; and (B) becomes manifest within the period, if any, prescribed in such regulations in a veteran who was exposed to that toxic substance during active military, naval, or air service. (3) (A) For purposes of this subsection, the Secretary may presume that a veteran who has a disease specified in paragraph (2) was exposed to the toxic substance for which the Secretary has determined under paragraph (2)(A) warrants a presumption of service connection by reason of having sufficient evidence of positive association with exposure to the toxic substance, notwithstanding that there is no record of evidence of such exposure, if the Secretary determines based on one or more of the factors set forth in subparagraph (B) that a presumption of exposure is warranted. (B) The factors set forth in this subparagraph are as follows: (i) The duty location of the veteran. (ii) The length of service of the veteran. (iii) Such other factors as the Secretary considers appropriate. (b) Determinations relating to diseases (1) Whenever the Secretary determines, on the basis of sound medical and scientific evidence, that sufficient evidence of positive association exists between the exposure of humans to a toxic substance and the occurrence of a disease in humans, the Secretary shall prescribe regulations providing that a presumption of service connection is warranted for that disease for purposes of this section. (2) In making determinations under paragraph (1), the Secretary shall take into account— (A) reports received by the Secretary from the National Academies of Sciences, Engineering, and Medicine under section 202(g) of the Toxic Exposure in the American Military Act of 2020