Hi lanzmum,
It sounds like you’ve read this already but there is more about this in post #10 here. If you believe that your combined “risk factors” justify it or if you have had any mold or mildew issues in your home then you could always add a bed rug on top of the solid surface.
If your mattress uses wool in the top layer under a cotton fabric then you would probably have less reason to worry because wool is very breathable and resistant to mold and mildew … particularly if you air it out in the sun on occasion. It may also be worth talking to the manufacturer of your mattress to see what they think about your options because they will be more familiar with it than anyone else.
In the end … this is really a “best judgement” call. I don’t think you can quantify the risk exactly but I would also consider that how you feel about your choice and how much you may “worry” about it would also be important. If I was in your shoes I would go with the suggestions of the manufacturer and since a solid platform is very supportive … if yours is in good condition I would probably consider the bed rug if you think the risk justifies the extra cost.
Phoenix