Halitosis is caused by putrefaction. Bacteria grow on dead cells an food particles which collect on the tongue and in pockets in the teeth. They generate hydrogen sulphide, methyl mercaptan and dimethyl sulphide. Volatile compounds of onions, alcohol and garlic are absorbed and excreted through the lungs causing bad breath. Usually those with bad breath are unaware of it and those wrongly convinced they have it need psychotherapy.
As an indication, the market for mouthwashes (whether antiseptic, cosmetic or medicinal) in the UK in 1986 was £10.5 million.