Recent research has claimed that an excess of positive ions in the air can have an ill effect on people' s physical or psychological health. What are positive ions? Well, the air is full of ions, electrically charged particles, and generally there is a rough balance between the positive and the negative charges (电荷). But sometimes this balance becomes disturbed and a larger proportion of positive ions are found. This happens naturally before thunderstorms, earthquakes or when winds such as the Mistral, Foehn, Hamsin or Sharav are blowing in certain countries. Or it can be caused by a build-up of static electricity indoors from carpets or clothing made of man-made fibres, or from TV sets, duplicators or computer display screens. When a large number of positive ions are present in the air, many people experience unpleasant effects such as headaches, fatigue, irritability, and some particularly sensitive people suffer nausea or even mental disturbance. Animals are also found to be affected, particularly before earthquakes snakes have been observed to come out of hibernation, rats to flee from their burrows, dog howl and cats jump about unaccountably. This has led the U.S. Geographical Survey to fund a network of volunteers to watch animals in an effort to foresee such disasters before they hit vulnerable areas, such as California. Conversely, when large numbers of negative ions are present, people have a feeling of well-being. Natural conditions that produce these large amounts are near the sea, close to waterfalls or fountains, or in any place where water is sprayed, or forms a spray. This probably accounts for the beneficial effect of a holiday by the sea, or in the mountains with tumbling streams or waterfalls. To increase the supply of negative ions indoors, some scientists recommend the use of ionisers: small portable machines which generate negative ions. They claim that ionisers not only clean and refresh the air, but also improve the health of people sensitive to excess positive ions. Of course, there are the detractors, other scientists, who dismiss such claims and are skeptical about negative/positive ion research. Therefore people can only make up their own minds by observing the effects on themselves, or on others, of a negative rich or poor environment. After all, it is debatable whether depending on seismic (地震的) readings to anticipate earthquakes is more effective than watching the cat. According to the passage, static electricity can be caused by ______.