Pollution-Free Aviation Sites Act or the PFAS Act This bill directs the Department of Transportation (DOT) to provide grants to Part 139 airports for a per- and polyfluoroalkyl substances (PFAS) replacement program for firefighting foams and related equipment. (Part 139 airports include airports that serve scheduled and unscheduled air carrier operations for aircraft with more than 30 seats or serve scheduled air carrier operations for aircraft with 10-30 seats.) DOT must establish the PFAS replacement program within 90 days of the Department of Defense approving a fluorine-free firefighting agent (i.e., an agent that does not contain any PFAS). Under the program, DOT must reimburse eligible airport sponsors for the reasonable and appropriate costs associated with the one-time initial acquisition of fluorine-free firefighting alternatives, the disposal of PFAS products as part of the transition to a fluorine-free agent, cleaning or disposal of existing equipment or components, and replacement of aircraft rescue and firefighting equipment. DOT must prioritize grants of up to $2 million to replace aircraft rescue and firefighting vehicles at specific smaller airports (i.e., Index A airports) that do not have existing capabilities to produce fluorine-free foam. DOT must determine the eligibility of costs payable under the PFAS replacement program by taking into account all engineering, technical, and environmental protocols and generally accepted industry standards that are developed or established for fluorine-free foams.