The nervous system and the lungs have little ability to repair themselves if harmed during development. The nervous system developes throughout childhood, making it especially vulnerable. Immature organ systems may not be capable of metabolizing or neutralizing certain foreign materials quite as effectively for purposes of excretion, to rid the body of waste. Also, an immature immune system will not respond as quickly to remove or neutralize foreign materials harmful to the body. Some exposure pathways are significant for children due to their habits.
Children spend more time playing outdoors than adults.
Also, they play in soil and standing water found in fields, empty lots, and near canals which may contain pesticides or industrial waste. Per pound of body weight a child ingests more food, drinks more water, and inhales more air than an adult. So exposure to dangerous substances translates into higher doses for children under the same exposure scenario as adults. Some drugs elicit an odd response in children when compared to the average adult. Typically these drugs sedate adults, in many children they have been known to cause hyperactivity.