The administration now says "scientific indicators are unclear," implying that the new standard was not justified. I was in charge of the Environmental Protection Agency office that developed these new standards under rigorous scientific review. Arsenic exposure is closely linked to lung and bladder cancer and many other adverse health effects. The Environmental Protection Agency approved the new permissible standard for arsenic in drinking water of 10 parts per billion in January, after a decade's worth of work and a lengthy public process. The old standard of 50 parts per billion was established in 1942, long before new research on arsenic's effects.

The National Academy of Sciences completed the most recent analysis of arsenic in 1999, concluding that the old standard was more than 100 times less protective than other drinking water standards. The academy did not recommend a new number. But it urged the federal government to move quickly to revise the World War II-era rule to protect public health. Even Congress expressed frustration with the slow pace of revising the arsenic standard, and in 1997, Congress directed the E.P.A. to set a new arsenic standard.

Now the Bush administration has justified its sudden reversal on arsenic by suggesting that there was no "consensus" on a specific numeric standard. Unfortunately, there rarely is consensus on such numbers. The 10 parts per billion standard for arsenic, however, is widely supported by drinking water utilities, states, scientists, public health officials and environmentalists, though not by the mining industry, some Western states and some scientists. Regulators always strive for consensus, as I did. But it is simply not possible to achieve absolute consensus in this case, so we opted for the scientifically sound standard that would protect public health.

Ifthe E.P.A. ultimately rescinds the new standard, there may be no standard whatsoever for arsenic. Under the new rule, all communities would have to be in compliance with the new standard within five years. But if that rule is repealed, there is a question as to whether the old standard would automatically go back into effect five years from now.

Communities need time to plan capital investments to improve drinking water quality. They need certainty in a regulatory environment. By reversing course, the E.P.A. would cause serious delays in their planning processes. Approximately 3,000 communities throughout the country, many of them in the Southwest, are in need of federal guidance in upgrading their water systems. But it could be many years before the agency makes another decision on arsenic levels.

The Bush administration apparently will go back to the drawing board and try to find a new consensus. While I have great faith in the agency's staff, the mining industry and public health professionals are not likely to agree. The questions facing the agency a few years from now will be fundamentally the same that exist today. We answered those questions with a commitment to public health protection. I certainly hope that the new administration will approach this issue in the same manner.

Chuck Fox is a former assistant administrator for water at the Environmental Protection Agency.