Water, air and soil pollution, along with other environmental factors, contribute to 40 percent of deaths worldwide each year, a new study concludes.
In a review of research into the effects of environmental pollutants and other sources of environmental degradation, Cornell University ecologist David Pimentel estimates that 62 million deaths per year (40 percent of all that occur) can be attributed to environmental factors, particularly organic and chemical pollutants that accumulate in the air we breathe and the water we drink.
Though scientists and organizations such as the United Nations and the World Health Organization have begun keeping tabs on the role environmental pollution plays and were aware of the enormous impact that some pollutants have, "we were surprised with the number," Pimentel said.
This "suggests the importance of the environment as it's related to our deaths," he told LiveScience.
With an estimated 1.1 billion people in the world lacking access to clean water (according to WHO estimates), it is little wonder that waterborne infections account for 80 percent of all infectious diseases in the world.
"Water is one of the major concerns, without any question," Pimentel said, because everyone must use it for drinking, cooking, washing and bathing.
Water contaminated with untreated sewage and fecal matter can facilitate the transmission of diarrheal diseases such as cholera (bacteria that live in feces), intestinal infections (which can compound health issues by causing malnutrition) and other diseases—all of which kill millions every year, especially children.
A 2004 study by the Population Resource Center found that 2.2 million infants and children die each year from diarrhea, caused largely by contaminated water and food. And, according to their estimates, polluted water in Africa and India causes 1.4 million deaths each year as a result of diarrheal diseases such as cholera and dysentery.
"Water sanitation and hygiene are, considered globally, one of the big, big causes of disease," said WHO scientist Annette Prüss-Üstün.
Most of the problems from contaminated water are an issue in developing countries, where there is little infrastructure to deal with sewage and other water sanitation issues—people in developing countries dump 95 percent of their untreated urban sewage into the same lakes and rivers they use for drinking and bathing, according to the United Nations.
"While in many countries there is still [a] water supply, proper disposal and treatment of sewage is a little bit less common in developing countries," Prüss-Üstün said.
In India, for example, only a handful of cities have water treatment facilities, according to Pimentel.
"It's a challenge just to get clean water," he said.
Air pollution is another big killer. The WHO ranks it as the eighth most important risk in the burden of disease and deems it responsible for 3 million deaths each year through diseases such as pneumonia, chronic bronchitis and lung cancer.
In developing countries, indoor air pollution is a major problem because most people rely on open stoves fueled by dung, wood, crop waste or coal to cook and heat poorly-ventilated homes.
A little more than half of the world's households use these solid fuels for cooking, "which is huge," Prüss-Üstün said. The smoke from these stoves accumulates in abodes, exposing those inside—mainly women and children—to the hazardous pollutants released from the fuel.
"In some houses you enter into the kitchen, and even though you might even have a permanent opening in the house … you can hardly see … the wall on the other side, so thick is the smoke," Prüss-Üstün said.
More than 200 different chemicals can be found in the smoke, and 14 of them are known carcinogens, Pimentel said. Every year, this indoor air pollution kills 1.6 million people (or one person every 20 seconds), according to the WHO.
Outdoor air pollution, on the other hand, accounts for some 800,000 deaths per year—about half as many as for indoor air—because the pollutants are much less concentrated.
"Indoor air pollution can be 100 times more concentrated," Prüss-Üstün said. "There's really a big difference."
But outdoor air pollution still impacts health—both in developing countries and cities in the developed world—through chronic respiratory problems, acute problems (such as asthma) in children and "a long list of cardiopulmonary diseases in adults," Prüss-Üstün said.
And as the world's population continues to grow and shifts to urban areas, outdoor air pollution will become a more serious health threat, Pimentel added.
Health is also affected by the tens of thousands of chemicals put into the environment by industrial processes and other sources.
With most of these chemicals, the environmental and biological effects they may have, including their toxicity to humans, is largely unknown, particularly because so many are used in combination.
"It's impossible to estimate," Prüss-Üstün said. "It's truly impossible to estimate more precisely, because … there are so many toxins."
But according to studies cited in Pimentel's review, detailed in a recent issue of the journal Human Ecology, chemical exposures can contribute to cancers, birth defects, immune system defects, behavioral problems, altered sex hormones and dysfunctions in specific organs.
Americans of all ages carry at least 116 foreign chemicals in their bodies, according to Pimentel's review, including the pesticide DDT (which still persists though it was banned three decades ago in the United States), lead and mercury (with coal-powered plants being the largest source of mercury pollution).
The uncertainty in some of these connections has led to lower, more conservative estimates of the part that environmental pollution plays in the global burden of disease. WHO estimates have linked only 25 percent of the global disease burden to pollution, because they have included only the more firmly known links, Prüss-Üstün said.
But what is clear, both Prüss-Üstün and Pimentel say, is that a large number of deaths could be prevented if developing countries were educated on and helped with the problems posed by water and indoor air pollution, which while fairly easy to solve in the developed world are beyond the means of those affected in developing countries to fix on their own.
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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.