How much humidity can a microcontroller/embedded device withstand?
Conformally coat your PCB. Conformal coating, which you can for instance buy as a spray-on substance (Plastik 70 is a popular choice that is stocked with most electronics distributors), creates a moisture, dust and grime-resistant coating over your entire PCB. The 'conformal' in the name means that it is especially engineered to form a continuous coat without holes or voids.