Downwinders Parity Act of 2021 This bill expands a program that compensates individuals who were exposed to radiation from atmospheric nuclear testing and subsequently developed specified cancers. It also extends for five additional years the Radiation Exposure Compensation Trust Fund. Under current law, the program compensates, among others, individuals who were present in a designated geographic area during a period of nuclear testing. The bill expands that area to include all parts of Clark County, Nevada, and Mohave County, Arizona. Current law restricts eligibility to certain parts of those counties.