Hi Lara,
As the acting C2C Care community monitor, I'd like to thank you for posting this question to the community. For oil paintings, relative humidity levels above 70-75% elevate the risk of mold growth and structural instability. If the painting is on a wood stretcher, the wood is at risk of expanding while the canvas is at risk of contracting/shrinking increasing the strain on and destabilization of the paint layers. Here are links to CCI Notes 10/4 Environmental Guidelines for Paintings and Structural Response to Relative Humidity for your reference.
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Sharon Fickeissen
Connecting to Collections Care Monitor
Senior Library Conservation Technician
Hagley Museum & Library
Wilmington DE
sfickeissen@hagley.org------------------------------