Building regulations mean that we have to install extractor fans in high humidity rooms like bathrooms, kitchens and toilets. This is especially important in non-breathing structures where the walls cannot assist with moisture management.
The extractors are designed to take out warm moist air to help ensure that damp and condensation issues are minimised. There is an issue about size and type of extractors as most are simple on-off that are linked to the light switch. They are also standard sizes that have little to do with the amount of moisture in the room. Bathroom extractors are 100mm fans and kitchen ones are 150mm. If they are anything like the ones in my house they tend to be too small to be really effective when showering or having more than two rings going on the hob.
The effectiveness of the extractors can be increased by using the ones with timers on, or ideally humidity sensors. These then keep the fans on until the humidity levels reach a low enough level. The timed ones at least give a bit more time for the warm moist air to be extracted to the outside.
All of them, though, can be even less effective if they are placed on the walls that face the prevailing winds. Their already weak motors cannot compete with the prevailing wind. For example the ones in my house are on the west and south facing walls and the south westerly winds that occasionally blow in Wales play merry hell with them. When not in use, the external shutters are constantly being blown around by the wind, which can be very annoying at night. Clatter clatter, humph!
So my advice is to think about this when deciding how to manage humidity in these areas of the home. It might be better to vent through the ceiling and out through the eaves rather than drilling through the closest wall. If venting through the eaves, again look at doing this to the East or North if possible. This might affect the power rating required for the fan and its cost, but it is better to have something that actually works rather than not.
This is a simple piece of knowledge and logic that is not always simple to enact, however it is worth bearing in mind if designing a new bathroom, refurbishing an old one etc.