Hong Kong was caught by surprise yesterday when a sandstorm from parched northern China descended on the city, pushing readings off the top of the 500-point air pollution index and prompting questions about why no early warnings were issued to the public. But environment and health officials said not much could be done to alleviate the effects of the choking murk, which was borne in on easterly winds along the coast early yesterday causing already-high pollution readings to jump sixfold. At 9pm, the readings at 10 out of the 14 air quality monitoring stations - including the three roadside stations in Causeway Bay, Central and Mong Kok - had gone off the scale, eclipsing the previous highest reading of 201 in Tung Chung in 2004. The Environmental Protection Department warns people with heart or respiratory conditions to stay indoors when the index exceeds 100. The concentration of the main pollutants - tiny suspended particles that can penetrate deep into the lungs - was at least 15 times the World Health Organisation's recommended maximum. Doctors said there was little people could do to protect themselves because ordinary surgical masks could not block the particles, and even staying indoors with the air conditioning on was no guarantee of safety. Weather forecasters have predicted some relief today as the sand-bearing easterly is replaced by a southeasterly sea breeze, but pollution is tipped to stay severe at up to 490 points. Environment officials said they had spotted an unusual rise in particle concentration at about 8pm on Sunday but did not issue a warning through the government information services until six hours later at 2.36am, when most people were asleep. Hahn Chu Hon-keung, environmental affairs manager of Friends of the Earth, said it was regrettable that the government had issued the warning so late, as the dust storm had been widely reported in mainland media for two days. 'The government neglected the need for advance caution. If the situation was known a day in advance, something could have been planned in the transport and power generation sectors to avoid worsening the pollution,' Chu said. Prentice Koo Wai-muk, a Greenpeace campaigner, said an earlier warning might allow workers and students with respiratory problems to take sick leave. Koo suggested that outdoor work should be temporarily suspended once the index hit 250. Officials said different departments had been in contact during the night but could not say why the warning had been delayed. The pollution - which made headlines in international media that often highlight Hong Kong's air quality problems - has raised questions about why nothing has been done to set up a warning system for pollution - similar to typhoon warnings - that has been under study for three years. Such a system was proposed by the Council for Sustainable Development in 2007, but the Environment Bureau is still studying it. Chandran Nair, a member of the council's study group on air quality, said the colour-coded alert system had been delayed by the government's air quality objectives review. The dominant pollutant is respirable suspended particles - also known as PM10 - no bigger than 10 microns, a 10th of the diameter of a human hair. In Eastern district, where air quality often tops that of other places, the concentration topped 770 micrograms per cubic metre of air at least 15 times the World Health Organisation guidelines, which is 50 micrograms per cubic metre of air. The government has secured support from the two power companies to use cleaner fuel in an effort to cut the pollution, but this cannot begin for about two days. Secretary for the Environment Edward Yau Tang-wah urged the public to use more public transport, stop smoking and switch off idling engines to help improve air quality. As the news spread, at least one school cut short its sports day and many others cancelled physical education classes for the rest of the day. But the Education Bureau said there was no need to suspend schools. Anthony Hedley, chair professor in community medicine at the University of Hong Kong, said the index was based on a set of outdated air quality objectives that would remain too loose even after the review. Mike Kilburn, environmental programme manager of think tank Civic Exchange, said the sandstorm should not overshadow the problem of roadside pollution. 'We must not lose sight of the fact that roadside air pollution remains the single biggest threat to public health in Hong Kong.' Lawmaker Gary Chan Hak-kan, of the Democratic Alliance for the Betterment and Progress of Hong Kong, called on the government to set up contingency plans to use according to the air pollution index level. 'We should consider a detailed plan to tell under which circumstances classes should be suspended and outdoor workers could earn a holiday and stay at home,' he said.