Beijingers do not need warnings from meteorologists to judge the smog in their city. All they have to do is count how many skyscrapers disappear from the skyline. On a day of average haze, some of the more distant tower blocks fade from view. During the worst periods, every building in this city of 14 million people seems to melt into the grey air. Look out of any window at such times and it is only the ghostly outline of the closest buildings that distinguishes the landscape from that of the Gobi Desert during a storm.
Such days are supposed to be getting fewer. Over the past five years, the Beijing municipal government has spent 67 billion yuan ($8.1bn) on relocating some of the dirtiest factories, tightening rules on sulphur emissions and introducing a fleet of electric and gas-fuelled buses. But if anything, the problem appears to have become worse in the past year. According to the state-run media, smog affected the city on more than one in every three days during the first ten months of 2004. With the peak coal-burning winter season approaching, that proportion is likely to grow even higher. The number of "clear-air days" in 2004 is almost certain to turn out to have been even fewer than the 223 recorded in 2003.
As with so many other policies relating to China's environment, the problem is not so much the central government's policies - which are increasingly eco-friendly - but their implementation in an economy where short-term profit takes priority over all else, including pollution regulations and long-term health concerns.
Take the sulphurous yellow air in Beijing. Despite efforts to clean up several of the old sources of pollution - notably the low-grade coal that heats homes in the old alleyways - the biggest culprit, the Shougang steel factory, remains untouched. And the steel factory has been joined by millions of new eco-villains: cars.
China's State Environmental Protection Administration estimates that road vehicles are responsible for 70 per cent of the sulphur in Beijing's air. The fast-growing car industry, however, is a major engine for economic development. Consequently, the Beijing authorities have resisted the tougher measures taken by Shanghai to restrict sales of new vehicles. In the past five years, the number of cars on the capital's roads has doubled, reaching two million. By 2008, it is expected to pass the three million mark.
Manufacturers, most of which have tie-ups with foreign car-makers such as Volkswagen, General Motors and Honda, boast that fuel efficiency is improving all the time. But the gains are not coming fast enough to offset the rise in traffic volume. To make matters worse, Sinopec, China's biggest oil company, has been buying cheaper, more sulphurous "sour crude" on the international market to save money, in a year when global oil prices hit heights not seen for decades.
The situation is by no means unique to Beijing. Acid rain now falls on a third of the Chinese land mass. According to the World Bank, China is home to 16 of the planet's 20 worst cities for air quality. Some are considerably worse than the capital. The smog in Linfen, in Shanxi Province, is exacerbated by the local coal and steel industries - a common problem in a country that still relies on coal to meet three-quarters of its energy needs. In Lanzhou, the capital of Gansu Province, pollution frequently gets trapped between the mountains on either side of the city, blotting out the sun for weeks on end.
It is not just an eyesore. The health costs are increasingly apparent. Respiratory diseases are the leading cause of death in China. Doctors blame smog for sharp rises in cases of bronchitis, pulmonary fibrosis, tuberculosis and lung cancer.
Water has suffered the same fate as air. China's rivers are increasingly likely to be exploited for dams and dump sites, and it is estimated that three-quarters of the rivers running through Chinese cities are so polluted that they cannot be used for drinking or fishing. The Yellow River - seen as the cradle of Chinese civilisation because the Han tribes who founded the nation lived along its banks - is testimony to unsustainable development. Once the country's second-biggest source of fresh water, it is now so polluted that 70 per cent of its waters are hazardous to drink, and it is so overexploited that it dries up for a third of the year before it reaches the sea. Even the mighty Yangtze - the third-longest river in the world - is struggling to sustain the 400 million people who live in its delta. In the past 25 years, 46 big dams have either been built or commenced construction on the Yangtze or its tributaries. Farmers are increasingly dumping phosphate fertilisers into its waters. The volume of human waste is also rising because of the explosive growth both of river traffic and of super-cities on its banks, such as Chongqing. The conservation organisation WWF reckons that as a result, this river, which starts off in the Tibetan plateau as some of the purest water in the world, ends as the biggest source of marine pollution in the Pacific.
That is the bad news. The good news is that the government is not only aware of the problems but, tentatively, starting to do something about them. The prime minister, Wen Jiabao, and the president, Hu Jintao, have stressed the need for "balanced development" that takes more account of the environmental costs of economic growth.
They are increasingly willing to consult international non- governmental organisations, including Greenpeace and WWF, and to tolerate the growing number of small domestic groups of green activists. Rafts of new regulations have been announced to curb factory pollution, improve car-exhaust emissions and clean up the waste produced by cities.
But all too often, fine-sounding central government policies are ignored at provincial or township level, where local officials are more interested in generating revenue. At times, it seems that, in the battle between the two, the central government finds it useful to call upon NGOs and the media to enforce its own rules and raise awareness of environmental issues.
Earlier this year, the prime minister won plaudits for halting the construction of a series of dams on the Nu River in Yunnan Province, which had been the subject of international scrutiny. This autumn, the State Environmental Protection Administration threw its support behind a campaign by NGOs and local residents to block the building of a dam near Tiger Leaping Gorge, a place of spectacular beauty and rich biological and ethnic diversity on an upper tributary of the Yangtze that is thought to be the inspiration for Shangri-la in James Hilton's novel Lost Horizon.
Even compared to a year ago, there are far more stories about the environment in the domestic media. However, this consciousness-raising has yet to result in a change of priorities. There may be tinkering at the margins - such as the efforts to clean up the air of Beijing - but the national goal is still the same. Economic growth always comes first, even if the country has to become filthy in order to become rich.
Jonathan Watts is the Guardian's Beijing correspondent