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Dirty air and Watch Psych 2: Lassie Come Home Onlinewater aren't just unpleasant, they have deadly consequences for the world's youngest inhabitants.

More than 1 in 4 global deaths of children under 5 years old are due to polluted environments, including smoggy air, sullied water and poor sanitation, the World Health Organization (WHO) said on Monday.

SEE ALSO: 2 billion children are breathing toxic air, UNICEF reports

Unhealthy living conditions can result in fatal cases of pneumonia, malaria and diarrhea, which together kill some 1.7 million children every year, the U.N. health agency found in two new reports.

"A polluted environment is a deadly one, particularly for young children," WHO Director-General Margaret Chan said in a statement.

"Their developing organs and immune systems, and smaller bodies and airways, make them especially vulnerable to dirty air and water," she said.

A young boy scavenges in the Anlong Pi landfill in Siem Reap, Cambodia. Credit: omar havana/Getty Images

Not all of WHO's findings are so grim.

The agency said child deaths have dropped drastically in recent decades, from 12.7 million under-five deaths in 1990 to about 5.9 million in 2015. Improved sanitation, better access to drinking water, lead-free fuels and other efforts are visibly improving children's well-being.

WHO pointed to five main causes of pollution-related deaths. They include:

Air pollution

Outdoor pollution from factories, power plants and vehicle tailpipes cloud the air with soot, carbon dioxide, mercury and a host of other harmful chemicals. Cooking inside over open flames and second-hand smoke bring dangerous air pollution indoors.

Around 570,000 children under five die each year from respiratory illnesses, including pneumonia and asthma attacks.

Original image has been replaced. Credit: Mashable

Outdoor pollution is particularly pronounced in industrial parts of eastern China and India. Household air pollution is highest in Sub-Saharan Africa and South Asia, where many families burn dung, wood and charcoal to cook food and heat their homes.

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Poor access to clean water and sanitation

Things that many of us take for granted—clean tap water, flushing toilets, hand soap and closed landfills—are still widely unavailable in communities around the world. As a result, children pick up an array of harmful bacteria, and the consequences can be fatal.

Diarrhea alone kills about 361,000 children under five each year; it's a result of poor access to clean water, sanitation and hygiene, according to WHO.

Children wash themselves and fetch water at a camp in Sittwe, Myanmar. Credit: Jonas Gratzer/Getty Images

Arsenic poisoning is a particularly large threat in Southeast Asia. Many people pump their water supplies from the ground, which contains high levels of the cancer-causing chemical in Bangladesh and parts of neighboring countries.

However, conditions are improving in many places.

In 2015, about 91 percent of the global population used "improved" drinking water, meaning water protected from outside contamination like fecal matter. That's up from 76 percent of the population in 1990. Since that year, about 2.1 billion people have also gained access to improved sanitation.

Prenatal conditions

Dirty air and water can harm children even before they're born. Prenatal exposure to arsenic, second-hand smoke and air pollution hinders a child's development in the mother's womb, resulting in life-threatening complications.

Unsanitary conditions at health clinics also threaten newborns in their earliest days.

A Congolese baby, ill with cholera, receives treatment in a clinic in the town of Goma. Credit: Uriel Sinai/Getty Images

Some 270,000 children die during their first month of life from prematurity and other conditions that could be prevented if families lived in cleaner environments or had access to cleaner health facilities, WHO said.

Malaria

Children in Africa disproportionally suffer from this vector-borne disease. Of the 200,000 children under five who die from malaria each year, about 95 percent live in an African country, WHO found.

Fortunately, childhood malaria deaths are falling overall.

Via Giphy

From 2000 to 2015, the number of malaria deaths in under-five children fell by 58 percent, both globally and in Africa. In Sub-Saharan countries, 68 percent of young children slept under insecticide-treated nets in 2015, a huge increase from only 2 percent in 2000.

Unintentional injuries

Poisoning, falls and drowning are all environment-related threats that can result in unintentional injuries. About 200,000 children under 5 years old die each year from such an accident, WHO found.

In its report, the U.N. agency offered a number of ways that government sectors can collaborate to improve the health and survival of the world's young children.

For instance: ensuring cleaner fuel sources for home cookstoves; providing clean water and reliable electricity in health facilities; avoiding using hazardous pesticides in agriculture; and creating more green spaces and safe walking paths in urban areas.

Health experts also warned of two rising threats to the health of children, and everyone else: Climate change and the world's growing heap of toxic, leaking electronic waste.

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Topics United Nations