Heating and humidity
The United States Environmental Protection Agency (EPA) recommends that indoor humidity levels should be between 30% and 50%, depending on the outdoor temperature: more humidity when it is warmer and less when it is colder. Exposure to very dry air can irritate the mucus membrane and cause throat dryness leading to a greater risk of catching a cold and flu, while exposure to very humid air can cause breathing difficulties and breathing allergies. It can also cause mildew and mites.
To prevent the air getting too dry, you should try to keep the temperature at around 20 degrees (if it is too hot the air will become too dry) or place a bowl of water on top of radiators. You can also install humidifiers in certain rooms or at certain times, for example, if you have a baby with mucus or breathing problems.
Temperature versus Humidity
On the other hand, excessive humidity can be avoided by airing rooms well, particularly in the kitchen when cooking or in bathrooms after a shower. Always use the fume extractor in the kitchen. It may be useful to install an extractor in the bathroom that starts up when the light is switched on for example. You should also avoid drying clothes indoors (or if you have no choice, place a dehumidifier in the room when you are drying them) and avoid keeping plants in the room. If high humidity persists, you can install a mechanical ventilation system, however, you should seek professional advice to determine the cause of the humidity.
In any event, it is wise to obtain a good hygrometer to measure the exact relative humidity in each room.