Adequate ventilation is essential in any building but can be particularly important in humid areas such as bathrooms and spaces used by the public or for business purposes. The air change rate is the number of times the air in a given space is replaced in an hour. Bathrooms and shower rooms should have an air change rate of between two to eight while commercial kitchens should have 20-plus, according to the University of Liverpool. Fans and air conditioning systems are rated by the volume of air they are capable of cycling per minute. If you know this capacity and the volume of the room, you can work out the number of air exchanges per hour.

Measure the room and calculate the volume. For a regular square or rectangular room, volume = length x width x height.

Check your fan or air conditioning unit manual to find the unit's capacity. This may be in metric units (litres per second) or imperial units (cubic feet per minute).

If you are using metric units, divide the capacity of the fan in litres per second by 1000 to get the rate in cubic metres per second. Multiply this figure by 3600 to get the rate in cubic metres per hour. Divide this by the volume of the room in cubic metres to get the air exchange rate.

If you are using imperial units, multiply the capacity of the fan in cubic feet per minute by 60 to get the rate in cubic feet per hour. Divide this by the volume of the room in cubic feet to get the air exchange rate.