; (2) efficiencies of allowing the safe transportation of hazardous materials by unmanned aircraft systems and whether such transportation complies with the hazardous materials regulations under subchapter C of chapter I of title 49, Code of Federal Regulations, including any changes to such regulations issued pursuant to this section; (3) the risk profile of the transportation of hazardous materials by unmanned aircraft systems, taking into consideration the risk associated with differing weights, quantities, and packing group classifications of hazardous materials; (4) mitigations to the risk of the hazardous materials being transported, based on the weight, amount, and type of materials being transported and the characteristics of the operation, including operational and aircraft-based mitigations; and (5) the altitude at which unmanned aircraft operations are conducted. (c) Safety risk assessments The Secretary may require unmanned aircraft systems operators to submit a safety risk assessment acceptable to the Administrator, as part of the operator certification process, in order for such operators to perform the carriage of hazardous materials as authorized under this section. (d) Conformity of hazardous materials regulations The Secretary shall make such changes as are necessary to conform the hazardous materials regulations under parts 173 and 175 of title 49, Code of Federal Regulations, to this section. Such changes shall be made concurrently with the activities described in subsection (a). (e) Stakeholder input on changes to the hazardous materials regulations (1) Implementation Not later than 180 days of the date of enactment of this Act, the Secretary shall hold a public meeting to obtain input on changes necessary to implement this section. (2) Periodic Updates The Secretary shall— (A) periodically review, as necessary, amounts of hazardous materials allowed to be carried by unmanned aircraft systems pursuant to this section