"Natural disaster" is an abnormal phenomenon in the nature on which human beings depend, and the harm caused by natural disasters to human society is often shocking. Among them are earthquakes, volcanic eruptions, mudslides, tsunamis, typhoons, floods and other sudden disasters; There are also gradual disasters, such as land subsidence, land desertification, drought, coastline changes, etc., which can only gradually appear in a long time; There are also environmental disasters caused by human activities such as ozone layer change, water pollution, soil erosion and acid rain. There is a complex relationship between these natural disasters and environmental damage. It has become the common theme of the international community to scientifically understand the occurrence and development of these disasters and minimize their harm.

Natural variations on the earth, including those induced by human activities, occur all the time. When this mutation brings harm to human society, it constitutes a natural disaster. Because it has brought different degrees of damage to human production and life, including the relationship between man and nature through labor and the interpersonal relationship related to it. Disasters are negative or destructive. Therefore, natural disasters are a manifestation of the contradiction between man and nature, with the dual attributes of nature and society, and are one of the most severe challenges faced by mankind in the past, present and future.

The main sudden natural disasters in the world are: drought, flood, typhoon, storm surge, freezing injury, hail, tsunami, earthquake, volcano, landslide, debris flow, forest fire, agricultural and forestry diseases and insect pests, etc.

There are many kinds of natural disasters in China. Earthquake, typhoon, rainstorm, flood, waterlogging, high temperature, lightning, fog, haze, debris flow, landslide, tsunami, road icing, tornado, hail, snowstorm, collapse, land subsidence, sandstorm, etc. It occurs in all parts of the country and local areas every year, causing extensive damage or devastating blows in local areas.

Many natural disasters, especially high-grade and high-intensity natural disasters, often induce a series of other disasters to occur one after another. This phenomenon is called disaster chain. The earliest and effective disaster in the disaster chain is called primary disaster; Disasters induced by primary disasters are called secondary disasters. After natural disasters, the harmonious conditions of human existence are destroyed, and a series of other disasters will be triggered, which are generally called derivative disasters. For example, after the drought, the extreme shortage of fresh water on the surface and shallow parts forced people to drink deep groundwater with high fluorine content, which led to fluorosis. These are called derivative disasters.

Of course, the process of disasters is often very complicated. Sometimes a disaster can be triggered by several disasters, or a disaster can trigger several different disasters at the same time. At this time, the determination of disaster types depends on the dominant causes and their main manifestations.

Sudden and unpredictable. Natural disasters are usually violent and devastating. The duration is long and short. Disasters include many factors, which will cause casualties, huge property losses and considerable chaos. The longer the disaster lasts, the greater the threat to the victims and the greater the impact of the incident. Another main feature that affects the degree of disaster is whether people get enough early warning.

Natural disasters have many important characteristics. They are sudden, powerful and uncontrollable, causing destruction and chaos. They are usually short-lived and the lowest point, sometimes predictable.

There are many ways in which disasters affect behavior and mental health:

(1) Disasters will bring substantial trauma and mental disorders;

(2) Most of the pain disappears within one or two years after the disaster, and people can adjust themselves;

(3) Chronic mental disorders caused by disasters are very rare;

(4) The overall impact of some disasters may be positive, because it may increase social cohesion;

5. Disasters have disrupted organizations, families and personal lives.

Natural disasters can lead to stress, anxiety, depression and other emotional and perceptual problems. The time of the impact and why some people can't adapt as soon as possible are still unknown. After floods, tornadoes, hurricanes and other natural disasters, victims will show evil thoughts, anxiety, depression and other emotional problems, which may last for a year.

The persistent influence of an extreme disaster is called post-traumatic stress disorder, that is, persistent, unnecessary and uncontrollable thoughts about irrelevant events after trauma, strong desire to avoid mentioning events, sleep disorder, social withdrawal and anxiety disorder with strong vigilance.