CHARLESTON, W.Va. -- West Virginians should have been given clearer information that the 1-part-per-million screening level for the toxic chemical "Crude MCHM" was not a "bright line" between what exposures are safe and unsafe, a top U.S. Centers for Disease Control scientist said Wednesday.Dr. Vikas Kapil, chief medical officer for the CDC's National Center for Environmental Health, also acknowledged that government officials could have moved more quickly in issuing an advisory that pregnant women drink only bottled water until chemical levels were zero in the West Virginia American Water system.In an interview, Kapil said that the CDC was working with very limited data and in an emergency situation, but that agency officials could have communicated the uncertainties more carefully to the 300,000 residents whose water was contaminated."It would have been probably preferable to provide that kind of information up front," Kapil said."There are always things we can do to improve," he said. "It's a moving, dynamic situation where we really are doing the best we can."West Virginia officials first mentioned the 1-part-per-million figure at a press conference on Friday, Jan. 10, the day after the Elk River spill. But both state officials and federal government representatives initially provided few details about how the number was derived.Using the CDC's guidance, state officials and West Virginia American cleared residents to begin drinking the tap water starting on Monday evening, Jan. 13.Then, on the night of Wednesday, Jan. 15, the state Department of Health and Human Resources announced that it was warning pregnant women to drink only bottled water -- at least for now.Previously, officials have said that they added the advisory to pregnant women only out of "an abundance of caution" to protect developing fetuses. But a letter from the CDC to the DHHR suggested federal officials had obtained some additional studies that led to the advisory.Kapil said he remains confident that the 1-part-per-million guidance is protective for most people, and that the region's water is safe, given that state sampling continues to show decreasing levels of Crude MCHM, with more and more samples listed with "non-detect" results.In a statement issued Wednesday, West Virginia American Water President Jeff McIntyre said that "the majority of samples" are reading non-detectable."In the areas where sample results show levels above the non-detectable limit, they are still extremely low and only a fraction of the CDC-established 1 ppm health-protective limit," McIntyre said.In a Wednesday letter to McIntyre, Sen. Jay Rockefeller, D-W.Va., asked West Virginia American for "maximum transparency," including Internet posting of results in "real time," including geographic coordinates of the samples, and the exact substances for which samples are being taken.West Virginia officials said they are able to detect levels of the chemical down to 10 parts per billion, while Louisville Water Co. officials in Kentucky have said they can detect down to 1 part per billion.Kapil said he was not familiar with the differing sampling methods being used"There are always these kinds of differences between methods," Kapil said. "When you get down to numbers that low, to some extent, I really don't think it's an issue."Asked to explain the delay in issuing the advisory to pregnant women, Kapil said that, "the way to answer that question is that there is some misconception among people about the screening value."Kapil said that the screening value doesn't mean people exposed to lower levels will absolutely not experience health effects, or that people exposed to higher levels will get sick."The screening value isn't a sort of bright line between safe and unsafe," he said. "That's one of the misconceptions."The CDC's number was based largely on an April 1990 Eastman study in which rats were exposed to varying levels of 4-methylcyclohexanemethanol. The study concluded that a concentration of 100 milligrams per kilogram of the chemical was the "No Observable Adverse Effects Level, or NOAEL," for the material.CDC officials divided that 100-milligram-per-kilogram level by 10 three times, once each to account for differences between rats and humans, differences between different humans and the lack of data on the chemical.Then, they set the final screening level for water based on a standard estimate for a 1-year-old child of about 22 pounds of body weight and consumption of a little more than a quart of water per day.Glenys Webster, an epidemiologist and postdoctoral fellow at Simon Fraser University, has explained that what the CDC's exercise actually does is set an estimated safe dose for daily exposure to the chemical of 0.1 milligrams per kilogram of body weight.An average adult weighing about 150 pounds and drinking a little more than two quarts of water a day would get an exposure dose of about 0.03 milligrams per kilogram of body weight, Webster said. That amounts to three times lower than the level that the CDC considers as unacceptable under its screening level.But for a 1-year-old child weighing 22 pounds and drinking about a quart of water per day, she said, the daily dose would be 0.1 milligrams per kilogram of body weight. That's right at the level the CDC set with its screening level.Reach Ken Ward Jr. at email@example.com or 304-348-1702.