To regulate or not to regulate? What to do with more toxic disinfection by-products?

Since the first regulation of disinfection by-products (DBPs) in the 1970s, > 700 DBPs have been identified, and many of these are much more toxic than those regulated. Moreover, drinking water today is not the same as it was in the past, with increasing use of alternative disinfectants like chloramine, ozone, chlorine dioxide, and UV (which can form other types of DBPs), as well as new impacts on our source waters from climate change, population increases, wastewater intrusion, and energy exploration. The question today is whether we are regulating the right DBPs to protect human health, and if not, what should be done. New approaches may involve (1) the use of in vitro data and a Precautionary Principle approach, (2) using surrogate metrics of finished waters, such as total organic bromine/iodine, total nitrosamines, or total organic nitrogen rather than creating longer lists of regulated DBPs, and (3) using toxicity assays for whole drinking water extracts to pinpoint potential problems, then using chemical analyses to identify the toxic agents, and finally, (4) invoking different treatment strategies to reduce the toxicity. While the early regulations likely significantly improved the safety of drinking water, DBP exposure is a constant in modern life, and there is more that we can do.