38 U.S.C. § 1168

(a) Medical Examinations and Medical Opinions .— (1) Except as provided in subsection (b), if a veteran submits to the Secretary a claim for compensation for a service-connected disability under section 1110 of this title with evidence of a disability and evidence of participation in a toxic exposure risk activity during active military, naval, air, or space service, and such evidence is not sufficient to establish a service connection for the disability, the Secretary shall— (A) provide the veteran with a medical examination under section 5103A(d) of this title ; and (B) obtain a medical opinion (to be requested by the Secretary in connection with the medical examination under subparagraph (A)) as to whether it is at least as likely as not that there is a nexus between the disability and the toxic exposure risk activity. (2) When providing the Secretary with a medical opinion under paragraph (1)(B) for a veteran, the health care provider shall consider— (A) the total potential exposure through all applicable military deployments of the veteran; and (B) the synergistic, combined effect of all toxic exposure risk activities of the veteran. (3) The requirement under paragraph (2)(B) shall not be construed as requiring a health care provider to consider the synergistic, combined effect of each of the substances, chemicals, and airborne hazards identified in the list under section 1119(b)(2) of this title . (b) Exception .— Subsection (a) shall not apply if the Secretary determines there is no indication of an association between the disability claimed by the veteran and the toxic exposure risk activity for which the veteran submitted evidence. (c) Toxic Exposure Risk Activity Defined .— In this section, the term “toxic exposure risk activity” has the meaning given that term in section 1710(e)(4) of this title .