As ozone is a toxic gas, users should try their best to avoid contact ozone with body tissues and organs, especially the nose and eyes. Exposure to ozone for a long time is dangerous. Exposure to ozone below 20ppm for no more than 2h has no permanent harm to human body. As the odor of ozone is very strong, people feel it when the concentration is 4.46×10-9mol/L（0.1ppm）. Therefore, there has been no report of death caused by ozone poisoning since the ozone has been used for more than 100 years in the world.