room to form a uniform and dense nanoparticle layer on the wall, so that the indoor walls can stably and long-term release high-concentration small-particle negative oxygen ions.[16]

Generation mechanism

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Common gases that produce negative air ions include single-component gases such as nitrogen, oxygen, carbon dioxide, water vapor, rare gases, 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[17].

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-·(H2O)n, O2-·(H2O)n, O3-·(H2O)n, OH-·(H2O)n, CO3-·(H2O)n, HCO3-·(H2O)n, CO4-·(H2O)n, HCO4-·(H2O)n, NO2-·(H2O)n, NO3-·(H2O)n, HNO3-, NO3-·(HNO3),etc.[18][19][20][21] 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.[22][23]



When trace gases such as SO2 and NH3 are mixed into the gas, the addition of these highly reactive gases will trigger a series of more complex molecular ion reactions, thereby producing a richer variety of gaseous negative ions.

NAI is O2−(H2O)n, or OH−(H2O)n, CO4(H2O)2

O-·(H2O)n, O2-·(H2O)n, O3-·(H2O)n, OH-·(H2O)n, CO3-·(H2O)n, HCO3-·(H2O)n, CO4-·(H2O)n, HCO4-·(H2O)n, NO2-·(H2O)n, NO3-·(H2O)n, HNO3-, NO3-·(HNO3)

·(H2O)n, O ·


addition to nitrogen, oxygen, carbon dioxide, water vapor and various aerosol particles, there is also some ionized air in the atmosphere. Ionized air includes negative and positive ions. Air molecules are composed of atoms, which are made up of nuclei and electrons. The nuclei carry positive charges and the electrons carry negative charges. When air molecules are ionized and receive enough energy, the outer electrons that are freed from the nuclei become free electrons, and the neutral molecules or nuclei that lose electrons become positive air ions. When neutral molecules or atoms in the air capture the free electrons that escape, they become negative air ions.[24]

NAIs include single gas molecules and light ion clusters with negative charges. NAIs and positive air ions exist simultaneously in the atmosphere. According to the size of the atmospheric ions, they can be divided into large ions, medium ions and small ions. Small ions are ions of molecular size; large ions are ions that are adsorbed by aerosol particles and carry positive or negative charges; and when several neutral molecules gather around air ions, they become medium ions, which are between small ions and large ions. Most of the NAIs discussed in current research refer to small negatively charged air ions.[25]

Generation methods

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Negative air ions can be produced by two methods: natural or artificial.The methods of producing gaseous negative ions in nature include the waterfall effect, lightning ionization, plant tip discharge, etc. Natural methods can produce a large number of fresh gaseous negative ions. The artificial means of producing gaseous negative ions include corona discharge, ultraviolet irradiation, and other methods.[26] Compared with the gaseous negative ions produced in nature, although artificial methods can produce high levels of gaseous negative ions, there are specific differences in the types and concentrations of gaseous negative ions, which makes the gaseous negative ions produced by artificial methods may not achieve the excellent environmental health effects of gaseous negative ions produced in nature.[27] Improving the artificial method to produce ecological-level negative ions is necessary.

Natural environments

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  • Waterfall method : When people are in a water-rich environment such as a waterfall, fountain, or seaside, they usually feel relaxed and relieved of stress, which is related to many negative air ions in the environment. As one of the most common methods for preparing negative air ions in nature, the scope of the waterfall method is not limited to the surrounding areas of waterfalls. Rainfall, lake ripples, city fountains, etc., can also cause charge redistribution.[28][29]
The mechanism of producing negative air ions by the waterfall method was first discovered by German scientist Lenard in 1915.When a waterfall hits a rock or water surface, the water flow will continue to split into ultra-fine droplets due to the impact. During the droplet-splitting process, the charge on the droplet surface will be redistributed.[30] The surface of the larger droplet will carry a positive charge, while the surface of the smaller droplet will carry a partial negative charge, thus forming hydrated negative air ions. This is called the Lenard Effect. Under the Lenard Effect, the splitting of droplets can ensure the continuous generation of hydrated negative air ions, thus forming an oxygen-rich environment in a local space such as a waterfall.[31][32]
The Lenard effect is achieved through two methods: the rupture of the "ring-bag" structure and the local protrusion separation.[33][34] 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.[31][35][36]
  • 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.[37] 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.[38] 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. The lightning strike method is one of the most important methods of providing negative air ions in nature.[39][40]
  • Plant tip discharge : People living in cities are usually negatively affected by urban air pollution, urban heat island effect, etc., so forest wetlands and other green spaces have become one of the best places for leisure and entertainment during holidays. The reason is not only that the air in forests, grasslands, wetlands and other environments is fresh, but also that the concentration of negative ions is significantly higher than that in urban environments, which can effectively relieve fatigue and regulate metabolism.
The tip of the leaves of vegetation will discharge under the action of the photoelectric effect, which can ionize nearby air molecules under the action of the atmospheric electric field. 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 [37].

