“I wouldn’t drink that if I were you.”

I’m sprawled inelegantly on the concrete banks of the Thames, leaning down to scoop some of its green, murky liquid into an empty water bottle. Already, it’s a scene of utter chaos. An unusually large wave has caught me off guard and sloshed water all up my sleeve. Hungry seagulls are swooping down from every direction. A large crowd of ducks has come to investigate – and two formidable geese are making hissing noises.

The advice is proffered by a man on a nearby boat, who has been watching me with an expression of amused bewilderment for some time. Alas, he disappears below deck before I can tell him that drinking the water is precisely what I’m going to do.

My mad plan begins with a startling fact. According to the World Health Organisation, some 2.1 billion people on our pale blue planet do not have a safe source of water to drink. Consequently, more people die from drinking contaminated water every year than from any form of violence, including war.

Read more on bbc.com.