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The human hair hygrometer was invented by H.B. De Saussure in the 18th century and has stood the test of time as a reliable and accurate method of measuring relative humidity, not actual amount of water vapor in the air.

The increase in length of human hair as relative humidity changes from 0 to 100% is 2.5% of the overall hair length. Hundreds of strands of untreated human hair are collected from remote areas of the world, treated to remove fats, washed and dried and used in each Belfort Recording Hygrothermograph to provide a consistent and accurate (1% RH at room temperature) measure of relative humidity over many years of service in museums, library’s and industry.