Artificial ionization

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  • Corona discharge method : Currently, the most common artificial method uses corona discharge to generate gaseous negative ions. Corona discharge is a local, self-sustaining discharge phenomenon generated by gas in an uneven electric field. The tip of the discharge electrode has a very small radius of curvature, which makes the local electric field strength very high. When the field strength is greater than the field strength required for gas ionization, the gas will be excited and ionized, resulting in a corona discharge phenomenon [22-23].
The specific process of the corona discharge method to produce gaseous negative ions is to connect the high-voltage negative electrode to a skinny needle-shaped wire or a conductor with a minimal radius of curvature so that a strong electric field will be generated near the electrode to release high-speed electrons. Its speed is enough to drive the electrons to collide with gas molecules, further ionizing to generate new free electrons and positive ions. The newly generated free electrons will repeat the previous process and continue to collide and ionize to generate new negative air ions. This process will be repeated many times. In the area near the electrode, positive ions and negative ions with the same charge will be evenly distributed. Due to the interaction between charges, ions with positive charges will migrate to the negative electrode so that the negative ions in the local space will be relatively excessive at a certain moment. And the mutual repulsion of like charges, negative air ions will diffuse to the surroundings so that negative air ions are continuously released from the tip electrode.


Artificial method


Its reaction mechanism is:

 
 
 
 
 
 



  • Areas with lower air pressure and humidity : The concentration of ions is higher in areas with lower air pressure and humidity.[41]
  • Forests, waterfalls, and beaches : Forests, waterfalls, and beaches are places with very high concentrations of negative oxygen ions. Trees release negative ions, and a large number of negative ions are also generated when water droplets break.[42]
  • Thunderstorms: Oxygen and nitrogen undergo ionization when they are subjected to high-speed motion or an electric field, resulting in the production of both negative and positive ions. For example, thunderstorms and lightning during electrical storms release a large number of ions into the atmosphere.[43]

Artificial ionization

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There are several ways to produce artificial ions in the air, including corona discharge, thermal electron emission from hot metal electrodes or photoelectrodes, radiation from radioactive isotopes, ultraviolet light, etc.[44]



Natural environments

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  • Solar radiation : Solar radiation can generate a significant amount of ions in the air, especially in high-altitude areas and coastal regions.[45]
  • Areas with lower air pressure and humidity : The concentration of ions is higher in areas with lower air pressure and humidity.[46]
  • Forests, waterfalls, and beaches : Forests, waterfalls, and beaches are places with very high concentrations of negative oxygen ions. Trees release negative ions, and a large number of negative ions are also generated when water droplets break.[47]
  • Thunderstorms: Oxygen and nitrogen undergo ionization when they are subjected to high-speed motion or an electric field, resulting in the production of both negative and positive ions. For example, thunderstorms and lightning during electrical storms release a large number of ions into the atmosphere.[48]

Artificial ionization

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There are several ways to produce artificial ions in the air, including corona discharge, thermal electron emission from hot metal electrodes or photoelectrodes, radiation from radioactive isotopes, ultraviolet light, etc.[49]

Application of negative air ions

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Health Promotion

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The effects of NAIs on human/animal health are mainly concentrated on the cardiovascular and respiratory systems and mental health. The impacts of NAIs on the cardiovascular system include improving red blood cell deformability and aerobic metabolism [24] and lowering blood pressure. In terms of mental health, experiments have shown that after exposure to NAIs, performance on all experimenter test tasks (mirror drawing, rotation tracking, visual reaction time and hearing) was significantly improved [86], and symptoms of seasonal affective disorder (SAD) were alleviated [87]. The effects of NAIs in relieving mood disorder symptoms are similar to those of antidepressant non-drug treatment trials [38], and NAIs have also shown effective treatment for chronic depression [88].

