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 charged. But sometimes this balance becomes disturbed and a larger proportion of positive ions are found. This happens naturally before thunderstorm , earthquakes when winds such as the Mistral , 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 affected , particularly before earthquakes , snakes have been observed to come out of hibernation , rats to flee from their burrows , dogs howl and cats jump about unaccountably. This has led the US 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 , then 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.examda 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 readings to anticipate earthquakes is more effective than watching the cat.