Household bleach is a 3-6% solution of the chemical sodium hypochlorite. The municipal water system chlorinates city water with a 15% solution of sodium hypochlorite and the city parks department chlorinates the public pools with a 30% solution of sodium hypochlorite. The chemical was discovered by a French chemist in 1787. Another French scientist, Louis Pasteur, discovered its disinfectant properties about a hundred years later.
Sodium hypochlorite is an oxidizer, which is a chemical, usually containing oxygen, that readily shares oxygen with another compound. Once on the loose in the wash, sodium hypochlorite attacks the stain in two ways. First, it combines with pieces of the stain and breaks it down into smaller, water soluble bits, much like standard laundry detergents do. But sodium hypochlorite gets its shine as a stain fighter from its second line of attack. Molecules get their color from chromophores — which are arrangements of atoms that absorb most colors of light and reflect back the color we see. Bleach messes with the structure of chromophores and reduces their ability to reflect light. Whatever bits of the stain that remain after the first attack lose their ability to reflect visible light. After a good bleaching a stain is not necessarily gone; it’s invisible.
Unfortunately, sodium hypochlorite does not distinguish between stains and fabric dyes. Colorsafe bleaches, on the other hand, contain much weaker oxidizers (hydrogen peroxide or sodium percarbonate) which, given enough time, are strong enough to tackle common stains but are over matched by durable industrial dyes.
Pasteur figured out how to use sodium hypochlorite as a disinfectant more than hundred years ago, but why it is so effective at germ killing is only now coming to light. Recent research has shown that sodium hypochlorite causes proteins that bacterial cells require for growth to lose their shape and clump into large, insoluble aggregates. The cell cannot make use of the larger, flatter proteins, causing the cell to die. Human immune systems, it turns out, produce hypochlorite when battling bacteria infections. As with shirt stains, hypochlorite attacks healthy cells along with bacterial cells, leading to tissue damage in areas of inflammation. Maybe our immune systems can be encouraged to switch to cell-safe bleach?