Environmental Improvement

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Negative air ions can be effectively used to remove dust and settle harmful pollutants such as PM particles. 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 ion generator to conduct experiments on particle 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 [138, 139, 140, 141, 142]. Other experimental studies have found that the air in the sea is much purer than the air in the city, with about 100 times fewer pathogens. In addition, experimental studies have shown that gaseous negative 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 [38].


oxygen ions have high reactivity and strong redox properties. They can disrupt the cell membranes of bacteria or the activity of cytoplasmic enzymes, thus achieving antibacterial and sterilization effects. Research has found that negative oxygen ions can combine with positively charged particles such as bacteria, dust, and smoke, causing them to aggregate into balls that fall to the ground. This helps in killing germs and eliminating odors (such as those from cigarette smoke and harmful gases released from construction materials).[50]





Health Promotion

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When it comes to eliminating smoke and dust, both large-sized negative ion and small-sized negative ion can directly remove smoke. The difference lies in the fact that small-sized ions have higher reactivity, so they can eliminate smoke quickly, while large-sized ions have lower reactivity, leading to a slower removal of smoke. The World Health Organization and the China Meteorological Administration both define the concentration of negative oxygen ions in the air as an important condition for measuring air quality. They both agree that only when the concentration of negative oxygen ions in the air reaches 1000-1500/cm³ or above can it be considered fresh air. Research by the EPA laboratory of the United States Environmental Protection Agency and many experts in the field of air environmental protection of the United Nations have shown that high-concentration negative oxygen ions can effectively eliminate the following pollution sources:[51]

  • Dust removal:

Ecological-grade negative oxygen ions can actively capture various small dust particles in the air, causing the pollution sources to condense and precipitate, effectively removing dust particles of 2.5 microns (pm2.5) and below in the air, and even 1 micron particles, thereby reducing the harm of pm2.5 to human health.[52]

  • Decomposition of toxic gases :

Ecological-grade negative oxygen ions can actively remove indoor air pollution, react chemically with carcinogens such as formaldehyde and benzene, and decompose into non-toxic and odorless carbon dioxide and water.[53]

 

△ Chemical formula of formaldehyde decomposition by negative oxygen ions

  • Antibacterial, sterilization, and virus elimination :

Ecological-grade air-negative oxygen ions have high activity and strong redox effects. According to tests, in a high-concentration negative oxygen ion environment, the number of mold and bacteria in the air can be reduced by more than 90%.[54]

Promoting health

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The relationship between the concentration of negative ions and human health in different regions

Negative oxygen ions have high reactivity and strong redox properties. They can disrupt the cell membranes of bacteria or the activity of cytoplasmic enzymes, thus achieving antibacterial and sterilization effects. Research has found that negative oxygen ions can combine with positively charged particles such as bacteria, dust, and smoke, causing them to aggregate into balls that fall to the ground. This helps in killing germs and eliminating odors (such as those from cigarette smoke and harmful gases released from construction materials).[55]

Determination method

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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−,O2−,O3−,CO3−,HCO3−,NO3−, etc.[56][57][58]



NAI determination is divided into NAI determination and NAI identification. NAI determination can be achieved by measuring the change in atmospheric conductivity when NAI passes through a conductive tube. NAI identification is achieved by identifying ions produced by mass spectrometry using a corona source, which can effectively measure the characteristics of a single molecule. This method has been used to identify a variety of negative ions, including O−,O2−,O3−,CO4−,NO2−, and NO3−.[59]

Evaluation method

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There is no unified standard for the evaluation of negative ions in the air at home and abroad. The main evaluation indexes include the monopolar coefficient, the ratio of heavy ions to light ions, the Abe air quality evaluation coefficient (CI), and the relative density of air ions. Among them, the monopolar coefficient and the Abe air quality evaluation coefficient are the most widely used evaluation indicators .[60]

Monopole coefficient (q)

