Safety with low voltage high current safety
5V, as said, is safe by all means (less than 60V, by IEC, CENELEC ad UL standards, ad less than 50 V, OSHA).
John D mentioned correctly the problem of burns, if you wear a metal ring.
Then you have to consider the way you interrupt the 42A and if there is a connector that should bring that current and may be extracted with current on. Usually I put several contacts in parallel. With 5V however arcing should be every limited. If I have to switch off 42A dc I would use relays with contacts in parallel: I have a heap of Omron G2RL and they perform quite well.