A bold environmental code becomes law in San Francisco today, one whose overarching framework is called the Precautionary Principle. Through it, San Francisco is taking a significant step away from the Bush administration's anti-environmental policies.

The Precautionary Principle sets out to improve the way we make environmental decisions. While the Bush team asks, "How much environmental harm will be allowed?," in San Francisco, decision-makers will ask a very different question: "How little harm is possible?"

San Francisco is a leader in making choices based on the least environmentally harmful alternatives, thereby challenging traditional assumptions about risk management. The 11 existing laws consolidated in the Environment Code have introduced more than 700 zero- or low-emission vehicles to the city's fleet, conserved 6,800 trees and more than half-a-million gallons of water each year by purchasing recycled content paper, cut toxic- pesticide use in half and protected worker health by designing buildings that use less energy and other precious natural resources.

We acknowledge that our world will never be free from risk. However, a risk that is unnecessary, and not freely chosen, is never acceptable. San Francisco's Precautionary Principle, enacted as part of the Environment Code, insists that environmental decision-making be based on rigorous science -- science that is explicit about what is known, what is not known and what may never be known about potential hazards.

Unfortunately, in today's regulatory system lack of proof of harm is usually misinterpreted as proof of safety. In San Francisco, we want to create a means to take action despite scientific uncertainty about the degree of a given risk. Too often, regulatory agencies get stuck in "paralysis by analysis"; the new framework removes excuses for inaction on the grounds of scientific uncertainty.

The costs of not taking precautionary action are often very high, as we've seen in the case of tobacco, lead and asbestos. Early scientific warnings about risks to health went unheeded by government agencies. As a result, billions of dollars have been spent to deal with the consequences of these problems. Costs include health care and health insurance, lost economic productivity, absenteeism, lost wages and cleanup. The Precautionary Principle process also requires decision-makers to consider possible impact to the local economy.

Our Precautionary Principle calls for a careful analysis of a range of alternatives using the best available information. The goal of this process is to determine whether a potentially hazardous activity is necessary, and whether less hazardous options are available. For instance, our pesticide reduction program eliminated all of the most toxic chemicals used by city gardeners and identified less-toxic ways to solve weed and pest problems, some as benign as using goats to clear weed-choked hillsides or heat cannons to kill termites in walls. Science provides vital evidence for making these decisions. However, elected officials will ultimately use a combination of scientific data and judgments of what is necessary, useful and fair to make environmental decisions.

Both locally and internationally, the public bears the direct consequences of environmental decisions. A government's course of action is necessarily enriched by broadly based public participation when a range of alternatives is considered. This concept of environmental democracy is deeply ingrained in San Francisco's Precautionary Principle.

At the World Trade Organization, the Bush administration is fighting the European Union's right to restrict imports of genetically modified foods; beef containing hormones, and proposed legislation that would require some 30,000 chemicals now in use to be immediately registered with EU authorities. The failure of the United States to adopt the Precautionary Principle is yet another way in which we are ostracizing ourselves from the rest of the planet.

San Francisco's Precautionary Principle presents a historic opportunity to refocus environmental decision-making on reducing harm. In doing so, we are sending a message to Washington: The days of letting polluters and industries set our health and environmental agenda may be over sooner than you think.