There are three core clinical symptoms of ADHD, which are attention deficit disorder, hyperactivity and impulsivity. Children with ADHD exhibit inattention disproportionate to their age and are easily distracted by environmental distractions. Hyperactivity is characterized by excessive activity, lack of quietness, more nonsense, inability to sit quietly in class, often standing up, and in severe cases, leaving the seat and walking around, affecting the learning of other students. Impulsivity, or self-control, is very poor, and children are often overexcited by external stimuli. Some children with severe ADHD may also be clumsy, unable to walk on their own, and have inflexible movements. The cause of ADHD is not yet fully understood, and most believe that it is the result of a combination of genetic and environmental factors. Genetic factors are also largely known in ADHD, and there is a significant family clustering phenomenon. Environmental factors, such as maternal smoking and alcohol consumption during pregnancy, are associated with a higher incidence of ADHD in children. Children with lead exposure, i.e., clinical lead poisoning, also have a significantly higher incidence of ADHD than normal children. There are also abnormalities in brain development, neuroanatomical abnormalities such as abnormal frontal lobe development, as well as neurophysiological and neurobiochemical abnormalities, such as low dopamine and norepinephrine neurotransmitter function in the central nervous system and 5-hydroxytryptamine hyperfunction, which are also closely associated with the onset of ADHD. Other factors such as psychosocial factors, parental discord, inappropriate family education, family economic difficulties, separation from parents during childhood, and inappropriate schooling are also associated with a higher incidence of ADHD than normal children. Zinc and iron deficiency can also significantly increase the incidence of children with ADHD. Excessive intake of artificial colors in food and food containing too much lead can also lead to ADHD episodes, although not to the extent of lead poisoning.