In the world’s poorest regions, one in five children will not live to age five. This is mostly due to environmental hazards, the World Health Organization (WHO) reported last week, in its first study of children’s unique susceptibility to harmful chemicals at various stages of development.

The comprehensive report authored by 24 scientists found that more than 30% of disease in children, who constitute one third of the world’s population, is attributable to harmful environmental factors such as water and air pollutants, pesticide residue in food and lead in soil, among many others. Children most affected are those who lack proper nutrition and live without access to clean water or sanitation.

The study also revealed that the stage at which developing children are exposed to environmental hazards is just as important as the degree to which they are exposed.

“Children are not just small adults,” said Dr. Terri Damstra, the WHO team leader for the Interregional Research Unit. “Children are especially vulnerable and respond differently from adults when exposed to environment factors, and this response may differ according to the different periods of development they are going through.”

New evidence shows that exposure to chemicals during childhood may increase the risk of contracting diseases such as cancer and heart disease later in life.

To learn the all-important cause behind the earth’s current contaminated state—and how it ultimately will be solved—read our extensive report This POLLUTED EARTH.