Thanks for your rich input. There's so much to learn from this forum but also Node-RED as a tool to process data from various devices/sensors around the house.
Have gotten this to work by setting the output of the MQTT In node to 'a parsed Json object' into a switch node to access the humidity property and either turning the fan on (MQTT Out) of off (MQTT Out) based on a threshold.
Solves me one challenge I had with my wife as I always forget to switch on the fan after a shower
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