When danced well, Standard ballroom is for the most part very low impact. So if you feel that your knees are giving you trouble as a result of, shall we say, impactuous action, then you may want to have a professional look at the quality of your movement from foot to foot, and give you some exercises for improving it.
On the other hand, if you have knee problems of any kind such as an injury or arthritis, there is a definite possibility that simply the repetetive action of bending and straightening the knees is affecting them. In this case, some sort of physical therapy of medical treatment might be in order, and you'd be best to consult a professional.
In other words, it might be a good idea to consult a professional dancer, a professional medical doctor, or better yet, both.
And yes, dancing on a non-sprung non-wood floor is probably not helping either. So if you have any control over that (such as in choosing where you go dancing), use it in any way you can.
I hope this helps.
Sincerely,
Jonathan Atkinson