I recommend replacing the fan with a constant flow unit such as the panasonic whispergreen fan.
Mold in bathroom ceiling and walls.
Mold can often be found in the shower and the bathtub.
Wearing gloves and a mask can protect people from inhaling any mold spores as.
Typically a bathtub shower or entire bathroom remains damp enough for mold growth just from showering or bathing common causes of bathroom mold.
The lack of light in the bathroom would not directly cause the mold growth.
Scrub thoroughly and wipe clean then spray again and let sit for 10 minutes before wiping.
Nothing beats this practical solution to mold in the bathroom ceiling.
Liberally apply on painted walls.
If you apply the mold resistant paint to the ceiling you already have a fool proof solution to mold problems in that region of your bathroom.
And of course there are abundant water sources for mold created by the running water and steam.
If that s outside of your budget you can install a timer switch on the existing fan.
Grime from body oils and soap scum which is washed off and onto the shower or tub create a food source for mold to feed on.
Squeegee shower walls and glass doors after every use to help remove the moisture that encourages the growth of mold on your walls and also prevent unsightly hard water and soap scum buildup.
Removing mold from painted or wallpapered walls.
Pour into a spray bottle.
Here is a link.
The main cause of mold on a bathroom ceiling is a broken or ineffective exhaust fan.
Combine two tablespoons of borax with 1 4 cup white vinegar and two cups of hot water in a bowl.