It causes cancer. In extremely high doses, it can poison a person. No one seems to argue that.

But there may not be consensus among public officials as to the effect of arsenic levels in Twin Falls County that violate the relatively new federal limit - 10 parts per billion, or the equivalent of an eyedropper of liquid in a swimming pool, according to one fact sheet.

That tight limit has been a headache for many Idaho towns. The federal limit, proposed in the late 1990s and approved in 2001, replaced a much roomier limit of 50 ppb. Its official adoption in 2006 placed several Idaho towns - including Twin Falls, Buhl and Castleford - in violation until they were able to negotiate time to upgrade their water systems with the Idaho Department of Environmental Quality.

But the debate over renovation costs and how best to get rid of arsenic has overshadowed another - that of the potential health threat to Idaho residents.

Legislators such as State Sen. Tom Gannon, R-Buhl, have protested the reasoning behind the U.S. Environmental Protection Agency's decision, and DEQ, city and public health officials have no shortage of opinions about what threat arsenic actually poses to southern Idahoans.

In Idaho, the DEQ is in charge of monitoring arsenic levels. Doug Howard, DEQ regional administrator, doesn't really have a choice about what standards to follow - the state, after all, can't have laxer restrictions than the federal government. But EPA studies, he said, provide good reasoning for the 10 ppb standard.

"Most of the time, it will boil down to deaths per 100,000 population," Howard said. "It's a real low number (for the arsenic standard)."

And some area wells, particularly for Twin Falls, stay close to that level anyway. The city tests its wells every month, water department superintendent Mike Schroeder said, and results from December's tests put most wells at 11 to 13 ppb, though peaks over the past two years have reached 18 ppb. The Blue Lakes area, source of up to two-thirds of the city's drinking water in the summer, regularly tests at less than 5 ppb.

Mixed together in the pipes, Schroeder said, the city's water supply is pretty close to meeting standards. And health and environmental officials he's spoken with agree, he said.

"They feel in large it's pretty safe," he said.

The story's not quite the same for nearby towns: Castleford's water has tested as high as 22 ppb in the recent past. But they may not be any worse off than Twin Falls in the grand scheme of things. Even 10 ppb carries some risk of cancer, Idaho Department of Health and Welfare spokeswoman Emily Simnitt said.

"Obviously that's significant, and it is enough to cause some concern over a chronic, long-term exposure," Simnitt said. "But there are parts of Idaho where it's much, much higher than that."

Those parts include places like Owyhee County, which is labeled with arsenic rates between 50 to 1,000 ppb on a map of rates accessible on DEQ's Web site. Those areas pale in comparison to the site of the studies quoted during the EPA review that led to the lower limit: In Taiwan, an ocean away, exposure in some people reached more than 600 ppb.

Tracking cancer rates associated specifically with arsenic exposure is tricky, Simnitt said. But numbers from the National Research Council indicate a risk of about 60 extra incidences of skin, kidney or bladder cancers per 10,000 people than lower levels, she said. Though she lacked similar data for the 50 ppb level, she said, the information shows that long-term exposure to any level of arsenic carries some risk.

"Any efforts to reduce the arsenic contaminants will reduce those risks," she said. "But there's no magic bullet that will protect us from the environment and our own genes."

Nate Poppino can be reached at 735-3237 or npoppino@magicvalley.com.