According to the news this morning (http://www.wbur.org/npr/262217754/water-bans-lifted-in-several-west-virginia-areas), something like 20% of the effected population has been told that their water is "safe" and been given instructions on how to flush the water lines (and the water lines of their appliances) in their homes/businesses, with the caveat that there isn't enough water in the supply lines for everyone to do this at once. This decision is based on reports that "An interagency team including the W.Va. Bureau for Public Health, West Virginia American Water and U.S. Army Corps of Engineers announced that extensive testing produced results showing that levels of MCHM (4-methylcyclohexane-methanol) are below one part per million (1 ppm)" (though how they decided that <1ppm is a safe level is unclear).
From NPR (http://www.npr.org/2014/01/13/262185930/mysteries-persist-surrounding-west-virginia-chemical-spill):
At the time of the accident, the CDC didn't have a standard for how much of this chemical in water is safe to drink.
So the agency had to come up with one.
The agency relied
on the little research that had been done on the chemical — an animal study that established the lethal dose for rats.
"And from that
you would decrease the proposed level down further and further taking
into account all the uncertainties," says Vikas Kapil, chief medical
officer at the CDC's National Center for Environmental Health.
For instance, the CDC built a safety
factor into the limits it set because health officials were uncertain if
people are more sensitive than rats to the chemical. And they added an
additional margin of safety to account for certain populations — such as
infants and the elderly – that might be especially vulnerable.
acknowledges that there was very little data to go on. Still, he says,
drinking water that meets the CDC guideline of one part per million, is
"generally not likely to be associated with any adverse health effects."
Virginia officials say they also turned to safety information
companies, which are required to provide information on the chemicals
they possess. But that so-called Material Safety Data Sheet included
very little data, in this case.
"The entries were largely 'data not available' for this particular compound," says Sharon Meyer, a toxicologist from the University of Louisiana at Monroe.
I don't see this as terribly reassuring really.
The water company map showing which areas are not considered safe and those still under a do not use advisory is available at http://www.amwater.com/about-us/news.html
The current map looks like this ...