The Environmental Protection Agency on Thursday set stringent new standards for airborne lead particles, following the recommendations of its science advisers and cutting the maximum allowable concentrations to a tenth of the previous standard. It was the first change in federal lead standards in three decades. But the cleanup of areas with excessive lead levels is not required for more than eight years, and the system of monitors that detect the toxic contaminant is frayed. Currently, 133 monitors are in operation nationwide, down from about 800 in 1980, an E.P.A. spokeswoman, Cathy Milbourn, said. The agency is working on rebuilding this network, to include more than 300 monitors, Ms. Milbourn said. The new standards set the limits for exposure at 0.15 micrograms per cubic meter of air, down from 1.5 micrograms, and well within the outer limit of 0.2 micrograms recommended by the advisers. The agency