This plateau - which still puts the city in the middle-to-high bracket globally - might not tell the whole story, says Dr Alfred Tam Yat-cheung, chairman of the Hong Kong Asthma Society. 'Although the prevalence has remained more or less the same, patients are becoming younger,' he says. 'And the number of admissions in public hospitals is progressively increasing, which may mean we're treating the same number of people, but their symptoms are more severe.'

In most developed countries, Tam says, the rate of asthma deaths is progressively coming down, owing to effective treatment and management of the condition. But it has remained the same here for the past few decades, at an estimated 90 to 100 a year. That translates to a higher death rate relative to the number of sufferers than a country such as the United States, even though asthma is far more prevalent there.

'Air pollution is an important factor. More [asthma] patients are admitted to hospital when air pollution is worse. And if you look at the relationship of roadside pollution with the prevalence of asthma ... studies show that in many other countries, living by the roadside has been shown to increase the risk of developing asthma by 60 to 200 per cent,' Tam says.

According to the WHO, the fundamental causes of asthma are 'not completely understood', and the condition is 'under-diagnosed and under-treated'. Risk factors include genetics, pollution, indoor living conditions and exposure to certain chemicals, medicines and food additives. 'Asthma, as we understand it now, is very heterogeneous,' Tam says. 'Different mechanisms can result in the same pathology.'

Because allergic asthma typically develops in childhood, 'how the immune system interacts with conditions during childhood is very important', says Dr Adrian Wu, specialist in immunology and allergy at Matilda International Hospital. In the US, for example, asthma rates vary depending on when people are born throughout the year; they are slightly higher in pollen season.

'It's the same in Hong Kong: you're at a higher risk if you have a high rate of exposure to indoor allergens, or if you grow up with smokers,' Wu says.

In Chinese medicine, it is believed that asthmatics have deeply harboured phlegm in the lung inherently, says Professor Feng Weibin, a Chinese medicine consultant at Kwong Wah Hospital. 'This may be due to heredity, physique and the invasion of the six evils - wind, cold, heat, dampness, dryness and fire.'

Many people in Hong Kong spend much of their time indoors - in conditions that can also worsen the condition. Dust mites are asthmatics' No1 indoor enemy - and the city's hot, wet, humid climate makes sofas, beds, carpets and soft toys perfect breeding grounds for them. Then there are the ubiquitous cockroaches, the second-biggest threat, and pets.

'There needs to be more education on indoor pollutant control,' says Dr Wong Wing-ching, associate consultant at the department of medicine and geriatrics at Kwong Wah. 'That includes cleaning to limit dust mites, controlling humidity, better ventilation during cooking and keeping pets out of the home - or at least out of the bedroom.'

The small size of many Hong Kong flats exacerbates another antagonist: fumes from poorly ventilated gas cookers. And the city's crowded living conditions also help diseases spread quickly. 'Infection is a big problem, especially for small children,' says Tam. 'Viruses, in particular, tend to irritate the airways.'

The workplace is less of an issue in Hong Kong, mostly because a lot of high-risk industries involving chemicals and plastics have migrated onto the mainland, although Wong says that occupational asthma still goes under-reported in the city. Construction workers, decorators, carpenters and even hairdressers and people who work in food processing are among those potentially at risk.

Other factors can include stress, diet and certain medicines. Tam says that many attacks, particularly among children and young people, are triggered by harsh words from a parent, teacher or boss. About 5 per cent of asthmatics are sensitive to sulphites, preservatives used in wine and dried fruit. Also, 10 per cent of asthmatics are sensitive to beta blockers and aspirin. 'Always tell your doctor that you're asthmatic, or it can be very dangerous,' says Dr Simon Ip Kar-shun, respiratory consultant at Hong Kong Baptist Hospital.

Asthma can also be induced by exercise - but, taken the right way, exercise can also help sufferers increase their respiratory capacity. 'It's extremely dangerous if you exercise and it's not under control,' Ip says. 'But asthmatics who have their condition under control can do any exercise they like.'

But that relies on clean air. Doctors agree that lowering the city's air pollution would alleviate asthmatics' suffering. '[It] would do so much to reduce the number of cases, their severity, and the number of deaths,' Tam says.

The long-awaited Motor Vehicle Idling Bill, Ip notes, will be a step in the right direction.

Other important measures include stopping smoking, improving living and working environments, maintaining a healthy diet, and the prevention and appropriate treatment of respiratory tract infections, Feng says.

Tam adds: 'There needs to be holistic management that takes in lifestyle factors. The patient, together with the doctor, needs to know the severity of their asthma and if there are trigger factors that make it worse, and then can devise appropriate management treatment.'

Roadside risk

200%

Maximum risk increase that roadside residents in other countries develop asthma, say studies cited by Dr Alfred Tam

Minimise symptoms

- Don't smoke.

- Exercise regularly, as long as your asthma is under control.

- Don't have carpets and soft toys in your home, and reduce the number of soft furnishings.

- Use covers on bedding and other furniture to control dust mites.

- Don't keep pets or, if you can't bear to be without them, at least don't sleep in the same room with them.

- Ensure your home is as dry as possible.

- Eat foods containing antioxidants, zinc, selenium, copper and omega-3 fatty acids.

- Don't consume anything containing sulphites unless you know your asthma isn't triggered by them.

- Certain medicines can trigger asthma, so let your doctor know that you're asthmatic.

- If you have an attack, use your inhaler, stay still, sit upright and take plenty of fluids.