To help your indoor greens grow healthy, you must maintain an ideal humidity level of 70-80%, which is a challenge to preserve for the tropical species.  For some of them, the more is the relative humidity the better it is for their survival to almost 90%. However, the succulents and cacti can survive happily under 10-40% of relative humidity and may show signs of rotting in extreme moisture.  To ensure a comfortable indoor for both you and your various species of houseplants, try to maintain an ideal humidity between 60-80% and keep adjusting the level during hot and humid summers.