Coal tar liquor, a by-product of the coking industry, was the dominant source of commercial benzene. Crude oil contains small amounts of benzene, toluene and xylene, but recovery is uneconomic. The products are predominantly benzene, toluene and xylenes. The major sources of benzene are: 35% from pyrolysis gasoline, 30% from catalytic reforming, and 16% by toluene dealkylation. The aromatic-rich solvent is drawn from the bottom of the column into a stripper where benzene, toluene and xylene are collected overhead and the solvent is returned to the extractor. The ratio of benzene to xylene can be varied depending on the feedstock composition. The products are predominantly benzene, toluene, ethylbenzenes and xylenes together with considerable quantities of hydrogen. In cold weather, benzene solidifies to a white crystalline mass. Demand for benzene is expected to grow at 2–3% in the 1990s. Benzene easily forms explosive, flammable mixtures with air; because of its heavy vapours, flashback is an additional hazard.