K2 Veterans Toxic Exposure Accountability Act of 2020 This bill addresses toxic exposure of members of the Armed Forces deployed to Karshi-Khanabad (K-2) Air Base in Uzbekistan between October 1, 2001, and December 31, 2005. Specifically, the bill requires the Department of Defense (DOD) to conduct a study and report on such exposure. Based on the results of the study, the Department of Veterans Affairs (VA) must determine (1) whether a positive association exists between such exposure and the occurrence of a diagnosed illness in humans, and (2) whether a presumption of service-connection is warranted. Under a presumption of service-connection, specific conditions diagnosed in certain veterans are presumed to have been caused by the circumstances of their military service. Health care benefits and disability compensation may then be awarded. If the VA determines a presumption of service-connection is not warranted, notice must be published and include an explanation of the basis for the determination. The bill also requires the VA to remove previous presumptions if evidence indicates such presumption is not warranted. The bill requires the VA to establish and maintain a registry for individuals who were deployed to K-2 during the specified time frame. Finally, DOD must provide access to information upon request by the National Academies of Sciences, Engineering, and Medicine for purposes of a study regarding the effects of exposure to toxic substances at K-2.