This finding comes from the CDC's "Fourth National Report on Human Exposure to Environmental Chemicals", which, according to the federal agency, is "the most comprehensive assessment to date of the exposure of the US population to chemicals in our environment".
For the report, CD scientists analyzed blood and urine samples from participants taking part in the CDC's National Health and Nutrition Examination Survey (NHANES), an ongoing survey that every two years samples members of the US population. The Fourth Report includes results from surveys covering 1999 - 2000, 2001 - 2002, and 2003 - 2004.
For the Fourth Report, CDC scientists measured total mercury in the blood of over 8,500 participants aged one year and over taking part in NHANES during 2003 - 2004. The scientists also measured mercury in the urine of over 2,500 participants aged 6 years and over, in the NHANES 2003 - 2004.
Total blood mercury mainly assesses methylmercury exposure, while urine mercury is a measure of inorganic mercury exposure. You need to measure both blood and urine levels of mercury to assess how much is in the body.
The CDC scientists found or concluded that:
- Most of the participants had a measureable amount of mercury in their bodies.
- Both blood and urine mercury levels tended to increase with age.
- All blood mercury levels were less than 33 µg/L.
- Blood and urine mercury in the US population were similar to levels seen in other developed countries.
However, in 2000, the National Research Council of the National Academy of Sciences suggested that a level of 85 micrograms per liter (µg/L) in cord blood was linked with early neurodevelopmental effects, and the lower limit of the 95 per cent confidence range of this estimate was 58 µg/L (95 per cent confidence range is a statistical measure of robustness that says if you were to do this experiment or study 100 times, on 95 of those times you would get a figure ranging between these two limits).
The CDC note that this lower 95 per cent confidence limit 58 µg/L is above the 33 µg/L that the Fourth Report estimates as the maximum present in the US population, and point out that:
"Finding a measurable amount of mercury in blood or urine does not mean that levels of mercury cause an adverse health effect."
Mercury is a metal that exists in soil, water and air in three forms, each with different properties, usage and toxicity: elemental (or metallic) mercury, inorganic mercury compounds, and organic mercury compounds.
Elemental mercury is probably the one we are most familiar with: it is a liquid at room temperature, and resembles the liquid metal form assumed by the T-1000 terminator played by Robert Patrick in the film Terminator 2.
In real life, elemental mercury is present in everyday goods including dental amalgams, some light bulbs and thermometers; it is also used in industry and mining and gets into the air when we burn coal and other fossil fuels. Elemental mercury enters our bodies when we breathe in air containing mercury vapour: this is mostly in workplaces like dental offices, industrial smelting plants, or where mercury has been spilled. In the body, elemental mercury usually forms inorganic compounds.
Inorganic mercury compounds occur when the metal combines with elements like sulfur or oxygen, to form compounds or salts that can occur naturally in the environment. These compounds are used in industry, where most human exposure occurs. Also, in some countries, but not the US, inorganic mercury compounds are used to make cosmetic skin creams.
Organic mercury compounds occur when the metal combines with carbon, such as when micro-organisms in water and soil make methylmercury from elemental and inorganic mercury: this compound accumulates in the food chain. People are exposed to methylmercury when they eat fish or shellfish. Methylmercury can also pass through the placenta into a developing fetus.
Other types of organic mercury include thimerosal and phenylmercuric acetate, made for use as preservatives (the former being the mercury compound at the center of the controversy about autism and certain childhood vaccines).
According to the CDC, we don't know what effect low levels of elemental mercury exposure have on human health, but very high levels of vapour concentrations can quickly lead to severe lung damage, and exposure to low vapour concentrations over a long period can result in neurological disturbances, memory problems, skin rash, and kidney abnormalities.
Some inorganic mercury compounds can be irritating and corrosive to the digestive system if eaten in large amounts, and if this happens repeatedly, or if the compounds are applied to the skin over a long period, the result is much the same as long term exposure to mercury vapour, including neurological disturbances, memory problems, skin rash, and kidney abnormalities.
The CDC says that eating large quantities of methylmercury over weeks to months damages the nervous system. For example, there are cases of babies born to women poisoned with methylmercury who had developmental abnormalities and cerebral palsy.
For the Fourth Report, CDC scientists measured 212 chemicals in people's blood or urine. Many of them, like mercury, have been monitored since the reporting started, but 75 of the chemicals have never been monitored in the US population before: these include acrylamide, arsenic, environmental phenols, including bisphenol A and triclosan, and perchlorate.
"National Report on Human Exposure to Environmental Chemicals."
CDC, National Center for Environmental Health, 2009.
US Department of Health and Human Services.
Error correction: please note that an earlier edition of this article contained an error suggesting that the MMR vaccine contained the preservative thimerosal.