Bleach whitens mold. It doesn't kill roots.

If the affected area is large – roughly bigger than a square metre – if the mould is on plaster, plasterboard or a ceiling rather than on tiles and sealant, if it's connected to a leak or damp coming through a wall, or if it keeps returning after proper treatment and better ventilation. Also if anyone in the house has asthma, a respiratory condition or a weakened immune system. In those cases the cause is usually structural, and cleaning it is treating a symptom.

Never combine bleach with vinegar, with any acid, or with hydrogen peroxide – those combinations release harmful gases. Don't mix hydrogen peroxide and vinegar into one solution either; use one, rinse thoroughly, then the other if you want to. Work with the door open and a window if you have one, wear gloves, and keep children and pets out until everything is dry.

It lightens the stain effectively, which is why the surface looks clean afterwards. On porous materials like grout and silicone it's generally not the recommended approach, and the discolouration tends to come back within a few weeks. Hydrogen peroxide is the usual chlorine‑free alternative for these surfaces, and it leaves no residue as it breaks down.

Hold a sheet of toilet paper against the vent while it runs – if it holds there, the suction is reasonable; if it drops, the fan is weak or the duct is blocked. The other test is the mirror: if it's still fogged fifteen minutes after a shower with the fan on, the room isn't clearing. Cleaning the dust off the vent is often all it needs.

Clean it if the discolouration lifts with a treatment or two. Replace it if black remains after several attempts, if it's cracked or pulling away from the joint, or if it's simply old. Replacing silicone along a bath or shower tray is an afternoon's work with an inexpensive tube, and it's usually the thing that ends the problem in that spot.