Who pays for air duct cleaning?

[Alex Shipp]

Have tenants who claim they have breathing problems and requested the air ducts to be cleaned. I looked it up and there is no scientific evidence that cleaning air ducts improves air quality according to the EPA.

Air ducts is kind of in the grey area of whether it's regular maintenance or not. Should I say this isn't regular maintenance and refuse to pay for it, split the cost 50/50, or pay the whole cost?

Part of me thinks, I should pay for the whole cost just to maintain a good tenant relations. 

What would you do?