The main cause of allergic asthma in Singapore is the house dust mite, scientists in Singapore have found.
Invisible to the naked eye, it feeds on flakes of human skin and is found in pillows, mattresses and carpets.
A study of about 8,000 people here showed about 80 per cent of them reacted to the mites, and 15 per cent had asthma symptoms.
Nearly 40 per cent had allergic rhinitis symptoms - when the nose is inflamed, causing itchiness, sneezing, and a runny or blocked nose.
In Singapore, about 5 per cent of adults and 20 per cent of children suffer from asthma, according to the Health Promotion Board.
Attacks can be triggered by causes including allergies and infections.
The study's authors said: "Allergic diseases can be caused by a number of common allergens present in considerable concentrations in Singapore, but we found that they are in fact largely dominated by a single allergen, the house dust mites."
Scientists from the National University of Singapore (NUS) and Agency for Science, Technology and Research (A*Star) said the findings could improve allergy prevention and treatment.
Their work was published in science journal Allergy this month.
"Now that we know that the house dust mite is the major problem, we can narrow down allergy tests
for people," said Research Associate Professor Wang De Yun from the NUS Yong Loo Lin School of Medicine.
"If we know a patient is susceptible to allergic rhinitis or asthma, we can target the house dust mite since it has the major impact."
Another significant finding was that the longer a person from a temperate zone lives in Singapore's tropical climate, the more sensitive he becomes to the dust mites.
The scientists studied only ethnic Chinese people - mostly young adult NUS students - to rule out differences caused by race.
Of the migrants born in temperate China, less than 20 per cent of those who had lived here for under three years reacted to the mites.
But this proportion rose to 50 per cent for those who had been here for more than eight years.
In contrast, there was no difference between Singapore-born participants and those from Malaysia.
Dr Wang said the mites' growth is maximised in hot and humid conditions typical in tropical climates.
By looking at different allergens, the scientists also concluded "lifestyle is an important contributor to the apparent bias towards house dust mite reactivity".
"Clearly, in Singapore, more time is spent indoors, especially in air-conditioned spaces," they said, adding that previous research showed that asthma patients living in mechanically-ventilated rather than air-conditioned homes had better health.
