The Downwinders Act amends the Radiation Exposure Compensation Act to expand compensation for people exposed to radiation from nuclear weapons testing and related activities. The bill expands the definition of affected areas to include additional counties in New Mexico and extends compensation eligibility to people exposed during the Trinity nuclear test in July 1945. It also adds a new category of compensation for people who lived in specific Missouri ZIP codes near the Coldwater Creek area for at least two years after 1949 and contracted radiation-related diseases, offering between $50,000 and additional medical expense reimbursement for living claimants or $25,000 for beneficiaries of deceased individuals. The bill extends the deadline for filing claims and the termination date of the compensation fund from 2 years to 8 years after the bill's enactment. Additionally, it requires the Department of Health and Human Services to study which counties in Nevada and New Mexico were affected by radiation fallout from nuclear testing and report findings to Congress within one year.