The U.S. Food and Drug Administration's (FDA) increased attention to food imports from China is an indicator of safety concerns as imported food becomes more common in the United States. U.S. food imports from China more than tripled in value between 2001 and 2008. Addressing safety risks associated with these imports is difficult because of the vast array of products from China, China's weak enforcement of food safety standards, its heavy use of agricultural chemicals, and its considerable environmental pollution. FDA import refusal data highlight food safety problems that appear to recur in trade and where FDA has focused its import alerts and monitoring efforts. FDA refusals of food shipments from China suggest recurring problems with "filth," unsafe additives, labeling (typically introduced in food processing and handling), and veterinary drug residues in fish and shellfish (introduced at the farm). Chinese authorities try to control food export safety by certifying exporters and the farms that supply them. However, monitoring such a wide range of products for the different hazards that can arise at varying points in the supply chain is a difficult challenge for Chinese and U.S. officials.