One-quarter of the children under 5 years old who die are killed by unhealthy environments, the World Health Organization (WHO) found in reports released Monday. About 1.7 million child deaths each year can be attributed to these unhealthy environments.

The environmental risks that can lead to these deaths included "indoor and outdoor air pollution, second-hand smoke, unsafe water, lack of sanitation, and inadequate hygiene," WHO noted in a press release. Small children often die from ailments like diarrhea, malaria and pneumonia, which could be prevented by giving kids access to good quality air and water.

"A polluted environment is a deadly one – particularly for young children," WHO Director-General Dr. 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."

Some 570,000 children under 5 years old die every year just from respiratory infections, which are caused by air pollution and second-hand smoke, according to WHO. Promoting cleaner air could save the lives of children.

"A polluted environment results in a heavy toll on the health of our children," Dr. Maria Neira, WHO director of the department of public health, environmental and social determinants of health, said in a statement. "Investing in the removal of environmental risks to health, such as improving water quality or using cleaner fuels, will result in massive health benefits."

Air pollution leads to some 7 million total premature deaths each year, a previous WHO study noted in 2014. The Global Burden of Disease project pegged that number last year at about 5.5 million. A large portion of those deaths happened in China and India, two countries experiencing rapid growth.

"In Beijing or Delhi on a bad air pollution day, the number of fine particles (known as PM2.5) can be higher than 300 micrograms per cubic meter," said Dan Greenbaum from the Health Effects Institute, according to the BBC. "The number should be about 25 or 35 micrograms."