Public drinking-water providers now know to protect water supplies at the source and to use disinfectants to prevent growth of dangerous bacteria in the water distribution system. The maintenance of what is called a "residual" of disinfectant that stays in the water distribution system while it is delivered to peoples' homes is not just good public health practice; it is required by the Environmental Protection Agency. The EPA offers drinking water providers two disinfectant choices: Chlorine and chloramine.

Like chlorine, chloramine is not new. It has been used extensively throughout the world since the 1930s, and approximately one-third of all U.S. water agencies also use it for residual disinfection. This year the San Francisco Public Utilities Commission became the last large water agency in the Bay Area to make the change from chlorine to chloramine. The primary reason for the shift is a significantly lower level of contaminants formed in the drinking water.

Chlorine and chloramine have many similarities. Both provide effective residual disinfection with minimal risk to public health. Both are toxic to fish and reptiles, since both come in direct contact with the bloodstream through their gills.

The difference is that while chlorine can be allowed to dissipate with time from water added to aquariums and fishponds, chloramine must be chemically removed. Both chlorine and chloramine must be removed from water prior to use in dialysis machines, since water comes into direct contact with the bloodstream during treatment. When drinking water, people have no trouble digesting chlorine or chloramine at the levels found in our drinking water because this water is not introduced directly into the bloodstream. A comprehensive search of the medical literature does not reveal any studies showing that those with compromised immune systems, weak livers or medication requirements have any special problems metabolizing chloramine.

The principal advantage of chloramine is the reduction in certain harmful byproducts -- especially trihalomethanes and haloacetic acids -- formed by reaction with other compounds in the water. It is true that another byproduct -- N-nitrosodimethylamine, which is frequently brought up in current discussions of chloramine -- is formed by both disinfectants.

However, the NDMA concentration in drinking water is negligible in comparison to other NDMA sources for humans -- such as tobacco smoke, chewing tobacco, cured meats, beer, fish, cheese, toiletries, shampoos, cleansers, the interior air of cars and household pesticides.

The San Francisco Public Utilities Commission will consistently monitor for NDMA and other byproducts, now that the switch to chloramine has been completed, as part of its charge to provide a continued supply of healthy, high-quality water. But high levels are not anticipated, given the quality of San Francisco's water source and treatment practices.

In making any decision, the known and unknown risks need to be balanced with the known benefits. In switching from chlorine to chloramine, the SFPUC carefully weighed the choices. It then joined the water utility agencies of Alameda, Contra Costa, Santa Clara and Marin counties to pick the best disinfectant given the most current information available.