A monopoly on salt production was introduced as early as 685 B.C. in the state of Qi on the Shandong Peninsula, though it may have existed even earlier than that, the scholar Rowan K. Flad writes in “Salt Production and Social Hierarchy in Ancient China.”

Under the system, the government designated who could produce salt, and the shipping of salt outside authorized districts was banned. The salt trade was long a significant source of revenue for the state, and helped provide revenue and pay for troops in far-flung outposts of the Chinese empire.

As China has industrialized, the contribution of the salt monopoly to overall tax revenues has greatly diminished, but it has still served important functions. As recently as the mid-1990s, China experienced widespread problems of preventable developmental disabilities because of a lack of iodine in children’s food supply. In 1995, the country mandated that edible salt be iodized to reduce the problem, and the salt monopoly was used to enforce that rule.

Studies have found a significant reduction in levels of iodine deficiency since the requirement was put in place.

Consumers have long complained about efforts to end the salt monopoly. When a proposal was put forward to eliminate the system in 2009, the central government backed down in the face of online opinion surveys that showed a majority of respondents wanted the government controls to remain in place, the China scholar James Reilly wrote in his 2011 book “Strong Society, Smart State.” Those concerns have revived again, as online comments have raised concerns about the inclusion of toxic industrial salts being mixed with edible salt, the magazine Foreign Policy has noted.