Each safety factor was given a weight of 10. They were multiplied by each other to produce a total safety factor of 1,000. CDC officials then divided the 100 milligram per kilogram of body weight NOAEL by 1,000, and came up with a figure of 0.1 milligrams per kilogram.
CDC officials then assumed a child who weighs 10 kilograms (22 pounds) and drinks 1 liter (slightly more than a quart) per day of water, ending up with a recommended screening level of 1 part per million.
Kapil said that at every step, scientists used the most conservative estimates.
"Some people apply smaller factors, sometimes three, sometimes four, depending on what it is you're talking about, but we applied the highest, so we feel pretty comfortable that the 1-part-per-million number is a number that would not be associated with any adverse effects for humans, based on this methodology," he said.
Denison, though, said that what the CDC refers to as "safety factors" -- and tells the public add conservatism to the agency's risk assessment -- actually are more like "reality factors," which simply help translate animal studies of a limited range of health effects to the real world.
"They do not impose this super safety standard," Denison said. "That's a standard claim of industry."
If the CDC wanted to include additional levels of safety to be conservative toward protecting public health, Denison said, the agency would have divided the 0.1-milligram per kilogram number by 10 again, or by 100, to add more protections.
Denison said residents should remember the uncertainties involved, and make their own decisions about using the water. And they should know that, especially for children, how much water they consume is important.
"These numbers are certainly far from precise," Denison said. "What it does mean is that, if your child is drinking more than a liter a day, they are getting a larger dose."
When asked if he would drink the water in Charleston if it was at a level of 0.9 parts per million, CDC's Kapil responded, "Absolutely. I would have no hesitation whatsoever."
However, Kapil and Tom Skinner, a CDC senior press officer, stressed that they did not have as much information as they would like.
"Ideally, we would like to have a whole host of studies -- like human epidemiology studies, human toxicology studies -- all of that information available to us," Kapil said. "We do have limited animal data and, from that animal data, we have to somehow figure out a way to extrapolate for decision-making related to these types of human exposure."
Skinner said the CDC did not have much experience dealing with a fast-moving situation of this magnitude.
"This whole situation is very fluid, so it's almost as if we're learning as we go," he said. "This experience that we're involved with now is actually going to provide us with a lot of information to base future decisions on."
The studies the CDC has been using were produced by the Eastman Chemical Co., whose plant in Kingsport, Tenn., manufactured the MCHM.
Eastman officials, responding to email questions, initially refused to provide copies of any of their studies or data about the potential impacts of MCHM.
"Eastman's toxicological studies for crude MCHM are not published in scientific journals and therefore are not what are commonly referred to as peer-reviewed studies," wrote Maranda Demuth, an Eastman spokeswoman. "The studies, however, were conducted under Good Laboratory Practices, and according to OECD guidelines, at a reputable laboratory where rigorous internal review processes were performed."
On Thursday, though, the company announced that it would post the studies on its website, perhaps that night or early Friday.
That move came after Rep. Henry A. Waxman, ranking Democrat on the House Committee on Energy and Commerce, wrote to Eastman to request that the company release its MCHM studies.
Also, on Thursday afternoon, Sen. Joe Manchin, D-W.Va., and Rep. Shelley Moore Capito, R-W.Va., wrote to Frieden requesting the detailed methodology behind their calculations, the information that led them to change the advisory for pregnant women, the date when the CDC became concerned about lower-level exposure in pregnant women and the CDC's level of confidence in the 1-part-per-million standard going forward.
"It is particularly concerning that as many as 150,000 people who had been under a 'do-not-use' water order were told that their water supply was safe for use before the CDC's recommendation that pregnant women should consider an alternate drinking water source," they wrote. "We are deeply disappointed in the CDC for recommending a screening level before receiving all relevant studies and information, which has resulted in confusion, fear and mistrust among Kanawha Valley residents."
In a Facebook posting after the DHHR's advisory to pregnant women, West Virginia American Water commented that the company "does not set water quality standards."
"Our responsibility is to follow the rules and guidelines established by federal and state regulatory agencies, such as the CDC, EPA, West Virginia Department of Health and Human Resources' Bureau for Public Health and the West Virginia Department of Environmental Protection," the company posted. "West Virginia American Water has followed the regulatory guidelines and testing protocols required to lift the Do Not Use order ban, and we are fully confident that we are providing customers with drinking water that meets all regulatory standards."
Reach David Gutman at david.gut...@wvgazette.com or 304-348-5119.
Reach Ken Ward Jr. at kw...@wvgazette.com or 304-348-1702.