Atmospheric humidity is expressed as a percentage of the moisture saturation of air.
There are 2 DIY ways to provide increased humidity are (1) By attaching a humidifier to the heating or ventilating system in the home or (2) Placing gravel trays (in which an even moisture level is maintained) under the flower pots or containers.
This will increase the relative humidity in the vicinity of the containers. As the moisture around the pebbles evaporates, the relative humidity is raised.