To bleach something is to remove or lighten its color, sometimes as a preliminary step in the process of dyeing; a bleach is a chemical that produces these effects, often via oxidation. Common chemical bleaches include “chlorine bleach,” a solution of sodium hypochlorite (NaClO), and “oxygen bleach,” which contains hydrogen peroxide or a peroxide-releasing compound such as sodium perborate or sodium percarbonate. Bleaching powder is calcium hypochlorite.
A problem with chlorine is that it reacts with organic material to form trihalomethanes like chloroform, which is a well known carcinogen. There is debate over whether any risk from the chloroform in treated drinking water is worth the benefits. However, the use of elemental chlorine in industrial processes such as paper bleaching, with its attendant production of organochlorine-persistent organic pollutants (including dioxins), does not have any benefits. As a consequence over 80% of the woodpulp is nowadays bleached with chlorine dioxide, reducing the dioxin generation under detectable levels.
Chlorine is a respiratory irritant. It also attacks mucous membranes and burns the skin. As little as 3.5 ppm can be detected as an odour, and 1000 ppm is likely to be fatal after a few deep breaths. Exposure to chlorine should not exceed 0.5 ppm (8-hour time-weighted average – 40 hour week).
Another hazard is the formation of acrid chloramine fumes when hypochlorite bleach comes into contact with ammonia or urine, which, though not nearly as dangerous as chlorine, can cause severe respiratory distress. (source: Wikipedia.org)
