Nearly half the world's schools lack clean drinking water, toilets and hand-washing facilities, putting millions of children at risk of disease, experts warned Monday.

Almost 900 million children have to contend with a lack of basic hygiene facilities during their education, putting their health at risk and meaning some have to miss school.

"You can't have a quality learning environment without these basics," said Dr. Rick Johnston of the World Health Organization, a lead researcher on the project.