BioEd Online

Perchlorate, a toxic chemical in rocket fuel, is contaminating breast and cow's milk across the United States at levels that could harm human health. This situation may be mirrored elsewhere in the world, say the researchers. They recommend increasing the daily intake of iodine to combat the effects of the toxin.

Perchlorate, a toxic chemical in rocket fuel, is contaminating breast and dairy milk across the United States at levels that could harm human health. This situation may be mirrored elsewhere in the world, say researchers. They recommend increasing people's daily intake of iodine to combat the effects of the toxin.

Debate about perchlorate has raged in the United States since the late 1990s, when the chemical was discovered in many water supplies. It occurs naturally in soil, usually at low levels, but larger amounts have leached into the environment from the fuel used to launch missiles and space rockets.

Babies are thought to be at particular risk from the chemical. Perchlorate in high doses can cause mental retardation in fetuses and young children by interfering with production of the necessary thyroid hormones.

And recent findings suggest that perchlorate is seeping into people's diet in a more pervasive way than was previously thought.

Doubled dose

Chemist Purnendu Dasgupta of Texas Tech University in Lubbock and his team tested 36 breast-milk samples from women in 18 states, and 47 samples of shop-bought cow's milk.

Perchlorate was present in all of the breast-milk samples, at an average level of 10.5 micrograms per litre. The researchers also found it in all but one of the dairy samples, at levels five times lower. The results are reported in Environmental Science and Technology¹.

The researchers calculate that the quantities in breast milk could expose babies to more than the safe dose recently recommended by a US National Academy of Sciences panel.

The Environmental Protection Agency adopted this reference dose on 18 February, putting the threshold at 0.7 micrograms per kilogram of body weight per day. If, for example, a four-kilogram baby drinks 0.7 litres of milk containing 10.5 micrograms of perchlorate per litre, it will swallow 1.8 micrograms per kilogram of body weight, which is more than twice the safe dose.

Calls for clean-up

The debate over perchlorate has been concentrated in the United States, but perchlorate probably occurs all over the world, says Richard Johnston at the University of Colorado, Denver, who headed the panel on safe levels. "It's likely to be a problem wherever there is a military facility that uses rockets," he says.

Although the United States has agreed a reference dose, it lacks a national standard for safe levels of perchlorate in drinking water. Dasgupta notes, however, that women are probably getting most of their perchlorate from other sources, such as food grown in areas fed by contaminated water.

Environmental groups want the defence industry to clean up contaminated sites and introduce measures that prevent any more perchlorate seeping out from fuel reservoirs or dumps. "There's got to be an effort to stop it leaking into the environment. Prevention is better," Johnston says. Industry officials have pointed out that this process is expensive.

Iodide protection

Dasgupta's study also shows that the risks of consuming perchlorate may be exacerbated by shortages in the essential nutrient iodide. The perchlorate molecule is a very similar shape to iodide, and competes with it for uptake into both the thyroid gland and breast milk, often causing an iodide shortage.

His team found that the level of iodide in breast milk has declined dramatically compared with measurements taken in the 1980s. This is probably because perchlorate blocks iodide from passing into breast milk, and because women are absorbing less iodide from fresh food as they switch to processed alternatives.

To compensate for both iodide deficiencies and potential perchlorate contamination, Dasgupta recommends that pregnant and breast-feeding women should take a supplement containing iodide. He also suggests that the figure for the recommended daily intake of iodine in our diet should be raised.