In uk.d-i-y Julie wrote:

Yes I am making an open top light box with fan ventilation for
seedlings

and shared worries about using a light fitting designed for "normal"
permanent ceiling installation.

Me, I wouldn't worry too much about the humidity - the fittings will
run warm enough that condensation on them is most unlikely. You do need
to make sure you have decent strain relief on the flex which connects
into the light fitting - there isn't usually any provision for strain
relief in tube fittings, so you may want to adapt something using a
cable gland or similar (tightish cable ties would do at a pinch, though
not really best practice!); but presumably you'll need an incoming feed
which then splits or loops to both the fan and the lights, right? So more
than one place to securely anchor the incoming mains lead.

Other thing to watch for is keeping ordinary PVC cable away from the
hottest bits of the fitting, the "ballast" - you'll see when you open it
up that the terminal block for the incoming wires is some few inches or
more away from that, while the wires running to the ballast are usually
wrapped in heat-resisting insulation. (If you're not sure which bit of
the tube fitting will run hottest, connect it up temporarily and feel
which bits got hot!). Keep your ordinary flex routed away from that part,
and/or use a more heat-tolerant flex like the one used for immersion
heaters...

If you're using this in a shed, you really want to be sure that the
shed's power supply comes through an RCD...

Happy light-box building - Stefek