The causes include unsafe water, lack of sanitation, poor hygiene practices and indoor and outdoor pollution, as well as injuries.
The new numbers equate to these pollutants being the cause of one in four deaths of children 1 month to 5 years old.
One new report highlights that the most common causes of child death are preventable through interventions already available to the communities most affected. These causes are diarrhea, malaria and pneumonia, which can be prevented using insecticide-treated bed nets, clean cooking fuels and improved access to clean water.
"A polluted environment is a deadly one -- particularly for young children," Dr. Margaret Chan, the WHO director-general, said in a statement. "Their developing organs and immune systems, and smaller bodies and airways, make them especially vulnerable to dirty air and water."
Infants exposed to indoor or outdoor air pollution, including secondhand smoke, have an increased risk of pneumonia during childhood as well as an increased risk of chronic respiratory diseases -- such as asthma -- for the rest of their lives, one report states.
The global body also highlighted the increased risk of heart disease, stroke and cancer from exposure to air pollution.
More than 90% of the world's population is thought to breathe air that violates quality guidelines set by the WHO.
The reports further list ways in which these risk factors can be removed to prevent disease and death.
"Investing in the removal of environmental risks to health, such as improving water quality or using cleaner fuels, will result in massive health benefits," said Dr. Maria Neira, director of the WHO's Department of Public Health, Environmental and Social Determinants of Health. "A polluted environment results in a heavy toll on the health of our children."
The growth of electronic and electrical waste is also a concern, according to the report. If not disposed of correctly, waste can expose children to toxins that can harm intelligence and cause attention deficits, lung damage and cancer.
Also among the fears: an increasing risk of climate change, due to rising temperatures and carbon dioxide levels, boosting pollen growth and possibly asthma. An estimated 44% of asthma cases among children worldwide are thought to be related to environmental exposures, the reports say.
In addition to highlighting the burden borne by young children, the new reports suggest ways in which risk factors -- and therefore death rates -- can be reduced.
These include reducing air pollution, improving access to clean water and sanitation, protecting pregnant women from secondhand smoke and building safer environments in order to reduce accidents and injuries.
"Both indoor and outdoor air pollution have an important effect on the health and development of children, and not just in the stereotypical 'polluted cities' context but also for very poor rural families who cook indoors," said Joy Lawn, professor of maternal reproductive and child health epidemiology at the London School of Hygiene and Tropical Medicine.
"Clean water is taken for granted by families in high-income countries, and yet those children in the hottest climates, facing the greatest risks of infectious diseases, are the very ones with least access to clean water."
But Lawn added that pollution is not the only risk factor when it comes to child mortality.
"We also need to be careful in attributing these deaths just to dirty water or pollution," she said. "To prevent deaths from pneumonia, we also need vaccines and antibiotics; from malaria, we also need bed nets and anti-malarials. It Is not just about pollution."
Other potential solutions mentioned in the reports are removing mold and pests from housing, removing lead paint, ensuring sanitation and good nutrition at schools and using better urban planning to create more green spaces in cities. Safe management of industrial waste by industries is also highlighted, along with stopping the use of hazardous pesticides and child labor in agriculture.
The report "highlights the scale of the problem of how environmental pollution affects the health of children across the globe," said John Holloway, professor of allergy and respiratory genetics at the University of Southampton. Holloway recently authored a report on the lifelong impact of air pollution.
"We must also remember that it is not just the acute effects of pollution on children's health mentioned in the report that we need to be concerned about, it is also the potential long-term effects of exposure to pollutants in early life that can have lifelong effects on health and well-being," he said.
Holloway also stressed that this is not a concern solely for developing countries. "Exposures such as air pollution and secondhand tobacco smoke affects the health of children in developed countries such as the UK as well," he said.
But, like the WHO, he also stressed that things can be done to help solve the problem and said authorities and individuals should act now -- as well as think long-term -- to protect the health of future generations.
"We all have a responsibility for reducing environmental pollution," he said. "This is going to require changes in society such as better monitoring of pollution and taking into account the true long-term economic cost of pollution when assessing the cost of measures to reduce environmental pollution."