Hydrogen, with a chemical formula of H2, is a diatomic gas molecule composed of two hydrogen atoms by sharing a pair of electrons. Hydrogen is the smallest molecule and the lightest gas in nature. Its density is only one-fourth that of air. Hydrogen occupies a very important position in our lives, because hydrogen can selectively scavenge hydroxyl radicals (∙ OH) and peroxynitroso anions (ONOO−) without affecting other active oxygen species with physiological functions Hydrogen can be used in the medical field, and hydrogen has been proven to have significant preventive and therapeutic effects on many diseases. Hydrogen is an important industrial raw material. It can be used to synthesize some important compounds such as ammonia, methanol, and aldehydes. It can also be used as a reducing agent to reduce unsaturated bonds such as olefins, aldehydes, and ketones, especially in the presence of a chiral catalyst. The asymmetric hydrogenation is widely used in the synthesis of natural products and drugs. At the same time, hydrogen as a renewable clean energy source has attracted increasing attention. This article describes the history of hydrogen discovery and the important role of hydrogen in medical health and synthetic chemistry.
 
1 History of hydrogen discovery
As early as the 16th century, Swiss doctor Paracelsus put iron filings with acid to get a flammable gas. This gas is hydrogen, but because he did not do further research at that time, he missed Opportunity to remain famous in the history of chemical element discovery. By the 17th century, Helmont, a Belgian scientist, had also been exposed to hydrogen by accident, but did not separate it and collect it for research. Although British chemist Boyle also accidentally collected the gas, it has not been studied. The real opportunity was taken by British chemist Cavendish. In 1766, Cavendish accidentally dropped an iron piece into a hydrochloric acid solution in an experiment and found that many bubbles were generated in the solution. This phenomenon immediately aroused great interest in Cavendish. He collected the gas through the drainage method and carried out a careful study, and found that the gas will explode on encountering Mars when mixed with air, and will generate water and other experimental results after burning in the air. Since Cavendish was the first to collect and study hydrogen, he became a recognized discoverer of hydrogen in the history of chemical element discovery. However, due to the influence of fuel theory, Cavendish once thought that the gas was a fuel element. Until 1787, French chemist Lavoisier formally proposed that hydrogen is an element. Because hydrogen generates water after combustion, this flammable gas was named "hydrogen", which means the producer of water [1] .
 

2 Medical uses of hydrogen
Initially, hydrogen was considered to be a physiologically inert gas, and it was applied to the diving field to replace nitrogen in compressed air and eliminate the problem of nitrogen anesthesia caused by high-pressure nitrogen [2]. In 1975, Dole et al. [3] found in a study that the use of a mixture of 8 atmospheres containing 97.5% hydrogen and 2.5% oxygen has a therapeutic effect on squamous cell carcinoma in mice. This is the earliest report on the biological effect of hydrogen. However, due to harsh conditions and difficult operations, it has not attracted the attention of biologists. Until 2007, Ohsawa et al. [4] confirmed in a study that the use of 2% hydrogen has a therapeutic effect on cerebral ischemia-reperfusion injury in rats, and at the same time, it proposed that hydrogen has the characteristic of selective reduction, and it is only free of hydroxyl in the body (∙ OH) and peroxynitrite (ONOO−). As soon as the results were published, they attracted widespread attention from biologists and set off a wave of research on the biological effects of hydrogen. Due to the small molecular weight of hydrogen, hydrogen has the characteristics of strong penetration and penetrating ability and fast diffusion speed. After entering the human body, it can quickly diffuse to the required area to exert its antioxidant effect. At the same time, hydrogen has high biological safety. It is non-toxic and non-residual. Water can be obtained after interacting with oxidants, so hydrogen is a very ideal drug. More and more studies show that hydrogen has a good therapeutic effect on many diseases. Dongjie Liu and others from Taishan Medical College [5] introduced the research progress of hydrogen molecules in relieving ophthalmic diseases in recent years, and pointed out that hydrogen can reduce eye damage caused by oxidative stress and inflammation. Hydrogen-containing physiological saline can protect the retina and cornea, and has a therapeutic effect on cataracts. The author also explained the mechanism of hydrogen in the relief of ophthalmic diseases, providing a basis for the application and research of hydrogen in clinical ophthalmic diseases. Liu Jianjian et al. [6] of the School of Life Science and Technology of Xi'an Jiaotong University reviewed the therapeutic effects of hydrogen molecules in heart injury, and pointed out that hydrogen can effectively remove the OH produced by heart injury and enhance the body's antioxidant capacity. Can inhibit the pro-inflammatory factors produced by damaged tissues and activate the tumor necrosis factor TNF-α and interferon IFN produced by lymphocytes, thereby reducing the damage to the heart muscle caused by ionizing radiation, alleviating heart ischemia-reperfusion injury, heart transplantation, heart lung Heart injury caused by circulation surgery has good prevention and treatment effects. Hydrogen has a protective effect on colon damage, and has significant preventive and therapeutic effects on cardiovascular disease, digestive system, respiratory system and other diseases [7–11]. At present, hydrogen medicine has moved from basic research to clinical application. Xie Fei from Beijing University of Technology [12] introduced the current research progress of hydrogen molecules in clinical applications, pointing out that hydrogen molecules are used in tumors, type II diabetes and hyperuricemia and other metabolic related diseases, cerebral infarction and spinal cord injury, etc. Neurological diseases, gastroesophageal reflux disease, rheumatoid arthritis and other diseases have shown good results in clinical applications, providing a reference for large-scale, multi-center clinical trials of hydrogen molecules. Figure 1 summarizes the application fields of hydrogen in medicine. The main methods of administration of hydrogen include direct inhalation, drinking hydrogen-rich water, injecting hydrogen physiological saline, using hydrogen-containing eye drops, and hydrogen-containing bath products and cosmetics [13]. Hydrogen inhalation is the most direct method of inhalation. Through the ventilating effect of the alveoli, hydrogen can quickly reach various parts of the body through the blood circulation. Drinking hydrogen-rich water is the most convenient method, and it is expected to become an effective means for disease prevention and treatment. The intravenous injection and local use of hydrogen-containing solutions are the most widely used clinically. The injection of hydrogen saline has been applied in a variety of disease models including hemorrhagic shock models and acute pancreatitis. Hydrogen-containing eye drops can be used to reduce retinal damage [14].

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