The World Health Organization (WHO) has issued the first-ever report highlighting children's special susceptibility to harmful chemical exposures at different periods of their growth. This new volume of the Environmental Health Criteria series, titled Principles for Evaluating Health Risks in Children Associated with Exposure to Chemicals, is the most comprehensive work yet undertaken on the scientific principles to be considered in the assessment of health risks in children. It highlights the fact that for children, the stage in development when exposure occurs may be just as important as the magnitude of the exposure.

The scientific principles that the document proposes for use in evaluation of environmental health risks in children will help the health sector, researchers, and policy makers to protect children of all ages, through improved risk assessments, appropriate interventions, and focused research, to become healthy adults.

"Children are not just small adults," said Dr. Terri Damstra, team leader for WHO's Interregional Research Unit. "Children are especially vulnerable and respond differently from adults when exposed to environmental factors-and this response may differ according to the different periods of development they are going through. For example, their lungs are not fully developed at birth, or even at the age of eight, and lung maturation may be altered by air pollutants that induce acute respiratory effects in childhood and may be the origin of chronic respiratory disease later in life."

Air and water contaminants, pesticides in food, lead in soil, and many other environmental threats that alter the delicate organism of a growing child may cause or worsen disease and induce developmental problems. Over 30 percent of the global burden of disease in children can be attributed to environmental factors.

Children have different susceptibilities during different life stages because of their dynamic growth and developmental processes. Health effects resulting from developmental exposures that occur prenatally and at birth include miscarriage, still birth, low birth weight, and birth defects. Effects of exposure in young children include infant mortality, asthma, and neurobehavioral and immune impairment. Effects of exposure in adolescents include precocious or delayed puberty. Emerging evidence suggests that exposures to certain environmental chemicals during childhood may contribute to increased risk of certain disease in adulthood, including cancer and heart disease.

The vulnerability of children is increased in degraded and poor environments. Neglected and malnourished children suffer the most. These children often live in unhealthy housing, lack clean water and sanitation services, and have limited access to health care and education. For example, lead is known to be more toxic to children whose diets are deficient in calories, iron, and calcium. One in five children in the poorest parts of the world will not live longer than their fifth birthday-mainly because of environment-related diseases.

The central focus of the new study is on the child, including developing embryo, fetus, infant, and adolescent, and on the need for a good understanding of the interactions between exposure, biological susceptibility, and socioeconomic and nutritional factors at each stage of a child's development.

The work was undertaken by an advisory group of 24 scientific experts representing 18 countries and was convened to provide insight, expertise, and guidance, and to ensure scientific accuracy and objectivity. Once the text was finalized, it was sent to over 100 contact points throughout the world for review and comment, and was also made available on the Web site of WHO's International Programme of Chemical Safety for external review and comment for a period of two months.