Since a bathroom has many sources of humidity, properly working ventilation system is a must. Fully operational ventilation also allows one to safely use gas water heaters.
Polish standards specify the minimum outflow of air froma  bathroom at 50 cubic metres per hour, and from the toilet – 30 cubic metres per hour.
Bathroom ventilation can use natural forces (gravitational ventilation) or fans, i.e. devices that force airflow.
Natural gravitational ventilation is the most popular way of air exchange in households. Polish building regulations require that gravitational ventilation ducts be present in bathrooms and separate toilets. An air grate should be as far from the door as possible, so that the airflow can circulate across the whole bathroom. Bathroom doors should be equipped with grated holes draining air from the bathroom, and these grates should never be obstructed.
Natural gravitational ventilation is sufficient for smaller bathrooms, but in larger ones (above 5.5 cubic metres) it is necessary to use ventilation systems enforcing air circulation. In such bathrooms ventilation can be enhanced by using mechanical exhaust ventilation. The most popular solution features fans installed in exhaust ducts or on grates, working in a constant mode and turned off when there is a person in the bathroom or when steam accumulates. Remember, however, that such devices reduce the efficiency of natural gravitational ventilation when they are not being used.
It is also recommended to install an additional fan in shower cubicle, if enclosed with walls from top to bottom. The capacity of such a cabin should not be lower than 1.5 cubic metres.

Additional fans on ventilation grates and in air ducts must not be installed in the bathrooms from which gas ducts are extended (e.g. to a gas water heater). It is because installing additional fan may lead to drawing in exhaust gases from the ducts to the bathroom, which may became a life an health hazard to the bathroom users.