We are getting ready to make some colocation space one of our datacenters available to the public. It uses a hybrid system that uses air cooling most of the time with a mechanical system when it's very hot in the summer (Northwest North Carolina).
We filter the air for particles and add/remove humidity as needed, temperature is always between 69 and 73F.
We maintain humidity between 40% and 60%, does anybody look for tighter humidity controls and for what reason?
Corrosion and static electical charges are a big danger to hardware. Although some manufacturers allow their products to operate as low as 30% it may not be ideal for a general purpose datacenter. Our datacenter operators run between 45-50%.
Corrosion is the classic problem with having humidity that is too high.
The other problem you mention, static is the opposite, when humidity is too low and can cause electrostatic discharge (ESD) problems with people and components as well as the dreaded in-server ESD normally only seen in the driest conditions.
Most equipment can operate in a much wider range though. Below are the specs for your average piece of dacenter equipment.
I have seen it many times, what a fantastic facility. They let their humidity drift to 75% or more. Everything is fine at that humidity with server vendors, but tapes and tape drives are sometimes outside the mfg spec.