How Much Humidity Is Too Much?

[BruceSD]

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I keep my dozens of lenses in my finished basement.   I usually keep the humidity down there at 45 - 50% during our humid summers.  What is the humidity where you keep your lenses? 

Yeah, I use dessicants too, but I'm not wanting to start a discussion on that.

[Birna Rørslett]

I would be surprised to see relative humidity indoors go above 40%. Climate in my region is Nordic cool. I never have had any issues with lens fungus (other than with those lenses purchased second-hand from abroad, which once in a blue moon need fungus removal).

[pluton]

I have no expert knowledge, but 50% is the figure I've seen over the years quoted as "safe" or "should be safe". 
I think it is important not to concentrate whatever humidity is present by storing lenses in hermetically sealed bags, containers, leather bags (which seem to absorb humidity from the air), or any place without air circulation.
The North American continental climate is a humidity monster in the summer. Without air conditioning or dehumidification, it gets sticky, even indoors.
Is your basement ventilated, in addition to being de-humidified?  You have taken the necessary precautions against the accumulation of Radon gas, I presume?

[BruceSD]

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Based on Birna's comment I reduced the humidity from 50 to 40%

I don't see how radon levels would affect camera gear.  However, I do have an expensive radon/sub-floor/continuous-flow ventilation system installed in my finished basement.