Crops covering almost 36 million hectares - an area roughly the size of Germany - are irrigated with water from rivers and lakes used by cities within 40 km (25 miles) upstream to discharge sewage, according to an international team of researchers.
About 80 percent of these crops - 29 million hectares - are in countries with very limited wastewater treatment, such as China, India, Pakistan, Mexico, according to the paper published in the journal Environmental Research Letters.
The study was the first to use remote sensing and geographic information systems (GIS) for its data analysis, improving on earlier estimates based on case studies and guesswork, researchers said.
Untreated wastewater, even when diluted, poses health risks for both farmers and consumers, said Pay Drechsel, one of the authors.
"In wastewater we have a lot of faecal contaminants from excrement," Drechsel, a scientist at the International Water Management Institute (IWMI), told the Thomson Reuters Foundation by phone.
Farm workers can get skin infections from contact with contaminated water, while consumers are at risk of contracting worms, diarrhoea and even cholera from vegetables eaten raw, he said.
Pollution from human and animal waste affects nearly one in three rivers in Latin America, Asia and Africa, and some 3.4 million people die each year from diseases associated with pathogens in water, according to the United Nations.
The threat will get worse as the global population grows and informal settlements not supported by proper infrastructure spring up in rapidly expanding cities in developing countries.
"As long as investment in wastewater treatment lags far behind population growth, large numbers of consumers eating raw produce will face heightened threats to food safety," said the study's lead author Anne Thebo, of the University of California, Berkeley.