Butanol is the most complex of the alcohol-based fuels. It is a four-carbon alcohol with a structure of CH3CH2CH2CH2OH. Butanol is more toxic than either meth­anol or ethanol. The LD50 for oral consumption of butanol by a rat is 790 mg/kg. The LD50 for skin adsorption of butanol by a rabbit is 3400 mg/kg. The boiling point of butanol is 118°C and the melting point is -89°C. The density of butanol is 0.81 g/mL, so it is more dense than the other two alcohols, but less dense than water. Butanol is commonly used as a solvent, but is also a candidate for use as a fuel. Butanol can be made from either petroleum or fermentation of agricultural products. Originally, butanol was manufactured from agricultural products in a fermentation process referred to as ABE, because it produced Acetone-Butanol and Ethanol. Currently, most butanol is produced from petroleum, which causes butanol to cost more than ethanol, even though it has some favorable physical properties compared to ethanol. It has a higher energy content than ethanol. The vapor pressure of butanol is 0.33 psi, which is almost an order of magnitude less than ethanol (2.0 psi) and less than both methanol (4.6 psi) and gasoline (4.5 psi). This decrease in vapor pressure means that there are less problems with evaporation of butanol than the other fuels, which makes it safer and more environmentally friendly than the other fuels. Butanol has been proposed as a replacement for ethanol in blended fuels, but it is currently more costly than ethanol. Butanol has also been proposed for use in a direct butanol fuel cell, but the efficiency of the fuel cell is poor because incomplete oxidation products easily passivate the platinum catalyst in a traditional fuel cell.