China's Clean-Air Drive Likely to Take a Long Time SAN FRANCISCO - Every so often, an extraordinary image makes the rounds on the Internet that shows an ominous gray cloud over the Beijing area, as seen from a satellite. Rather than a rain cloud, it is a shroud of pollution, the type that has caused fear and anger in China's worst-hit cities. But China's pollution, while extremely severe, is not unique, and efforts by other countries, like Britain and the United States, to conquer dirty air may hold lessons for China's future. The Chinese government is working on the problem and recently announced new limits on pollutants along with a promise of increased monitoring. Public awareness has spiked, a necessary step toward ending the crisis. But the overriding message from other nations is a discouraging one: Serious change can take decades, especially when pollution is a byproduct of economic growth. The classic example is London. Persistent pollution problems caused by industrialization culminated in a 1952 disaster known as the Great Smog. At least 4,000 people, and probably many more, died from enduring five days of filthy air in December. Coal-fired furnaces in homes and factories had pumped out soot during a cold snap, contributing to the dense, smoky fog known as smog. London's pollution was probably blacker than China's haze today, according to Michael McElroy, a professor of environmental sciences at Harvard University, though exact quantitative comparisons across 60 years are difficult. London's problem, echoed in other British cities, had long been known. A century before the Great Smog, Charles Dickens wrote in "Bleak House" of "smoke lowering down from chimney-pots, making a soft black drizzle, with flakes of soot in it as big as full-grown snow-flakes." Anti-pollution activists tried to rally the populace as far back as Dickens's era, but changes were relatively toothless until Britain's Clean Air Act in 1956 led to the creation of "smokeless zones" around towns and cities, according to Stephen Mosley, a lecturer in history at Leeds Metropolitan University. The use of coal for home furnaces gave way to electricity and gas. "There was a sea change in public opinion, and smoke began to be associated more with waste than wealth," Dr. Mosley wrote in an email. Nonetheless, the transition away from coal took a few decades to complete after the Clean Air Act was implemented, Dr. Mosley said. In 1962, 10 years after the Great Smog, about 700 Londoners died during another awful, smoky fog. Even today, while smoky coal furnaces are long gone from the city center, air pollution is resurfacing as an issue in London. This time, the problem centers on emissions from vehicles. A crisis similar to the Great Smog took place in Pennsylvania in 1948. A high-pressure weather system settled over the factory town of Donora, resulting in five days of sickening, yellowish smog. Nearly half the town fell ill. To avoid such problems, American factories built taller smokestacks, according to Dr. McElroy. The idea was to send the smoke elsewhere so it would not cause problems locally - in accordance with the memorable motto, "the solution to pollution is dilution." But that contributed to another pollution problem: acid rain. This plagued the northeastern United States and parts of Canada for years, harming trees and fish. It took until 1990, when the United States adopted a cap-and-trade program for pollutants that contribute to acid rain, for the issue to be addressed in a serious way. "What we've tended to do is not solve the problem but fix a little piece of it - and then we find there's another problem," Dr. McElroy said. In China today, the solution is clear, environmental advocates say: No more coal. "China has to get away from its addiction to coal," Li Shuo, a climate and energy campaigner for Greenpeace East Asia, wrote in a recent email. The externalities of coal are staggering. The government is pushing other energy sources for power plants, including wind, solar, nuclear, hydropower and natural gas. But coal, which is a steady and cheap power source, is well-established in China and hard to turn off. "It's sort of a transition that can't occur instantaneously but has to happen as quickly as possible," Dr. McElroy said. As China moves to tighten regulations to clear the air, it might also be mindful of unintended consequences. In Mexico City, which struggles with ozone problems, a regulation took effect in 1989 that allowed drivers to use their car only four out of five weekdays, based on the vehicle's license number. One popular response, Dr. McElroy said, was to buy a junky second car - that emitted more pollution than the primary car - so that driving every day was still possible. The city of Beijing has just announced alternate-day driving regulations that will apply during bad smog days. It has imposed such rules previously. Even assuming that Beijing avoids the second-car problem - which seems likely, not least because the intermittent nature of Beijing's restrictions reduces the incentive to buy another vehicle - experts seem to agree that the city, and the nation, must enact many more changes before the gray cloak of smog goes away.