Early industrial manufacture of acetone was based on the thermal decomposition of calcium acetate, produced by treating pyroligneous acid, the condensate from the destructive distillation of wood, with lime. The ready availability of propylene in the 1960s led to the development of acetone processes using either iso-propyl alcohol or eumene as the intermediate. The cleavage mixture is cooled rapidly to prevent further reaction by the acetone formed, and then the residual acid is either neutralized with alkali or removed by passage through an ion exchange column. Isopropyl alcohol can be dehydrogenated to acetone over a catalyst which can be a metal, metal oxide or salt. Hydrogen passes overhead and is scrubbed to remove any residual isopropyl alcohol or acetone before being used as fuel. Steam stripping removes the acetone and by-product propionaldehyde, and the overhead product is condensed. Acetone vapour has a slight narcotic effect and excessive exposure can lead to dizziness, nausea and drowsiness.