Increasing the humidity above 7% causes a protective layer of water molecules on the surface of the reactants which prevent the chemical reaction taking place so efficiently .
apparently it is suggested that multiple layers of water molecules are formed at the TiO2 interface with air, even at low relative humidities. The multiple-layered film of water retards/prevents CB from reaching the reactive TiO2 surface or contacting radical species in the boundary layer.
Difference between affect and effect.
Getting drunk can 'affect' a person's balance. - Cause
Falling over is the 'effect' of too much to drink. - Result.
This only applies in a general everyday usage sense.
There are many more definitions of the two words, and ways of using them - such as -
'personal effects' = personal property
'affected attitude = an attitude of pretence.
As no.knowledge said, it is a minefield.
But for general purposes, remembering 'Cause' and 'Result' can help.