# Does hot water weaken bleach (for killing bacteria)? **URL:** \ **Category:** Factual Questions\ **Created:** [February 18, 2004, 5:42pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820 "2004-02-18T17:42:06Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![ready29003](https://avatars.discourse-cdn.com/v4/letter/r/71e660/32.png) [@ready29003](https://boards.straightdope.com/u/ready29003)\ **Post date:** [February 18, 2004, 5:42pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/1 "2004-02-18T17:42:06Z")
This was reported to me by a former Safeway meat room cleaner: Safeway forbids the people who clean their meat room with bleach and hot water. They say hot water destroys (or weakens) the ability of bleach to kill bacteria and all that stuff. What say you dopers? Is this really true? The people I was with were shocked to here this. They use bleach and hot water all the time while cleaning stuff. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [February 18, 2004, 6:02pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/2 "2004-02-18T18:02:56Z")
Well, heat will accelerate the breakdown and volatilization of the hypochlorite in bleach, but I’ve never measured the accelerated rate myself. I know that sitting at room temperature for a few months will render bleach worthless, so hot water might do it in hours, or it might not. Why use hot water anyway? Nothing coming out of a tap will do significant harm to the bacteria population, and it uses a lot of energy. ---
**Author:** ![ready29003](https://avatars.discourse-cdn.com/v4/letter/r/71e660/32.png) [@ready29003](https://boards.straightdope.com/u/ready29003)\ **Post date:** [March 10, 2004, 5:13pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/3 "2004-03-10T17:13:58Z")
My friends are very curious regarding the final answer to this question. Does anyone have any other information? Should we be cleaning stuff with cold water and bleach rather than hot water and bleach?? ---
**Author:** ![j666](https://avatars.discourse-cdn.com/v4/letter/j/9de0a6/32.png) [@j666](https://boards.straightdope.com/u/j666)\ **Post date:** [March 10, 2004, 5:32pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/4 "2004-03-10T17:32:10Z")
> [@Nametag](#): > > Why use hot water anyway? Nothing coming out of a tap will do significant harm to the bacteria population, and it uses a lot of energy. There is more to cleaning than killing bacteria; cold water, besides being very uncomfortable for the hands, does not dissolve grease and dried blood. Soaps/detergents dissolve better in warm water and therefore clean better in warm water. I never use bleach on anything but white clothes and the toilet. ---
**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [March 10, 2004, 5:35pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/5 "2004-03-10T17:35:11Z")
I used to work in a hospital maintaining electrical machinery and stuff, part of my work involved keeping industrial laundry equipment running. One of the tasks was to set up the bleach dispensation meters. In the context of laundry, bleach is most effective at temperatures lower than 90 degrees, around room temperature is the lower limit. The foulwash laundry (you don’t really want to know) would undergo a process of bleach wash at around 75 degrees, and then after a period of around 20 minutes, this would be raised to around boiling using steam injectors for around 40 minutes. In certain areas, like theatres intensive care and the neonatal unit, even this stuff once laundered and ironed would then be subject to autoclave sterilisation. ---
**Author:** ![Toddly](https://avatars.discourse-cdn.com/v4/letter/t/22d042/32.png) [@Toddly](https://boards.straightdope.com/u/Toddly)\ **Post date:** [March 10, 2004, 6:39pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/6 "2004-03-10T18:39:40Z")
I don’t think you would want to use hot water because it would irritate your eyes and nose. Looking at this [page](http://www.info.gov.hk/dh/diseases/ap/eng/useofbleach.htm) it seems to validate what the meat cutter said. “Cold water should be used for dilution as hot water decomposes the active ingredient of bleach and renders it ineffective.” ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [March 10, 2004, 10:35pm UTC](https://boards.straightdope.com/t/does-hot-water-weaken-bleach-for-killing-bacteria/229820/7 "2004-03-10T22:35:38Z")
> [@casdave](#): > > I used to work in a hospital maintaining electrical machinery and stuff, part of my work involved keeping industrial laundry equipment running. > > One of the tasks was to set up the bleach dispensation meters. > > In the context of laundry, bleach is most effective at temperatures lower than 90 degrees, around room temperature is the lower limit. > > The foulwash laundry (you don’t really want to know) would undergo a process of bleach wash at around 75 degrees, and then after a period of around 20 minutes, this would be raised to around boiling using steam injectors for around 40 minutes. > > In certain areas, like theatres intensive care and the neonatal unit, even this stuff once laundered and ironed would then be subject to autoclave sterilisation. In **clothes washing** the bleach is commonly sodium perborate, which releases hydrogen peroxide. At 95 degrees the perborate releases 90% of its oxygen and thus is a very effective bleach. At 55 degrees it is only about 60% effective. So clothes washing with this bleach is best done at higher temperatures. Also at low temperatures the enzyme catalase decomposes peroxide incredibly well. At high temperatures the enzyme is deactivated, thus maintining the bleaching power of the peroxide. **Ready etc** , - use the cold water. In addition to what has been said, you are exposing yourself to greater conc’s of chloramines which are volatilised, and probably more easily formed, at higher temperatures.