Research into how to make reuse of urine and feces safe in agriculture was carried out by Swedish researchers, for example Hakan Jönsson and his team, whose publication on "Guidelines on the Use of Urine and feces in Crop Production" was a milestone which was later incorporated into the WHO "Guidelines on Safe Reuse of Wastewater, Excreta and Greywater" from 2006. The multiple barrier concept to reuse, which is the key cornerstone of this publication, has led to a clear understanding on how excreta reuse can be done safely.