ADHD and the Environment

An article in today’s on-line version of Environmental Health Perspectives is creating quite a media buzz.

“Exposures to Environmental Toxicants and Attention Deficit Hyperactivity Disorder in US Children” (Braun, et al.) links specific environmental exposures (tobacco smoke, lead) to higher rates of ADHD. The idea is not a novel one, but the study does provide new evidence to support a widely-held belief, that the increase in neurodevelopmental disorders in the US is at least in part due to environmental toxicants, including second-hand smoke and heavy metal exposure (lead, mercury). One of the most worrisome aspects of the study is the association of ADHD with seemingly “mild” elevations in blood lead levels. Currently, we “accept” levels of 10 mcg/dl or less as “normal” in the US. Yet there is now mounting evidence that levels over 2-5 mcg/dl may be associated with neurological concerns.

According to the L.A. Times:

One-third of attention deficit hyperactivity disorder cases are linked to prenatal exposures to cigarette smoke or childhood exposures to lead, researchers reported Monday. The study, headed by researchers at Cincinnati Children’s Hospital Medical Center, was the first to estimate the number of ADHD cases attributable to environmental toxins. The report “provides further evidence that we need to find ways to dramatically reduce prenatal tobacco smoke exposures and childhood lead exposures,” said principal author Dr. Bruce Lanphear. ADHD is a condition marked by impulsivity, poor concentration and hyperactivity, making it difficult for children to pay attention in school. About 2 million children in the U.S. are treated for ADHD, according to government statistics. Researchers analyzed data gathered on 4,704 children ages 4 to 15 as part of the federal National Health and Nutrition Examination Survey, which was conducted from 1999 to 2002. About 8% of children in the study had been diagnosed with ADHD and 4.2% were prescribed drugs to treat the condition. The study contained information on prenatal exposure to cigarette smoke and the concentration of lead in blood samples taken from the children. The researchers, who published their findings in the journal Environmental Health Perspectives, linked blood lead concentrations of 2 micrograms per deciliter or greater to an increased risk of ADHD. Children in that group had an ADHD risk four times higher than children with the lowest blood lead levels — under 0.8 micrograms per deciliter. Federal standards consider blood lead levels below 10 micrograms per deciliter to be safe. The study confirmed the link found in previous studies between prenatal exposure to cigarette smoke and ADHD. The latest study found that children exposed to tobacco smoke prenatally had an ADHD risk 2.5 times higher than unexposed children.

Other articles related to the environment and neurodevelopmental disorders:

Koger SM, et al: Environmental toxicants and developmental disabilities. American Psychologist 60: 243-255, 2005.

Herbert MR, et al: Autism and environmental genomics. Neurotoxicology 27: 671-684. Epub 2006 Mar 28.

London E, Etzel R: The environment as an etiologic factor in autism: a new direction for research. Environ Health Perspect 108: S3, 2000.

Palmer RF, et al: Environmental mercury release, special education rates, and autism disorder: an ecological study of Texas. Health Place 12: 203-209, 2005.

Szpir M: Focus: New thinking on neurodevelopment. Environ Health Perspect 114: A100-107, 2006.

Trasande L, et al: Public health and economic consequences of methyl mercury toxicity to the developing brain.

Environ Health Perspect 113: 590-596, 2005.

Windham GC, et al: Autism spectrum disorders in relation to distribution of hazardous air pollutants in the San Francisco Bay area. Environ Health Perspect doi:10.1289/ehp.9120 available via http://dx.doi.org/ [Online 21 June 2006].