But some CDC studies suggest the thyroid sometimes reacts to low levels -- between 1 and 20 parts per billion (ppb) -- of perchlorate, EPA's Mayer said.
"Is that an adverse effect?" Mayer asked. "We're putting a lot of pieces together to come up with what might be a threat and at what levels it might be a threat."
How to regulate?
Uncertainty about perchlorate's origins -- and by extension, how people are exposed to it -- makes it difficult to regulate.
"Obviously, you need to get it out of drinking water, but universally there's obviously some other source," said Richard Wiles, executive director of the Environmental Working Group. "We have internal debates on the appropriate policy response, and it's really tricky."
Environmentalists and health groups have been pushing EPA to set a more stringent standard for drinking water, arguing that it would quickly reduce exposure levels for the largest number of people.
"This is a very widespread contaminant," Wiles said. "While it's true that some sources of contamination are difficult to pin down, when you have one you can, you should definitely address it."
In January, EPA said it would delay its final decision on perchlorate regulation until the National Academy of Sciences studied the matter. That announcement followed an agency determination that it would not limit perchlorate in drinking water, finding that there was no meaningful opportunity for reducing health risks through regulation.
But the Obama administration has opted to review decisions on perchlorate made by President George W. Bush's EPA.
The problem: Even if the government imposes strict drinking-water standards, people will still be exposed from a variety of sources. Perchlorate has been found in leafy vegetables and fruit, breast milk and infant formula.
Wiles suggested releasing a health advisory, similar to those issued by the Food and Drug Administration about high mercury-containing fish. That raises its own problems, because a healthy diet should include fruit and leafy vegetables.
But industry groups argue that exposure to perchlorate at low levels is not harmful.
Bill Romanelli of the Perchlorate Information Bureau said perchlorate has been studied for more than five decades and there is no evidence that low doses harm people.
"We have a real chasm of debate here about whether there is any health effect at all," Romanelli said. "It's not just about public health at that point. There are costs associated with regulating any kind of chemical or compound, and there are only a finite amount of resources to do it with."
The debate is far from over -- the Government Accountability Office is now starting a new study to look at the science surrounding perchlorate in drinking water and EPA's progress so far on regulating it, a GAO spokeswoman said. The report will also address another emerging contaminant, trichloroethylene.
Wiles, from the Environmental Working Group, said, "What do you do? There seems to be some source -- the old Chilean argument, maybe. I don't think people know. There's not enough research, although there's some."
But clues are emerging, and scientists are looking for answers everywhere -- even on Mars.
Reprinted from Greenwire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500