What is “frac” fluid and why is it scary?

According to Energy in Depth, a lobbying group representing independent oil and gas producers, “frac” fluid is comprised of about 99.5 percent water. The remaining .5 percent of the “frac” fluid is comprised of the following chemicals: Acids, Glutaraldehyde, Sodium Chloride, N, n-Dimethyl formamide, Borate salts, Polyacrylamide, Petroleum distillates, Guar gum, Citric Acid, Potassium chloride, Ammonium bisulfate, sodium or potassium carbonate, Proppant, Ethylene glycol, and Isopropanol. Many of these chemicals are found in everyday consumer goods. http://www.energyindepth.org/frac-fluid.pdf
However, according to environmentalist safety group such as Earthworks, some of the chemicals in “frac” are dangerous even in small quantities. One such chemical is benzene which is linked to cancer. http://www.earthworksaction.org/FracingDetails.cfm. As previously mentioned, approximately ½ of one percent of “frac” fluid is comprised of chemicals with the rest being comprised of water. And only a percentage of that ½ percent is considered harmful. Is there really anything to worry about? For example, low levels of arsenic are found in many of the foods we eat.
Ultimately, it is the handling of this “frac” fluid that is the difference. Irresponsible disposal and use of this fluid is bound to cause trouble. As put by researchers at MIT, “The environmental impacts of shale development are manageable but challenging.”