Early engine coolant antifreeze was methanol (methyl alcohol). Methanol was used as an automobile coolant antifreeze in the early 1900s. Various coolants have successfully been used, ranging from diesel fuel to antifreeze and water-soluble oil. Propylene glycol is used rather than ethylene glycol (ie antifreeze), as the latter is toxic. The industry term for the antifreeze mixture is "engine coolant". Diethylene glycol, an ingredient used in antifreeze, is toxic to humans. Many indirect systems use antifreeze (e.g., propylene glycol) in the heat transfer fluid. This included brands like Fram Filters, Prestone antifreeze and Autolite spark plugs. It is usually used with additives, like corrosion inhibitors and antifreeze. Another problem was clogged radiators from antifreeze mixtures incompatible with aluminum. Engine cooling system was modified to use antifreeze instead of water. It is used as an antifreeze in diesel engines. Transmission oil is heated/cooled by the engine antifreeze that runs through the cooler. Flat panels are regularly used in both pure water and antifreeze systems. The plant produced plastics, herbicides, and antifreeze.