This number provides a good measure of the gasoline’s ignition quality. A higher octane number corresponds to a smaller chance of premature combustion when subject to combustion in a standard engine. It is denoted as a percentage of isooctance in a mixture containing isooctane and normal heptane. Previously, the use of lead tetraethyl in gas to achieve a high octane number led to lead pollution or lead poisoning. Nowadays, without the use of lead tetraethyl, the same result is achieved through improvements to the engine. In addition, the addition of aromatic hydrocarbons and oxygenates increases the turbulence in the engine’s combustion chamber.