Shanghai is experiencing its worst level of pollution to date with yesterday's recorded air quality again at the top of the scale - severe, the same as Monday.

The last time pollution topped the five-level scale was on April 2, 2007 but for a shorter duration.

A short spell of rain yesterday morning resulted in muddy showers in many parts of the city leaving cars covered in a layer of wet dust washed down by the rain.

But the wet weather failed to substantially reduce the pollution, with the Shanghai Environmental Protection Bureau again recording its highest level of air pollutants.

Weather forecasters and the environmental bureau say that with more rain and favorable winds over the next few days, the air pollution, blamed mainly on sandstorms in the north of the country, should ease. Today's air pollution is forecast to be "slight," the third level on the five-level scale.

More residents could be seen wearing masks yesterday but for those with respiratory problems it may already have been too late.

Many were critical of the authorities, saying they had failed to guide people to take timely precautions.

Some urged the city to plant more trees or produce artificial rain to ease the problem in future.

But experts said that such measures would not help the current situation. They said the current air pollution had little to do with Shanghai's local environment but was caused by two sandstorms - one blown down from the north and the other blown in from the sea.

"The longer distance the sand-filled storm or wind travels, the smaller the particulate matters in the air. Given the distance the sandstorm has traveled before reaching Shanghai, rain can only help bring down a small portion of the relatively bigger particulate matter in the air to ease instead of effectively avoid pollution," said Dong Wenbo, an environmental engineering professor with Fudan University.

"And we must learn the route of the sandstorm in detail to precisely produce artificial rain, which is technically still quite demanding at present," Dong added.

He said increasing green areas in Shanghai would help improve local air quality but would do little to prevent such air pollution since it was caused by environmental conditions elsewhere.

Chen Wenhui, an IT worker and amateur meteorologist, said the authorities had reacted insufficiently to help the public deal with the pollution.

"They barely did enough. Many people around me said they did not know how serious the pollution was until they heard the evening news on Monday. For those in need of special precautions, it was too late," he said.

Chen added: "There should have been more widespread and obvious warnings so that the weak group such as those with respiratory diseases could be alerted beforehand and choose to stay at home."

The environmental bureau said the local air pollution index air stood at the 500 ceiling level for both the past two days, the worst situation yet recorded. Air quality is considered hazardous once the benchmark figure climbs to 300.

"I wonder what has been done effectively over the years. They said trees have been planted in the north but we are still suffering sandstorms every year," Chen said.

The environmental bureau said yesterday that they were cooperating with telecom suppliers so that warnings of extreme bad weather, including serious air pollution, would be passed on more quickly through communication channels such as microblogs and text messages.

From next month, air quality results would be updated with increased frequency each day to be more helpful to the public, compared to the current practice where the city's environmental bureau publishes results at noon every day on its official website.