Manganese heightens lead's effects on children.
Henn, BC, L Schnaas, AS Ettinger, J Shwartz, H Lamadrid-Figueroa, M Hernandez-Avila, C Amarasiriwardena, H Hu, D Bellinger, RO Wright and MM Tellex-Rojo. Association of early childhood manganese and lead co-exposure with neurodevelopment. Environmental Health Perspectives http://dx.doi.org/10.1289/ehp.1003300.
Scientists have known for decades that lead affects a child’s neurological development. Now new research shows that simultaneous exposure to the metal manganese creates even greater risk for 1-year-olds for reduced mental and motor skills later in life.
The researchers followed children from birth to 3 years old, measuring both lead and manganese levels. They found that simultaneous exposure to the two metals was associated with lower performance in developmental tests at age 3. Surprisingly though, the two exposures at age 2 had no association with the subsequent developmental test scores. It may be that the first year is a more critical growth stage for combined exposure to lead and manganese.
This study is consistent with an earlier study in South Korea showing brain development effects associated with exposure to both manganese and lead in children.
Even at low exposures, lead can affect a child's mental development. Whether co-exposure to other toxic metals – such as manganese – makes lead more harmful has not been known. Since lead and manganese both affect the central nervous system in children and exposure to both at the same time is likely, the two are good candidates to examine if co-exposure is linked to greater effects than each alone.
It is rare that children are exposed to only one contaminant, so the need to study multiple exposures is a high priority among environmental scientists.
In the United States, children are exposed to lead mainly from leaded paint in older buildings and in lead-contaminated soil and dust. In other parts of the world, leaded gasoline and lead-glazed pottery or ceramics are also main sources of lead exposure. Working in or living near places where car batteries and obsolete electronics are recycled can also increase lead exposure.
On the other hand, manganese is an essential nutrient. It is found naturally in a variety of foods, including leafy vegetables, nuts and teas, and is also released into the environment through industrial activities. Either too little or too much exposure is harmful.
These results add to growing evidence that manganese may affect child neurological development, especially at high levels. Studies have found associations between manganese exposure and several mental factors, including cognition, memory, behavior and motor function. Because manganese is needed for normal body function, it is important to understand the optimal manganese intake to prevent excessive exposure from environmental sources, such as mining, alloys and fossil fuels.
In this study, the researchers followed 455 babies born in Mexico City from their birth to 3 years of age. Lead and manganese were measured in the children's blood at 1 and 2 years old. The children were evaluated every six months between the ages of 1 and 3 using the Bayley Scales of Infant Development (BSID), 2nd version – a standardized test that measures both mental and motor development.
The average blood lead levels of 5 microgram per deciliter measured in the children were two to three times higher than U.S. children's average. Reported levels, however, are not unusual in U.S. children living in inner cities. The exposure levels of manganese in the study children were within the range of other studies of newborns and children.
The interaction of lead and manganese was mainly observed when the exposures were measured at age 1. The developmental effect on mental and motor development was much stronger in children with higher manganese exposures – those in the upper 20 percent, or greater than 28 micrograms per liter. In other words, in these co-exposed children, lead was associated with deeper deficits in mental and motor development than children with average manganese exposure.
For example, each microgram per deciliter of lead was associated with a 2.2-point mental developmental index deficit if the blood manganese was in the upper 20 percent. In contrast, when the manganese level was lower than that, the same amount of lead was merely associated with a 0.07-point deficit. The mental developmental index is a measure of mental ability in infants and toddlers. Therefore, manganese levels modified the association between lead and child mental development.
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