While normal humidifiers aren't particularly environmentally friendly, due to requiring lots of electricity to function, radiator humidifiers buck that trend. They could be considered the most environmentally friendly humidifiers since they work during the times that your heating is on already. When the heating is on and the temperature is naturally warm, allowing for the moisture to evaporate into the air. They simply work when you need them to the most, which is usually due to the heating in the home naturally drying out the air, and these clever devices combat that.

Simply fill the container and hang them of a radiator. When they heat up, the water evaporates and is released into the air negating the dryness caused by the heat. 

These humidifiers are beneficial for your skin and respiratory system, as well as being great for the health of plants and protect furniture, they guard against drying out when you have your heating on.