Seven billion. Sometime next week, that will become the number of human beings living on Earth, the United Nations projects. The world's population has grown incredibly fast in recent years, fueling both environmental and public health crises. It's a "sobering reminder of our planet's predicament," says Roger Martin at Britain's Guardian. Here, a brief guide, by the numbers to Earth's human inhabitants:

500 million
Estimated population of Earth in 1500

1.26 billion
Estimated population of Earth in 1850

2.53 billion
Estimated population of Earth in 1950

4.7 billion
Estimated global population in 1983, the year that we reached a point of consuming land, water, air, and other natural resources faster than they could be replenished

6 billion
Esimated global population in 1999

Number of people by which the global population increases every hour

Number by which the population of Qatar, the world's fastest-growing country, increases each day. "In developing nations, where improvements in health care and sanitation are seeing death rates fall, birth rates still remain relatively high," says the BBC.

Number by which the population of the fastest-shrinking country, Moldova, decreases each day. That's mainly due to emigration.

2.6 billion
Number of people around the world that lack any sanitation

200 million
Tons of human waste that go untreated each year

Percent of sewage in developing countries that is released directly into oceans, lakes, and rivers. "All this untreated sewage adds up to a major public health crisis," says Stephanie Pappas at MSNBC.

1.4 million
Estimated number of children that die each year because of that public health crisis. That's one child every 20 seconds. "Crapping in the bush," a.k.a. "open defecation," is a huge problem, says Rose George, author of The Big Necessity: The Unmentionable World of Human Waste and Why It Matters. Pathogens in human feces often end up contaminating the water supply.

Reduction in public health costs for every $1 invested in better sanitation, according to the U.N.

Daily living budget
for nearly half of the world's population

The number of Malawians it takes to equal the carbon footprint of one British person. By that calculation, a population increase of 10 million Britons would tax the environment as much as 220 million Malawians. "Our self-indulgent lifestyles are grossly inequitable, and must become much more modest," says Martin.

of sub-Saharan Africans that will exist for every European alive in 2100, according the U.N. In 1950, Europeans outnumbered sub-Saharan Africans 3 to 1. 

9.1 billion
Projected world population in 2050, according to the United Nations. That assumes that the current "global fertility rate" of 2.56 children per women will decline to 2.02 by 2040. In 1950, the rate was 4.92 children per woman. That's "an accomplishment of trial and sometimes brutally coercive error, but also a result of one woman at a time making her individual choices," says Julia Whitty at Mother Jones.

$6.7 billion
What it would cost, per year, to provide "modern family planning methods" to people who don't have access to them

$6.9 billion
Amount Americans are expected to spend on Halloween this year

Sources: BBC, Economic Times, Guardian, MSNBC, Mother Jones