 The benefits of each approach need to be considered in light of other sources of errors associated with emissions monitoring, notably vehicle-to-vehicle differences, and the emissions variability within the vehicle itself.PEMS need to be safe enough to use on public roads.During testing, portable emissions systems could attach extensions of the tailpipe, add lines and cables outside the vehicle, carry lead-acid batteries in the passenger compartment, have hot components accessible to bystanders, block emergency exits, or interfe with the driver, or have loose components that could be caught in moving parts.