"The experts then took this number and calculated the uncertainty factors," she wrote. "In this situation there were two. The first uncertainty factor was translating these results from rats to humans. The second uncertainty factor took into account sensitive populations. This includes the elderly, the sick, the immuno-compromised and children, amongst others.
"Uncertainty factors range from 5 to 10 percent," she wrote. "Given the dearth of data and an abundance of caution, both uncertainty factors were rated at 10 percent."
This, Tierney explained, changed the level that would cause death to 8.25 milligrams per kilogram of body weight.
LD50 figures, though, consider only death. They would tell officials nothing about what levels at which chemical exposure would cause other health effects, even serious ones. To address this, they changed the figure to 1 milligram per kilogram of body weight, which is equal to 1 part per million.
In his blog post, Denison took issue with two things that government officials did in putting together their number.
First, he wrote, the tenfold uncertainty factors should not have been applied to the LD50 value. In standard risk assessment, Denison wrote, those factors would instead be applied to something called the "no observable adverse effect level," or NOAEL, or, if that's not available, the Lowest Observable Adverse Effect, or LOAE.
"That is the dose at which no effect of a chemical exposure is observed," Denison wrote. "It doesn't take a risk assessor to recognize that the dose at which no effect is seen is going to typically be far, far lower than the dose that outright kills half of the exposed subjects."
Denison said that by using the LD50, they started out with the wrong value.
"No doubt they did so because the values they should have used -- the NOAEL or the LOAEL -- are not available for this chemical," Denison wrote. "But that's no excuse for not compensating for this major problem, at the very least through application of an additional large uncertainty factor."
Second, Denison wrote, officials assumed without any basis that any nonlethal effects of the chemical would occur at low doses that were at most 8.25-fold lower than the lethal dose that would kill half of the exposed subjects.
"This assumption can only have been pulled out of thin air," Denison wrote. "Put aside the convenience of selecting a factor that allowed a nice round number of 1 ppm to be set as the safe level.
"On what possible basis could it be assumed that the dose of the chemical that would, for example, be moderately toxic even in the short term to the liver or kidney, be only about one-eighth the dose that would kill someone outright in just 24 hours? Many health effects of chemicals occur at doses that are orders of magnitude lower than the lethal dose."
Denison concluded, "Now, let me be clear. I am not saying that the level of 1 ppm is unsafe.
"I am saying that we have no way of knowing whether or not it is safe. The data needed to make that assessment simply do not exist for this chemical."
Staff writers David Gutman and Rachel Molenda contributed to this report.
Reach Ken Ward Jr. at kw...@wvgazette.com or 304-348-1702.