The research community has observed that blood lead levels of 2 µg/dl and even lower are associated with various chronic diseases, learning and behavioral issues. In 2012, CDC supported the research community by making two critical changes. First it recognized that there is no known minimum level of lead exposure which can be defined as safe. And secondly CDC guideline language about children’s “acceptable lead exposure” was revised to “reference value”. The revision is feasible because it is now possible to reliably measure blood lead levels at 1 µg/dl and even lower. The reference value is connected to the findings of the 97.5th percentile of the National Health and Nutrition Evaluation Survey (NHANES) results for children’s blood lead in the U.S. Currently the top 2.5% of the children in the U.S. have a blood lead level of 5 µg/dl or higher. The NHANES is repeated every few years and because blood lead is declining the future blood lead reference value should also decline.

In the body, lead enters the bloodstream (99 percent is associated with red blood cells and 1 percent is in the plasma) where it is carried throughout the body. Lead meets one of two fates: 1) It can be excreted through the kidneys or intestines; or 2) It can be stored in soft tissue and bones and teeth where it is slowly released over decades.

More than 95 percent of total lead in the adult body is in bones and teeth. Throughout a lifetime, lead is mobilized back into the bloodstream in times of stress, chronic disease or pregnancy/lactation (a hazardous exposure source for unborn fetuses).

Even though single exposures are hazardous, it is chronic exposure that poses the biggest threat. Constant, long-term exposure from both the external environment and total body burden (all lead circulating or stored in a body) can cause adverse health effects. For instance, even if a person is removed from a lead source and blood-lead levels return to normal, the stored lead can be mobilized for decades, representing a continuous lead source that could cause lead poisoning.

Figure 1. Regional Brain Volume Loss for the Cincinnati Lead Study Participants show more A composite representation of regions with significant volume loss for male and female CLS participants (n = 157) associated with mean childhood blood lead concentrations is shown with red and yellow clusters overlaid upon a standard brain template (seen at multiple angles; the first row presents views from the midline of the left and right hemispheres, respectively; the second row demonstrates views from the back and front of the cerebrum, respectively; the third row shows the lateral right and left hemispheres; and the fourth row shows views from below and above the cerebrum.
Brain template source reference [51]. doi:10.1371/journal.pmed.0050112.g001.

Lead affects almost every bodily system, but susceptibility varies with age, gender and nutritional status.

Most vulnerable are unborn babies and young children. The heavy metal can damage growing brains and nervous systems that have not yet formed their natural protection systems. Unborn babies are exposed through their mother's blood. Through hand-to-mouth action, children from nine months to 6 years-years-old inadvertently eat lead in paint, dust or soil. They can absorb and retain up to 50% of the swallowed element.

Once in the body, children's health effects vary. With severe lead poisoning, problems range from permanent mental retardation to death.

Figure 1. Blood Lead Levels in Young Children ( USA & Selected States, 1996--1999 ). Centers for Disease Control and Prevention (CDC), MMWR.

Lower blood-lead levels also create a range of problems including a drop in IQ. One study found blood-lead levels of 10 µg/dl lowered IQ by 3 to 5 points. Additionally, constant exposure to low lead levels causes attention deficit, hearing loss, slowed growth, headaches and hyperactivity.

Some of these illnesses go unnoticed until a child first attends school. By that time, the irreversible brain damage may be enough to interfere with life-long learning. Adults who live or work with high amounts of lead also suffer. Related health problems include difficulties during pregnancy, reproductive problems, muscle and joint pain, digestive problems, memory and concentration problems, nerve disorders and high blood pressure.

Centers for Disease Control and Prevention 2014 MMWR: Lead Screening and Prevalence of Blood Lead Levels in Children Aged 1–2 Years — Child Blood Lead Surveillance System, United States, 2002–2010 and National Health and Nutrition Examination Survey, United States, 1999–2010

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6. From a speech at Earth Day 1997 at the Center for Bioenvironmental Research at Tulane and Xavier Universities in New Orleans, Louisiana.
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