The International Water Management Institute, which published the findings, said that more than half of farmland near 70 percent of cities in Third World countries is watered with sewage that threatens to spread epidemics.

An increasing demand for water and food has spurred the use of sewage to water crops but in many cases is the only form of irrigation for farmers who lack clean water, the study showed. It is mostly used to produce vegetables and cereals, and poses a major health risk to consumers of uncooked vegetables.