The ideal humidity level in a UK home should be around 50-55% relative humidity (rh) and slightly lower in winter months, around 40% to prevent condensation from forming on windows. However, the range in which optimal indoor humidity is achieved is quite broad, between 30 and 60% rh. It is important to monitor humidity levels regularly, which can be done using a hygrometer, a handheld device that can be purchased online or from a DIY store for under £10. Humidity levels can be affected by various factors such as air temperature, pressure, and moisture sources like cooking, showering, and drying clothes indoors. To maintain the ideal humidity level, it is recommended to ventilate the house by opening windows, using a dehumidifier, and avoiding drying clothes indoors without turning on the heating.