LONDON (Reuters) -- Childhood allergies are on the rise around the world, including in many developing countries where asthma, eczema and hay fever are emerging as important public health problems, scientists said.

Asthma, in particular, is responsible for millions of children missing school, ending up in hospital or even dying.

The underlying cause of the condition is inflammation of the airways but just what triggers the problem and why some people develop asthma and others do not is still poorly understood.

What is clear, however, is that more and more young children are suffering allergic disorders, with the prevalence of all allergies increasing notably in the past decade.

Researchers who carried out the International Study of Asthma and Allergies in Childhood in 1991 repeated the survey in 2002 and 2003 and found widespread increases in prevalence in 56 countries, especially among young children.

The study -- details of which were published in the latest edition of the Lancet medical journal -- surveyed parents of 193,000 children aged 6-7 years and 305,000 children aged 13-14 years in countries as diverse as South Africa, Brazil, Iran, Canada and Sweden.

The increases were greatest for eczema in the younger age group and for hay fever in both age groups.

In the older age group, however, where prevalence of asthma had been high, there were some signs of decreases.

Nonetheless, in Britain -- one of the developed countries with the worst allergy epidemics -- asthma prevalence went up to 20.9 from 18.4 percent, hay fever increased to 10.1 from 9.8 percent and eczema to 16 from 13 percent between 1991 and 2003.

Professor Innes Asher of the University of Auckland, New Zealand, lead author of the study, said the rise in prevalence in many countries was "concerning", especially as allergies often occurred in large population centres.

"Although changes in mean annual prevalence to the order of 0.5 percent might sound small, such changes could have substantial pubic health implications, especially since the increases took place most commonly in heavily populated countries," he said.

Experts say a host of factors including air pollution, diet, lifestyle and exposure to bacteria in early life are likely to be linked to the rise in asthma and other allergies, but the reasons are likely to vary from place to place.

Given the diversity of the condition, the Lancet said in a editorial that asthma was in fact unlikely to be a single disease and the term should be abolished altogether.

A separate study published in the journal reinforced the benefits of combination medicines in tackling asthma.

Dr. Klaus Rabe of Leiden University in the Netherlands said a 12-month study involving more than 3,000 people showed that giving asthma patients a top-up dose of AstraZeneca's two-in-one drug Symbicort to relieve their symptoms was more effective than giving them a single drug top-up.

All patients in the study were already on a maintenance dose of Symbicort, which combines a corticosteroid with a long-acting beta agonist.