Determine humidity level of PCB circuit
While there are standards for humidity testing, much will depend on your application. High humidity levels will cause leakage currents between nets. For very low power boards this may influence the circuit's operation, while for power boards (currents in the amperes range) the effect may be negligible.  

Note that humidity tests are often done with a salt spray. This not only increases conductivity, but also has a corrosive effect. The salt may also deposit on small mechanical parts which may cause them to get stuck.