To the list of problems caused by a declining economy something new soon may be added--tooth decay.

Washington and other cities face an impending shortage of the chemicals used in the fluoridation of drinking water. Unless those chemicals are found somewhere, public health officials warn, large numbers of children could have a lot more cavities.

The problem began last year when the nation's phosphate manufacturers were forced to cut production because of a reduction in orders for fertilizer from farmers hit by high interest rates and record operating costs. Among the byproducts of the phosphate manufacturing process are the chemicals used in fluoridation.

Washington area treatment plants have enough fluoride compounds for now, but the outlook for late summer and fall is uncertain.

"We have been notified by our supplier that they anticipate a shortage," said Harry C. Ways, chief of the Washington Aqueduct, the system that provides water for 1.1 million people in the District of Columbia and Northern Virginia.

"What that means is that sometime down the pike, if they can't supply as much as we need, we would stop putting it fluoride chemicals in the water," Ways said. "And that would mean children eventually would have three times as many cavities as they have now."

Officials of the Washington Suburban Sanitary Commission, which provides water for Montgomery and Prince George's counties, and the Fairfax County Water Authority, which serves parts of Northern Virginia, say they realize that a shortage is impending but so far have received their regular shipments of fluoride chemicals from their supplier.

The addition of fluoride chemicals to water doesn't make water any safer or healthier. But public health studies have shown that fluoridated water can reduce tooth decay by 50 to 60 percent, compared to fluoridated toothpaste which reduces decay by about 20 to 30 percent. No shortage exists in the supply of toothpaste fluoride, which is a chemical product made in the laboratory. The availability of fluoride chemicals used for water treatment, however, is tied to phosphate manufacturing. If the phosphates aren't needed for fertilizer, they aren't processed. And if the rocks aren't processed, the fluoride byproducts aren't produced.

Fewer phosphates have been needed because of the reduced demand for fertilizer.

"We sold 46 million tons of fertilizer to U. S. farmers this past year, compared to 53.4 million tons the previous year ," said Ed Wheeler, president of The Fertilizer Institute, a trade association. He said farmers had cut back on fertilizer in an effort to reduce operating costs and avoid high interest rates.

The two most common fluoride chemicals used to treat water are hydrofluosilicic acid, which comes in liquid form, and sodium silicofluoride, which is a powder. The Washington Aqueduct uses about 1,500 tons of the liquid fluoride each year, purchased from Chemtech Industries Inc. of St. Louis, at a cost of about $88 a ton--roughly 12 cents per person per year. Chemtech officials say they are putting their customers on 50 percent allocation because of the shortage. But so far, the Aqueduct has been receiving 100 percent of its orders, according to Ways.

For optimum results, fluoride chemicals are added to Washington area water in the amount of about one part chemical per one million parts water.

"When it drops below that level, you start to get losses of dental benefits," according to William Bock, the chief of dental disease prevention for the U. S. Centers for Disease Control. He warned that communities trying to stretch supplies by adding the fluoride chemicals in lesser amounts would be making a mistake. "A drop of two-tenths part per million below optimum can reduce benefits up to 50 percent over a period of a year," Bock said.

He said he is concerned that some communities will stop using fluoride chemicals because of the shortage and never start again, even after the shortage ends. At present, about 50 percent of the total U. S. population uses fluoridated water and has less tooth decay as a result, he said.

Sodium fluoride tablets can be added to a child's drinking water to protect against cavities, Bock said. And, if area water were to go untreated by fluoridation chemicals for a year or more, he said he would advise parents to buy the tablets, which are available for a few pennies apiece.

But those tablets generally are available only with a prescription. And studies of their use indicate that parents frequently forget to add the tablets to the child's water after the first few weeks of use.

Officials weren't sure how serious the shortage is or how long it will last. If fluoride chemicals become available again within a few months, then the potential decay problems would be minimal, Bock said. "It would take about a year (of no fluoridated water) before the effect would become apparent, and then it would be in smaller children," he said.