Medical experts are urging the government to take more effective action to clear the city's air after a new study found that air pollution restricts the growth of children's lungs. The preliminary analysis of a six-year study done in Britain indicates that children exposed to nitrogen dioxide and particulate matter have slower lung growth. Professor Frank Kelly, an environmental health expert at King's College, London, said a child could lose as much as 165ml of lung volume under high-level exposure to nitrogen dioxide. The professor said the impact of a smaller lung was lifelong and could "contribute to the development of chronic lung diseases such as asthma". Studies in Hong Kong have found similar results. Dr Alfred Tam Yat-cheung of the Hong Kong Society of Paediatric Respirology said that children living in polluted districts suffered from restricted lung growth, while girls had a more significant decline in lung volume than boys. A weaker and smaller lung means children might have more difficulty doing exercise and a lower resistance to respiratory diseases, said Tam. But the specifics of how pollutants restrict the development of lungs is still being studied. "Perhaps children have smaller lungs than adults, and therefore they inhale more air pollutants. However, this is still just speculation," Kelly said. Ruby Tse, 24, a long-term rhinitis patient, started to suffer from asthma three years ago when she was studying in the UK. The problem persisted when she returned to Hong Kong in 2012. She says her health improves when she avoids crowded areas such as Mong Kok or Causeway Bay on heavily polluted days. She also exercises indoors to strengthen her respiratory system and has reduced her reliance on an asthma inhaler from six times to twice a day. To improve lung function and build defences against pollution, medical experts recommended a diet rich in antioxidants, Vitamin C and E, and urged the government to take more action to improve air quality. Undersecretary for the Environment Christine Loh Kung-wai said major steps were being taken, including the replacement of catalytic converters in minibuses and taxis, and the replacement of more than 80,000 diesel vehicles by 2019.