Researchers in developing countries have discovered a free nuclear fusion-powered way to clean polluted water and kill disease-causing organisms. It’s called the “sun” and it’s available to anyone with relatively clear skies and a few plastic water bottles. Although first identified in the 1980s, researchers have been perfecting the technique–known as SODIS for solar water disinfection–and preaching its virtues around the world.

“The number of people regularly using SODIS for the treatment of their drinking water is steadily increasing,” says Regula Meierhofer of the Swiss Government’s Department of Water and Sanitation in Developing Countries, one of the groups most active promoting the idea. Today, at least 5 million people in about 30 countries disinfect their drinking water daily with SODIS, and 750,000 more join the ranks each year. SODIS works by exposing contaminated water to the sun’s UV rays, destroying the genetic material and cellular structure of viruses, bacteria, and protozoa. PET plastic bottles work best, as they’re both durable and allow much of the UV radiation to pass through them. Bottled water is left outside for at least six hours (or 48 hours if it’s cloudy), and then stored for future use. Since both polluted water and plastic bottles are abundant in many developing countries, SODIS is catching on and cutting the incidence of diarrhea by more than 85% in some places.

But that’s still just a drop in the proverbial bucket. At least 1.2 billion people are without clean water globally, and 1.8 million children die each year from diarrhea (often water-borne), the combined population under the age of 5 in New York and London, according to a study for the Swiss Government by Urs Heierli.

So why isn’t a cheap, practical and effective solution not an instant success? Heierli rephrased the question in a more telling way in his report: “Why is it so hard to get safe water to the poor–and so profitable to sell it to the rich?”

Basically, it’s a marketing problem. Even in places where SODIS training is readily available, only about half of the households trained in the technique actually adopt it. SODIS training cannot be stopped after the first year of promotion. “People need reminders to form solid habits,” says Meierhofer. They also need to be convinced of its value: Families capable of spending the modest time and money for SODIS often prioritize other things (such as buying soft drinks).

If SODIS and comparable treatment methods such as boiling, sand or ceramic filtration, and chlorination, can finally gain traction, it’s a safe bet that a profitable business model will be needed to solve the problem, argues Heierli. He suggests a sustainable supply chain, massive demand catalyzed through social marketing campaigns, and some good old fashioned salesmanship. Hopefully, those without clean water will soon be able to drink to that.