Developmental Disabilities and the Environment
People are exposed to a wide range of chemicals during their lives. Chemicals are present in food, clothes, furniture, housing materials, and more. Evidence suggests that many chemicals can cause damage to the central nervous system which can impact developmental disabilities. However, much of this evidence is based on studies of groups accidentally exposed or workers exposed to levels above those found in the environment. Read more about exposure and risk.
Avoiding or reducing children's exposure to harmful chemicals may help prevent some developmental disabilities. Most scientific studies about how the environment may impact developmental disabilities have been focused on intellectual disabilities and autism spectrum disorders (ASDs).
Exposures to lead, polychlorinated biphenyls (PCBs), and methylmercury are well-documented risk factors for intellectual disabilities. Lead is a well-studied toxin which affects the nervous system of children and adults. In children lead can cause learning disabilities and behavioral problems. At high levels, it can cause seizures, coma, or even death. Data and information about lead in children are available on the Tracking Network. Learn more about lead in children from CDC's Lead Poisoning Prevention Program.
Exposure to PCBs is associated with lowered intel�ligence. Eating PCB-contami�nated fish during pregnancy has been studied extensively and found to produce intellectual disabilities in children. Environmental exposure to very high doses of methylmercury also may affect intelligence and has been observed to cause intellectual disabilities. The most common way we are exposed to mercury in the U.S. is by eating contaminated fish and shellfish.
Most fish and shellfish contain small amounts of mercury. When people eat fish, some of this mercury can get into the body. The amount of mercury people take in depends on:
- What kind of fish and shellfish they eat,
- How often they eat fish and shellfish, and
- How much fish and shellfish they eat at each meal.
For women who are pregnant or about to become pregnant, eating too much fish that contains mercury can cause health problems for the developing baby. Eating too much fish that contains mercury could be a problem for young children. If a nursing mother eats too much fish that contains mercury, there could also be problems for the developing baby.
During the first several years of life, a child's brain develops rapidly. Exposure to mercury before birth and in the first few years of life may affect a child's development. This could cause problems with language, attention, memory, vision and coordination. Often, children have to be tested to find these problems. Read more about recommendations for eating fish safely.
Autism Spectrum Disorders
We do not know all of the causes of ASDs. However, we have learned that there are likely many causes for multiple types of ASDs. There may be many different factors that make a child more likely to have an ASD, including environmental, biologic and genetic factors. Read more about ASD research here.
What Research Tells Us
- Most scientists agree that genes are one of the risk factors that can make a person more likely to develop an ASD. Studies have shown that:
- among identical twins, if one child has an ASD, then the other will be affected about 60-96% of the time;
- in non-identical twins, if one child has an ASD, then the other is affected about 0-24% of the time; and
- parents who have a child with an ASD have a 2%–8% chance of having a second child who is also affected.
- ASDs tend to occur more often in people who have certain other medical conditions. About 10% of children with an ASD have an identifiable genetic, neurologic, or metabolic disorder, such as:
- Some harmful drugs taken during pregnancy also have been linked with a higher risk of ASDs, for example, the prescription drugs thalidomide and valproic acid.
- We know that the once common belief that poor parenting practices cause ASDs is not true.
- There is some evidence that the critical period for developing ASDs occurs before birth. However, concerns about vaccines and infections have led researchers to consider risk factors before and after birth.
There is still a lot to learn about environmental contributors to autism and research into some environmental factors is underway. Read more about research at CDC and the National Institute of Environmental Health Sciences.
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