The green revolution in the 1950s and ’60s dramatically increased global food production through expanded use of fertilizers, pesticides, and high-yield crop varieties. While this has made it possible to feed more than twice as many people as before, it has also caused widespread environmental damage. Phosphate chemicals in fertilizers, for example, helped to more than double cereal production from 1960 to 2000. At the same time, excess phosphates in runoff water are killing aquatic life by fueling explosive algae growth, causing billions of dollars of damage every year.

In response, some US state and local governments are banning the sale of household products containing phosphates, such as certain dishwasher detergents and lawn fertilizers. But while trendy, these laws have little impact on overall phosphate water pollution, according to University of Massachusetts at Amherst’s David Keiser.

Authorities are reluctant to impose limits on agricultural phosphate use, which accounts for 95 percent of the chemical’s consumption in the United States. The 1972 Clean Water Act doesn’t even regulate agricultural pollution. Meanwhile, lawmakers are eager to restrict household and industrial phosphate use, which accounts for the small remainder.

“It’s similar to a climate policy that’s trying to control carbon dioxide without regulating the burning of fossil fuels,” Keiser says. “It’s not a sound idea.”

Using data from the US Geological Survey, Keiser calculated the production of phosphate rock in the US for agricultural and industrial uses from 1932 to 2015. He then estimated the amount of phosphate left over to run off into waterways after crops consumed their share of the fertilizer. He also plotted phosphate use in industrial and consumer products, demonstrating that it peaked around 1970 and has remained meager since 2000.

Keiser’s finding that agriculture accounts for almost all of phosphate use in the US suggests that state and local bans on industrial and consumer products would make little difference in improving water quality across much of the country. Policy makers should study these disproportionate phosphate-use rates if they are serious about improving water quality, Keiser suggests.

Lawmakers should also consider how consumers respond to purchasing restrictions, Keiser says. If one county bans the sale of detergents containing phosphates, for example, residents may just buy them in the next county. In regions that impose seasonal restrictions on the sale of lawn fertilizers, residents may stock up before and after. Keiser illustrates this effect in a case study of dishwasher-detergent sales in towns in Washington and Idaho, which instituted phosphate bans two years apart. For this part of the research, he tapped into the Nielsen Datasets at Chicago Booth’s Kilts Center for Marketing.

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“If we implement these types of water-quality policies, we need to be cognizant of the fact that consumers may respond by engaging in actions that are going to reduce the effectiveness,” Keiser says.

Local leaders should also consider how phosphate bans will interact with other regulation, he argues. Wastewater treatment plants, for example, have little incentive to improve more than required under current standards. Unless a ban lowers phosphate levels so much that the standards are no longer binding, wastewater treatment facilities are likely to discharge water with the same phosphate levels as before, he writes.

Phosphate bans are more effective in some regions than others, Keiser says. But overall, policy makers should take a closer look at the evidence before proceeding with regulations.

“Phosphate bans have become a really popular tool for trying to address the nutrient pollution problem in the US,” he says. “But these kinds of policies that solely target household or industrial uses are not necessarily the answer.”

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