Pre-mixed 50 percent high temperature, nontoxic glycol antifreeze and water - Solar HD is a heat transfer fluid designed for low and high temperature range applications. SolarHD is based on the same chemical formula as propelyne glycol (1,2 Propanediol) but the structure has been modified to (1,3 Propanediol) to have less viscosity at low temperatures and is structured with less acid at high temperatures. A premixed nontoxic glycol solution, it is bio-based and is more sustainable for the environment all while providing great performance. SolarHD is manufactured in a sustainable process. It also is the only solar thermal glycol fluid to be NSF listed. NSF International is a public health and environmental organization that provides standards development, product certification and education on food safety requirements