Besides detonation resistance, however, other characteristics of a replacement fuel are scarcely less important. One is environmental safety; there is no point replacing the lead in avgas with another powerful toxin. Another is materials compatibility. Some potential fuel components, like ethanol and alcohol, are hard on elastomers used in engine, propeller and fuel system seals. If they had to be used, every airplane might have to have some parts, like O-rings and pump diaphragms, replaced. Another is lubricity; gasoline has lubricating properties that some potential antiknock fuel components, like toluene, lack. Another is vapor pressure, which represents resistance to bubbling at low ambient pressure and high temperature. Avgas has a much higher resistance to bubble formation than, for instance, auto fuel does. Excessive bubble formation, in the form of vapor lock, can make an engine run roughly or quit altogether.