There are an estimated more than 20 million domestic workers in China, most of whom are women, according to a report published in June last year by the Department of Trade in Services and Commercial Services of the Ministry of Commerce and the Chinese Academy of International Trade and Economic Cooperation.

Data from the National Population Census 2010, the most recent census, shows that there are about 652 million women on the Chinese mainland. This means about one in every 32 Chinese women, no matter how old they are, is a domestic worker.

And the country's aging population is pushing the ratio even higher. As of the end of 2014, people age 60 and older had reached 212 million, which is 15.5 percent of China's total population. And the number of seniors in the country is expected to continue to rise.

Tang Binyao, an associate professor of social work at the University of Jinan, said 16 to 17 percent of residents in Jinan, provincial capital of Shandong province, are older than 60. And there is great demand for helpers among these senior citizens, especially those who are living alone.

Traditionally, seniors tend to refuse to go to nursing homes after retirement in China, and there are not enough elderly care facilities.

The growing middle class is another group with rising demand for domestic workers, said Chen Jiyan, a program officer for domestic workers at the Beijing Hongyan Social Work Service Center.

The gap between the rich and the poor is growing, and women who would have traditionally cared for the home but now have good jobs are transferring this duty to women at the lower end of society, Chen said.

"People also don't trust each other as much as before. While children younger than 6 might have been left alone or in the care of neighbors before, many families now hire domestic workers to take care of them to ensure their safety," Chen said.

Gao Xin, author of Blooming Viola Philippica: The Oral History of Domestic Workers, said the cheap cost of labor in China makes hiring domestic workers affordable for many.

"My classmates in Canada cannot afford domestic workers, so they have to do all of housework themselves," she said.

The current minimum hourly wage for domestic helpers in Ontario, Canada, is C$12.40 ($9.60) per hour, with the rate rising to C$12.55 per hour on Oct 1, according to the website of the Ministry of Labor Ontario.

In Beijing, however, a live-in domestic worker will usually make about 4,500 yuan ($674) a month with four days off. So the hourly wage is only about 21 yuan if they work eight hours a day. It's not unusual for them to work more than eight hours a day.