A colourless liquid diene, CH2:C(CH3)CH:CH2. The systematic name is 2-methylbuta-1,3-diene. It is the structural unit in terpenes and natural rubber and is used in making synthetic rubbers. Isoprene is one of the principal volatile hydrocarbons emitted by plant foliage, especially at high temperatures on sunny days. This scatters sunlight, forming a bluish haze, and also reacts with nitrogen oxides in air, contributing to low-level ozone, which can cause respiratory problems.