Editor's note: Alexandra Harney is a Hong Kong-based writer and the author of the forthcoming book "The China Price: The True Cost of Chinese Competitive Advantage" (Penguin Press, 2008).
HONG KONG, China (CNN) -- In the crowds still stranded by snow at train stations around China stand some of the country's most valuable economic assets: migrant workers.
This group of 150 million to 200 million farmers -- more than the population of the United Kingdom, France and Australia combined -- account for the majority of employees in China's world-beating manufacturing sector, the bulk of its coal miners and most of its construction workers.
During the past two decades, according to a conservative estimate from UNESCO and the Chinese Academy of Social Sciences, migrants have contributed 16 percent of gross domestic product growth.
Living for years at a time in coastal cities, China's migrant workers have built the country's skyscrapers and assembled its exports, sending tens of billions of dollars in earnings home to their families in poor inland provinces. For the workers known as "factories without smoke," the Chinese New Year holiday is often their only annual vacation.
The forces that brought these smokeless factories to the cities took shape in the early 1980s, when Beijing, as part of an easing of central controls on the economy, loosened internal mobility regulations. Farmers have been pouring out of the countryside ever since, in what is believed to be the world's largest internal migration.
They leave for mostly economic reasons: wages in the cities are higher than what workers could earn at home. And life there, many find, is more exciting than back on the farm.
Today, migrants dominate the Chinese labor force in dirty and dangerous trades: 70 percent of construction workers, 68 percent of manufacturing employees, and 80 percent of coal miners are migrant workers. But not all are on their hands and knees. More than 60 percent of staff in the service trade, according to state media, are migrants as well.
On average, migrants tend to be among the best educated people in their villages. Still, many have little more than a junior high school diploma. Many migrate as teenagers, often with friends or neighbors, leaving behind their family in the countryside. More than half are men, but the toy and shoe factories of southern China prefer women -- they are easier to control, managers say, and their fingers more nimble.
Wages vary by city and company, but many migrants in export factories in the south take home about Rmb1,000 a month ($139) -- or even more. They sleep 12 to a room in bunkbed dormitories furnished by their employers, working six and sometimes seven days a week for months at a time. Wages are not always paid on time, occasionally not at all. Watch more about migrant workers' living conditions in dormitories »
Victims of occupational disease, lacking of insurance
As little as a fifth of migrant workers in southern China's Guangdong province, according to one Hong Kong non-governmental organization, have medical insurance. China's household registration or hukou system links social benefits to the place where one is registered, and most migrants are still registered in their rural hometowns, hundreds of miles away from where they work. About 90 percent of the victims of occupational disease in China are migrants.
These migrants' schedules are dictated by the fluctuations of demand from their foreign customers: winter is peak season for lawn furniture factories, for example. But most of the factories in southern China are busiest in summer, as they fill orders for the Christmas season.
Many of these plants close for the first months of the year and take the Chinese New Year holiday off, triggering an exodus of migrants as those who can afford the train and bus tickets travel home to see their families. Watch a migrant worker's 1,000-mile journey home »
Much has changed since Chinese farmers began arriving in the cities two decades ago. Some of today's migrant workers are "second generation" -- the sons and daughters of the first generation of migrant workers. Most were born after China introduced a family planning policy in 1979, so they come from smaller families. Second generation migrants tend to be more demanding employees: they are pickier about where they work, preferring factories with better facilities and wages.
Their preferences, along with a rapid growth in factories in the Yangtze River Delta around Shanghai and a rise in rural incomes, have contributed to labor shortages in Guangdong province in the last several years. In response, the government is raising the minimum wage and strengthening labor laws. Forced to compete for workers for the first time in more than a decade, factory managers are building basketball courts and libraries, installing air conditioners and improving their cafeteria menus.
Beijing, too, is realizing the importance of migrant workers as a political constituency. The state-controlled labor union, the All-China Federation of Trade Unions, is targeting migrants in a recruitment drive. The government is expanding insurance coverage for migrant workers.
State media cover their hardships regularly. "Migrant Workers: We Need Them Just Like They Need Us," read one headline in the China Daily last March. As the recent appearance of premier Wen Jiabao at the packed Guangzhou train station illustrated, migrants are crucial to keeping China's economic development on track. E-mail to a friend