Asthma is a controllable but not curable disease. The World Health Organisation (WHO) says 100 to 150 million people around the world are asthmatic and the number is growing by 50% every decade. It causes 180,000 deaths a year. What is asthma? Asthma is a chronic, inflammatory lung disease characterised by recurrent breathing problems. People with the disease suffer "attacks", or acute episodes, when the air passages in their lungs narrow and breathing becomes difficult. Attacks are caused by the airways over-reacting to certain environmental factors. They then become inflamed and clogged. They are described as feeling similar to taking deep breaths of very cold air in winter. Breathing becomes harder and may hurt, and there may be coughing. The air may make a wheezing or whistling sound. What causes it? A definitive cause for the lung abnormality at the root of asthma has so far eluded doctors. However, Professor Duncan Geddes of the National Asthma Campaign says there are plenty of reasons why prevalence of the disease is increasing. "There are simple ones - women who smoke during pregnancy are much more likely to have asthmatic children. "It's also something to do with the way we're living in modern housing - little ventilation, damp housing, more carpets and more dust make asthma more common." Exposure to illness in childhood could also play a role, he says. "There are some curious things like the pattern of early childhood infections. It may be that in the past when all children had a lot of viral infections their bodies were defending themselves against infection. "Now with fewer of them, their bodies are turning over to asthma instead." Some scientists have linked the increase in asthma with an increase in air pollution. However, this theory is hotly disputed. The most likely culprit are PM10s, microscopically small particles given off by diesel engines, coal burning, mining, construction and quarrying. These particles can penetrate deep into the lung and are known to worsen existing heart or respiratory problems. What triggers an asthma attack? There are plenty of known triggers for asthma attacks. They include: Allergens (substances to which some people are allergic) such as pollens, foods, dust, mould, feathers, or animal dander (small scales from animal hair or feathers); Irritants in the air such as dirt, cigarette smoke, gases, and odours; Respiratory infections such as colds, flu, sore throats, and bronchitis; Too much exertion such as running upstairs too fast or carrying heavy loads (although people with asthma can benefit from a moderate amount of exercise); Emotional stress such as excessive fear or excitement; Weather such as very cold air, windy weather, or sudden changes in weather; Medication such as aspirin or related drugs and some drugs used to treat glaucoma and high blood pressure. Each person with asthma reacts to a different set of triggers. Identifying which triggers apply is important if asthma attacks are to be prevented. What are the symptoms? The condition varies a great deal from one person to another. Symptoms range from mild to moderate to severe and can be life-threatening. Attacks can be only occasional or frequent. The symptoms of asthma are a major cause of time lost from school and work and sleep disturbances. However, with proper treatment these symptoms can almost always be controlled. Sometimes the only symptom is a chronic cough which may occur only at night or during exercise. Some people think they have recurrent bronchitis, since respiratory infections usually settle in the chest in a person predisposed to asthma. The condition can be diagnosed using: Spirometry, which uses an instrument that measures the air taken into and out of the lungs; Peak flow monitoring, which also measures the performance of the lungs; Chest x-rays; (Occasionally) blood and allergy tests. There are certain warning signs of an impending attack. These can occur hours or days before audible wheezing begins. They vary from person to person, but include: Light wheezing; Coughing pain; A tight feeling in the chest; Shortness of breath; Restlessness. Once a patient learns what their signs are they can become better at recognising an attack before it gets out of control. How can attacks be prevented? To prevent attacks, people with asthma should avoid their asthma triggers and take medicine appropriately. What is the treatment? Treatment consists of two main factors - environmental control and medication. Environmental control means simply keeping away from factors - like tobacco smoke or allergens - likely to trigger an attack. Medication is the mainstay of asthma treatment. Because patterns of asthma are different for different people, the specific type of drug treatment varies a lot depending on the frequency, severity, and particular triggers of each patient's episodes. For people with mild asthma, medication may only be needed before exposure to triggers or when they detect the onset of an attack. Those with more frequent symptoms may take daily medicine as well as using medicine for specific symptoms. In the case of severe, persistent asthma patients may need two or more doses of medicine each day. The major types of anti-asthma medicine are: Corticosteroids, which reduce the inflammation of the airways. They can be taken as pills or as an aerosol. Inhaled steroids have fewer side-effects, so oral steroids are usually reserved for those with severe asthma; Anti-allergy drugs, which can be used to prevent an attack, but are of no use after an attack has begun; Bronchodilators, of which there are several types. These provide temporary relief from asthma symptoms but do not tackle the underlying inflammation. They can be taken in liquid, inhaled or tablet form. Immunotherapy - where the patient is desensitised to their allergens - can be useful in cases where environmental control and medication have failed. Professor Geddes says: "A cure is always just around the corner." The National Asthma Campaign is one of many bodies sponsoring research into the causes of asthma, and scientists hope to produce a vaccine for the disease in the near future.