Posted By John Ridd I would guess the source of the 40-60% RH is BS 7179 Part 6 (1990); another source is CIBSE who offer a slightly different value I think. In the end it's what your staff are comfortable with - but then most don't immediately connect problems to low humidity. Another problem is that Building Services will often tell you that the system is performing 'within spec.' but staff are still having problems. This can happen because the humidity sensors are not positioned at desk level; often they are at the system output or in the roof of an open plan area - these often give different readings to that which might be collected at user level. I have often recorded relative humidity levels in the 20s in offices with many computers.

Posted By Laurie Like David, I think the "comfort level" is 40 - 70% Laurie