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In a normal atmosphere, the concentrations of positive and negative ions in the air are generally not equal. This characteristic is called the monopolarity of the atmosphere. Monopolarity is expressed by the monopolar coefficient, which is the ratio of positive ions to negative ions in the air, that is, q=n+/n-. The smaller the monopolar coefficient, the higher the concentration of negative ions in the air is than the concentration of positive ions, and the more beneficial it is to the human body. Research by Japanese scholars shows that when n- is greater than 1000 cm-3 and the q value is less than 1, the air is clean and comfortable. When q>1, the air is not clean, and when the q value increases to more than 3, people will feel irritable and uneasy. Generally, the q value in the lower atmosphere is between 1 and 1.2; the q value in areas with more vegetation is less than 1; and the q value on high mountains can be as low as 0.53.[61]

Abe Air Quality Evaluation Index (CI)

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Japanese scholar Abe established the Abe Air Ion Evaluation Index through research on air ions in urban residential areas. The Abe Air Quality Evaluation Index reflects the degree to which the ion concentration in the air is close to the natural air ionization level, CI=n-/1000q.[62]

CI is the air quality evaluation index, n- is the concentration of negative ions in the air (pcs·cm-3), q is the monopolar coefficient, and the number of negative ions of 1000 pcs/cm-3 is the most basic requirement standard for the human body. The air quality evaluation index takes negative ions in the air as an indicator, while also taking into account the composition ratio of positive and negative ions. It is relatively comprehensive and objective. Therefore, it has been widely used in the evaluation of urban air ions abroad.[63]

The larger the CI value, the better the air quality. It can be divided into the following five levels: when CI>1.00, the air quality is Class A, the cleanest; when CI is 1.00~0.70, the air quality is Class B, clean; when CI is 0.69~0.50, the air quality is Class C, medium; when CI is 0.49~0.30, the air quality is Class D, acceptable; when CI<0.29, the air quality is Class E, 0.29 is the critical value, and air below 0.29 is polluted air.[64]

Main influencing factors

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Environmental and meteorological factors

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The concentration of negative air ions is correlated with natural environmental meteorological factors. The content of negative ions in the air will increase significantly in waterfalls, fountains, coastal areas and after thunderstorms. Some studies also believe that the concentration of negative air ions is positively correlated with air humidity and negatively correlated with temperature.

Plant community factors

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The concentration of negative air ions in areas covered by green vegetation is much higher than that in other urban areas. The oxygen released by plant photosynthesis is easily affected by the photoelectric effect of short-wave ultraviolet rays to form negative oxygen ions, which will increase the concentration of negative air ions within a specific range. In addition, different vegetation types and plant communities, different forest ages and canopy densities have very different effects on the concentration of negative air ions.

Seasonal and diurnal factors

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The change in the concentration of negative air ions has obvious seasonal and diurnal variations. Generally, the seasonal variation of negative air ion concentration is highest in summer, followed by spring and autumn, and lowest in winter. The diurnal variation is that the average concentration of negative air ions during the day is higher than the average concentration of negative air ions at night. The concentration is highest in the early morning to the morning, followed by noon to the afternoon, and rises to a certain extent in the evening, while the concentration is lowest at night. [24]




Plant community pattern

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The oxygen released by the photosynthesis of plants is easily photoelectrically affected by short-wave ultraviolet radiation to form oxygen-negative ions, thereby increasing the level of negative air ions in a small area. The concentration of negative air ions in areas covered by green vegetation is much higher than in other areas. Different vegetation types and plant communities, different forest ages and canopy densities have very different effects on the concentration of negative air ions.[65]

Environmental meteorological factors

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There are many reports on the correlation between negative air ion concentration and meteorological factors. Studies generally believe that the concentration of negative air ions is positively correlated with humidity and negatively correlated with temperature. There is controversy over the effect of wind speed on the concentration of negative air ions. The presence of water bodies has a great influence on the concentration of negative ions. For example, the content of negative ions in the air in waterfalls, fountains, coastal areas and after thunderstorms will increase significantly. In addition, suspended matter, carbon dioxide, sulfur dioxide and nitrogen oxides in the air are significantly negatively correlated with air ions.[66]

Seasonal dynamics and diurnal dynamics

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The change in negative air ion concentration has significant seasonal dynamics and diurnal dynamics. Generally speaking, the seasonal dynamics of negative air ion concentration are highest in summer, followed by spring and autumn, and lowest in winter. The daily dynamics are that the average daytime negative air ion concentration is higher than the average nighttime negative air ion concentration, with the highest concentration in the early morning to morning, followed by noon to afternoon, and a certain rise in the evening, while the lowest concentration is at night.[67]

See also

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References

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