Last month, the entire human race changed adjectives. No longer are we a rural species; as of May 23, 2007, humanity is urban.

This news comes to us from the United Nations Populations Fund: for the first time in our history, the majority of our species lives in towns and cities. In fact, researchers at two American universities claim that May 23 was the tipping point — the very day the 3,300,000,001th person left his or her farm for life in the big city.

But when people crowd into cities, problems of sanitation follow. I describe the relationship in my book: “As a population’s density grows, so too grows the threat from its accumulated poop. {A population’s} safety depends on being able to destroy poop faster than it can produce it.” With that in mind, the UN’s report should bode ill for the sanitary health of our species.

But according to UNICEF, the trend is the opposite: as worldwide population density has increased, we humans have actually gotten better at managing human waste.

In their assessment of world sanitation and drinking water, UNICEF reported that 58% of the world has adequate sanitation as of 2002. That’s up from 49% in 1990.

As horrified as I am that 42% of the world — 2.6 billion people — don’t have access to decent sanitation, I still see in those numbers a staggeringly positive trend. One would think that the world’s population growing by 950 million people in a twelve year period would overwhelm whatever sanitary solutions civilization had thus far devised. But the results have contradicted expectations: fewer people go without sanitation today than did in 1990, despite the fact that almost a billion more people were added to the mix.

What does it mean to have sanitary coverage? It doesn’t mean that you have a toilet that flushes into the sewers. While globalization is spreading the Western-style toilet to the far reaches of the Earth (a trend I hope to examine in my next book; and as the Western-style sitter replaces the Eastern-style squatter, will Western-style rates of hemorrhoids and colon cancer be far behind?), toilets and sewers aren’t the only receptacles UNICEF deems appropriate for humanity’s business. According to UNICEF , “access to sanitary means of excreta disposal is estimated by the percentage of the population using improved sanitation facilities. Improved sanitation facilities are those more likely to ensure privacy and hygienic use.” The facilities UNICEF looks upon with approval include:

• A connection to a public sewer
• A connection to a septic system
• A pour-flush latrine
• A simple pit latrine
• A ventilated improved pit latrine

Unimproved sanitation facilities are technologies like public or shared latrines, open pit latrines, and bucket latrines.

Human beings, at least those on a Western diet, produce an average of a half-pound of poop and two quarts of urine a day. A latrine servicing a hundred people thus accumulate fifty pounds of poop and fifty gallons of urine every twenty-four hours; at this rate, unimproved sanitary facilities can easily lapse into a health threat. Poop is a vector for viruses, parasites, and bacteria, and poop attracts insects and vermin who carry more of the same. Before toilets and sewers grew widespread in the late nineteenth and early twentieth centuries, poor sanitation killed millions. Modern cities don’t much suffer typhoid or cholera anymore, but in many less developed places, outbreaks still occur.

In my nascent career as sewage eco-warrior, I’ve given a lot of criticism to toilets. “They waste water,” I’ve said. “They demand an expensive sewage infrastructure,” I’ve said. “They don’t allow us to safely reclaim our waste as fertilizer,” I’ve said. I’ve imagined a bathroom utopia based on a yet-to-be-invented technology in which our waste is kept out of the sewers to avoid contamination and perhaps even harnessed to generate electricity. But with every criticism I deliver I try to give toilets their due: they represent a tremendous step for humanity. This year’s shift from rural to urban would not be possible had we not shifted from cesspool to toilet one hundred years ago. The externalities the toilet has created are worth the lives it has saved. Our next step is to reduce these externalities without sacrificing the gains we’ve achieved in sanitation, convenience, and ease-of-use.

The UN’s population report expects another 1.7 billion people to make the urban shift by 2030, warning of increases in poverty, violence, and pollution as they do. UNICEF underscores these warnings with their analysis of the world’s access to clean water and sanitation. But UNICEF’s report provides much hope to the exploding masses of humanity: despite all the problems facing our species, we’re finally starting to get better at the basics.