BEIJING: Hospital admissions for respiratory complaints rose 20 percent during the latest choking smog to hit Beijing, reports said Thursday as state media demanded greater government openness on pollution.

This week's pollution across vast swathes of northern China -- the fourth serious case of toxic air in recent weeks -- has sparked anger online and prompted unusually outspoken calls for action even from official media.

The number of patients admitted to several hospitals in the capital for breathing problems rose by a fifth in recent days, the Beijing Morning Post reported.

Half of those admitted to a children's hospital in the city were suffering from respiratory infections, the newspaper said, citing doctors.

The China Daily urged the government to reveal details of the causes of the pollution, saying departments had yet to provide "credible data".

Without such information "the government's promise to tackle the problem may fail to materialise", it said.

The pollution in the capital has been blamed on emissions from coal-burning in power stations and exhaust fumes from vehicles on choked streets.

The elderly, young and those with health problems in the city of 20 million were urged to stay indoors earlier in the week -- or wear protective masks if they had to venture out -- while dozens of flights were cancelled after visibility fell drastically.

Beijing has ordered the emergency closure of factories and removed government vehicles from the streets to try to reduce the haze, but experts say more radical controls are needed to combat the problem effectively.

Real estate tycoon and Internet blogger Pan Shiyi -- who has 14 million followers on Sina Weibo, a Chinese version of Twitter -- has started a campaign for clean air legislation.

It had attracted more than 46,000 signatures as of Thursday afternoon.

Social media users reacted angrily to comments from an official at the Ministry of Environmental Protection, who said developed countries took up to 50 years to solve their pollution problems.

"It will take years and years and cost taxpayers all their money," one user wrote.

State broadcaster CCTV quoted Zhong Nanshan, the president of the China Medical Association who revealed China's cover-up of the SARS epidemic of 2002, as saying: "Air pollution is much more scary than SARS, and affects the heart and veins."

Traffic policemen urged officials to change the dress code and let them wear face masks on duty, the China Daily reported.

"We need masks on duty because of the serious air pollution, but we first need approval from the ministry of public security," it quoted a spokesman for traffic police in the southern city of Changsha as saying.

The US embassy's air quality index in Beijing stood at 207 on Thursday afternoon, or "very unhealthy", after it peaked at more than 500 on Tuesday.

The municipality's figure was 175 at 2pm Thursday, or "lightly polluted".

The meteorological agency said the smog in Beijing was likely to begin to disperse on Thursday evening when the city would be hit by strong winds.