kevin_tee,
Originally I was not sure what you were asking about. But now I understand you are asking about the candy becoming gooey from humidity - an important question in the candy making industry where one does not want the candy to become gooey to the touch all too soon, if at all.
Even important in sugar transport, where one does not want the sugar to cake together and become one big lump. For table salt, the processors add calcium silicate to keep the salt crystals free flowing, but for table sugar that most likely does not work, as at home my sugar has been known to lump together ( I don't have a bag of sugar to check for additional ingredients ).
Here,
http://www.thaiscience.info/journals/Article/Effects%20of%20grain%20size,%20reducing%20sugar%20content,%20temperature%20and%20pressure%20on%20caking%20of%20raw%20sugar.pdf
someone has done a bit of research on raw sugar.
They mention temperature, relative humidity, grain size, and the presence of reducing sugars as factors.
Reducing sugars
http://en.wikipedia.org/wiki/Reducing_sugar
would become syrupy and sticky with moisture, and one has the glucose and fructose acting reducing sugars, present from the caramelization of sucrose.
You can follow those leads and see what the candy industry does to their products to combat stickiness.