The nonmesonic weak decay of Λ hypernuclei using nonrelativistic nuclear matter is studied. As the basic building block we use the Polarization Propagator Method developed by Oset and Salcedo. It is shown that the exact calculation of exchange terms is required. Using the Local Density Approximation we evaluate the nonmesonic decay width for 12 Λ C and compare the result with a finite nucleus calculation, obtaining a qualitative agreement.PACS number: 21.80.+a, 25.80.Pw.