More than four billion people around the world lack access to adequate toilets and sanitation systems, according to the United Nations. An estimated 432,000 die from diarrhea annually, as the result of untreated human waste that gets into water system and food supply chain and spreads diseases. It's a crisis receiving particular international attention on World Toilet Day.

It's difficult to have a discussion about toilets without talking about human waste, but a giant inflatable toilet displayed every year at the United Nations helps start the conversation about how safely managed toilets remain out of reach for so many people.

"We still have 673 million practicing open defecation," said Tom Slaymaker, senior statistics and monitoring specialist at UNICEF, who worked with the World Health Organization on a joint progress update about the global sanitation situation. 

That's nine percent of the global population who lack safely-managed sanitation services — people who instead use bushes, rivers and beaches.

Despite huge strides made to provide everyone access to clean toilets and sanitation systems — including halving the rate of open defecation from 21 percent in 2000 to just nine percent today — the issue persists especially in Sub-Saharan Africa.

"Very often, it's the poorest and most marginalized populations who lack these services, and therefore they are facing the biggest risks," said Slaymaker.

"They are the ones who are exposed to unmanaged waste in their environment, and if they're growing up in that environment, then it can have very serious impacts, especially on young children."

Dr. Les Roberts, renown epidemiologist and professor of Population and Family Health at Columbia University, explained how poverty, malnutrition and the lack of sanitation make for a potent and deadly mix.

"Diarrhea kills children who are very poor, who when they get that little bit of nutritional deficit you get with an episode of diarrhea, they never catch up nutritionally, and that makes them more and more vulnerable to the next episode with diarrhea or to the next infection with pneumonia or something else."

The problem isn't just open defecation.

Toilets from which human waste is safely collected, treated and disposed of, remain out of reach for some 4.2 billion people, opening them up to the risk of diarrhea diseases and exposure to pathogens like cholera, typhoid and dysentery.

To fix this, the United Nations has ambitious plans to make sure everyone has access to clean, safe toilets and sanitation systems by 2030.

But the world is not on track to reach that Sustainable Development goal.

"I think we would need to increase the current rates of progress by about three times in order to meet the goals that world leaders have set for the next 11 years," warned Slaymaker.  

Inequities and a lack of education have been key barriers, said Dr. Roberts.

"When a community, especially a rural community doesn't want to use toilets, it is very hard for outsiders to make them use toilets. It takes education and changing social values to make hygiene and toilets become a norm." 

(Cover photo: Indian women hold latrines as they participate in the International Toilet Festival in New Delhi on November 18, 2014, the eve of World Toilet Day./ VCG Photo)