What are Negative Air Ions (NAIs)?
NAIs are negatively charged gas ions formed and produced by various sources such as sunlight, cosmic rays, plant-based energy sources, and other natural and artificial sources, also known as small particle size negative ions. They account for approximately 10-20% of the content of atmospheric negative ions1.
NAIs are important indicators of air quality, with high concentrations often found in natural environments like forests, waterfalls, and beaches2. This article aims to review research on the generation, distribution, and biological function of NAIs, and their impact on human health.
Generation of NAIs
NAIs are generated in three primary ways3. Firstly, they are formed when free electrons in the atmosphere, released by cosmic rays, ultraviolet rays, and other energy sources, are captured by air molecules. Secondly, NAIs are produced in environments with waterfalls, waves, and rainstorms, where negative charge is carried away by airflows from water mist. Lastly, plant "tip discharge" and photosynthesis release free electrons that combine with oxygen or water molecules to form NAIs4.
Spatial and Temporal Patterns of NAIs Concentration
The concentration of NAIs in the atmosphere fluctuates based on time, weather, and location5. Studies have found higher concentrations in the morning and night, lower in the afternoon, and variations across seasons, with higher levels in summer and autumn, and relatively lower in winter6. In addition, the concentration of NAIs is positively correlated with altitude7 and negatively correlated with air pollution8.
Biological Functions of NAIs
NAIs have been found to exhibit potential biological functions, such as regulating respiratory system function, sedation, hypnosis, hypotension, regulating mood, neurological function, metabolism, and endocrine function9. Their potential health benefits are discussed in the following subsections.
Research has suggested that NAIs can effectively inhibit the sympathetic nerve and activate the parasympathetic nerve10. This regulation of autonomic nerve function can adjust heart rate and blood pressure11.
High-concentration NAIs inhalation may improve lung function, regulate metabolism, and treat asthma symptoms12. Recent studies have demonstrated that NAIs could significantly reduce blood pressure, heart rate, and increase the high-frequency power of the HRV spectrum13.
High-concentration NAIs exposure may reduce the severity of depression, psychological stress, and anxiety to improve well-being14. However, evidence of these effects is still ambiguous, and there seems to be a lack of in-depth mechanistic research in humans15.
Omics-based Studies on NAIs
Recent technological advances in omics, a comprehensive suite of techniques used to explore the roles, relationships, and actions of the various types of molecules that make up the cells of an organism, have allowed for global and sensitive identification of molecular changes relevant for monitoring NAIs exposure16.
Research on NAIs has shown that they have potential health benefits, particularly in relation to cardiovascular and respiratory health, as well as mental well-being. However, the mechanisms underlying these benefits are not yet fully understood, and further research is needed, particularly using modern omics techniques. In addition, while NAIs are a natural component of the atmosphere and are often associated with clean, healthy air, their concentrations can be influenced by a variety of factors, including pollution, weather, and location. Therefore, more research is also needed to understand how these factors might impact the potential health benefits of NAIs.