To examine the measurement method for post-crash hydrogen or helium concentrations in the cabins and other enclosed spaces of vehicles which is provided in the UN Global Technical Regulation on hydrogen fuel cell vehicles (HFCV-gtr), the present study investigated 1) wind velocity conditions not affecting the hydrogen concentrations in the cabin, 2) the effect of the impact absorber of a moving deformable barrier, and 3) the feasibility of substituting the hydrogen concentration measurement with helium gas. The results indicated that the HFCV-gtr measurement method posed problems in its accuracy and reliability because hydrogen concentrations in the cabin varied under the influence of a 0.1 m/s wind and in the presence of an impact absorber in contact with the test vehicle. Furthermore it was found that although HFCV-gtr defines a permissible hydrogen concentration of 4vol% to be equivalent with a permissible helium concentration of 3vol%, this equivalence could not be verified. Consequently it is necessary to replace the HFCV-gtr measurement method for in-cabin hydrogen concentrations with a simpler method immune to external disturbances.