Dr Sulman also undertook a study of "weather sensitive" volunteers and showed that, during the time of the Sharav winds, their bodies would produce up to ten times their normal level of serotonin - a hormone associated with stress. He found that, in effect, they were being poisoned by their own serotonin, causing migraines, hot flushes, irritability, pains around the heart, difficulty in breathing and a worsening of bronchial complaints, anxiety and irrational tension. Also a slowing of reactions was observed. Interestingly, it was discovered that in many people, the body's initial response to positive ions is to produce adrenaline and noradrenaline - the "fight or flight" hormones - which produces short-term euphoria but eventually leads to a condition of exhaustion. (It is this condition that is thought to affect insects and animals into restless activity as the positive ions build up before a storm.) The research also showed that exposure to positive ions can trigger an over-production of histamine, which most people will immediately recognise as the body chemical that aggravates allergies. Statistically it was found that 25% of the population are quite strongly affected by levels of ions in the air. Of the remainder, 50% are affected considerably, although 25% do not appear sensitive at all.