An innovation in preconception and fertility products, the Fairhaven Health Glass Basal Thermometer is a mercury-free and non-toxic alternative to mercury thermometers, that is designed specifically for Basal Body Temperature (BBT) charting. Family and Earth-friendly. Contains absolutely no mercury. Ultra accurate and certified to exceed ASTM standards. No-break, easy-read magnifying case. Includes fertility charts and detailed instructions on BBT charting. Cleans up with soap and water.