It's an area about the size of Germany: 29.3 million hectares (72.4 million acres) of cropland worldwide is mainly irrigated with what's flushed down the toilet, a study published in "Environmental Research Letters" reports.
That translates into a major health risk for all people who eat the food that grows on that farmland. As many as 885 million urban residents are being exposed to dangerous pathogens, including parastites and bacteria, according to the researchers'. Untreated wastewater might contain parasite eggs or bacteria leading to cholera, but mostly bacteria causing diarrhea. According to Drechsel, children below the age of five are the most sensitive to diarrhea, often with fatal consequences. Apart from a health risk, it is also an environmental issue. If sewage enters lake and rivers, it can contaminate waters and harm wildlife. Wastewater also contains high amounts of phosphates and other substances which fuel algae growth, taking up even more oxygen and creating a so-called "dead zone" where fish and other animals cannot live.
Irrigation with untreated wastewater is especially an issue around bigger cities without proper sewage treatment, and - according to the recent study - particularly in China, India, Pakistan, Mexico and Iran.
In Ghana, farmers around large cities mainly grow vegetables, Drechsel says.
"Vegetables are cash crops number one, especially exotic leafy vegetables." Such crops require regular irrigation, unlike most staple foods such as grains.
Nearly a million people in urban Ghana eat vegetables produced with polluted water every day, he says.