There's finally a convenient way to protect your sensitive lenses from excess humidity and ambient relative humidity fluctuations: this Dehumidifying Rear Lens Cap for Canon from Brno. The cap holds a replaceable silica gel and protects your precious Canon equipment from moisture that can lead to fungus or mold.
Simply put the cap on a lens, and it will maintain the optimum relative humidity (RH) of 35% to 45%. The caps are ideal for protecting equipment from fluctuating RH and minimizing condensation build-up in climates with excessive changes in temperature--even moving from air conditioning to a hot humid environment will create unhealthy moisture in a lens or camera. Each silica gel changes color from orange to green when it needs changing. One rear lens cap for Canon lenses is included, along with 4 silica gels. Fits all current Canon lenses.
Dehumidifying Rear Lens Cap for Canon
Canon Rear Lens Dehumidifying Cap