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.