Antifreeze protects the cooling system against both freezing and boiling over. When used at normal strength (50-percent antifreeze, 50-percent water), it can lower the freezing point of the coolant to -34 degrees F. and raise its boiling temperature to 276 degrees F. Never use straight antifreeze in a cooling system. Always mix it with at least 50-percent water. Most antifreeze is 95-percent ethylene glycol (EG). The only differences between brands of antifreeze are the type and/or quantity of anti-corrosion additives used. Ethylene glycol never wears out but the corrosion inhibitors do. That's why antifreeze should be changed every two years (except for long-life antifreezes, which have special additive packages that allow them to go up to five years/150,000 miles between changes). For the environmentally concerned, propylene glycol (PG) antifreeze is also available at slightly higher cost. PG antifreeze is less toxic than ethylene glycol.