Arla Harrell Act This bill requires the Department of Veterans Affairs (VA) to reconsider and make a new determination regarding each claim for disability compensation in connection with exposure to mustard gas or lewisite during active military, naval, or air service during World War II that was denied before this bill's enactment. The VA or the Department of Defense (DOD): (1) shall presume that a veteran experienced full-body exposure to mustard gas or lewisite unless proven otherwise; and (2) may not use information contained in the DOD and VA Chemical Biological Warfare Database or any list of known testing sites for mustard gas or lewisite maintained by the VA or DOD as the sole reason for determining that the veteran did not experience such exposure. The VA shall report to Congress every 90 days on reconsidered claims that were denied. The VA and DOD shall jointly establish a policy for processing future claims in connection with such exposure. The bill requires DOD, for purposes of determining whether a site should be added to the list of DOD sites where mustard gas or lewisite testing occurred, to: (1) investigate and assess sites where the Army Corps of Engineers has uncovered evidence of mustard gas or lewisite testing or where more than two veterans submitted claims in connection with exposure that were denied; and (2) report on experiments conducted by DOD during World War II to assess the effects of mustard gas and lewisite. The VA shall: (1) investigate and assess VA actions to reach out to individuals who had been exposed in such experiments, the disability compensation claims that were filed, and the percentage of such claims that were denied; and (2) submit a comprehensive list of each location where such an experiment was conducted.