Contribution of heavy metals to developmental disabilities in children

Abstract

Children are exposed to heavy metals, since they occur naturally in the environment and levels of some are raised by pollution. Both lead and mercury are neurotoxic and there is a risk of developmental problems following exposure. High levels of exposure by either metal can cause mental retardation, and lower levels of exposure may cause cognitive and behavioral changes. The lowest level of lead exposure believed to adversely affect neurodevelopment has steadily decreased, and the current blood lead level considered safe is 10 μg/dl or less. Children are exposed to chronic low levels of inorganic mercury from dental amalgams and of methylmercury when they eat fish or fish products. Both mercury vapor and methylmercury cross the placental and blood–brain barriers and at high exposures cause brain damage. No safe level for mercury vapor or methylmercury exposure has been definitely established, but a peak maternal hair level of ≥10 ppm following methylmercury exposure is presently thought to present about a 5% risk of cognitive impairment. This level can be readily achieved by eating fish regularly. Determining the lowest level of heavy metal exposure associated with any degree of alteration in children's cognition or behavior presents a difficult research challenge. MRDD Research Reviews 1997;3:239–245. © 1997 Wiley-Liss, Inc.