Hey Stephanie –

This is a really interesting question – as I write this response I am suffering from a nasty case of the flu, so I am especially interested in microbicides! Your question is interesting because people are instructed by many health officials to dilute bleach by a factor of ten before using it. We use this in our own lab to disinfect surfaces, especially surfaces that might otherwise be damaged by undiluted bleach. It is worth noting that the high pH of undiluted bleach also ensures that the antimicrobial component of bleach, sodium hypochlorite, remains stable. At lower pH it can break down into hypochlorite ions and chloramines.

 

A group English of scientists made several dilutions of sodium hypochloriteand tested their efficacy in killing the yeast Candida albicans and the bacterium Enterococcus faecalis. They found that solution strengths from 5.25% to 0.5% were very effective at killing yeast. However, E. faecalis was a little more resilient. At 5.25% (the concentration of undiluted household bleach), 79% of bacteria were killed after 30 seconds exposure. However, at 0.5% (so…roughly 1:10 dilution) only 17% were killed after 30 seconds exposure.

 

So, all things being equal, undiluted bleach is more effective at killing stubborn bacteria. It is important to note, however, that after 30 minutes of exposure, 5.25% and 0.5% were equally effective at killing bacteria – 100% of them, in fact.

 

The data of viruses is a little more straight forward – viruses are simply not cut out to defend against the bleach onslaught! A group of scientists in New York studied the effects of very dilute concentrations of bleach on the respiratory syncytial virus (RSV). A tenfold dilution of bleach, which subsequently mixed with an equal volume of RSV-containing medium (so in fact a twentyfold dilution) eradicated all of the virus. A 100-fold dilution of bleach killed 100% of the virus half of the time, and decreased the number of live viral particles by greater than three logs in the other half of the tests. This was all after five minutes of treatment.

 

So at least for viruses, you can probably dilute the bleach tenfold without worrying too much about decreased antimicrobial activity.

 

I hope this helps with your studies – I’m still going to dilute my bleach because I can’t handle the smell!

Regards,

 

Billy.

 

 

Check out these papers for more information:

Radcliffe CE, Potouridou L, Qureshi R, Habahbeh N, Qualtrough A, Worthington H, and Drucker D. “Antimicrobial activity of varying concentrations of sodium hypochlorite on the endodontic microorganisms Actinomyces israelii, A. naeslundii, Candida albicans, and Enterococcus faecalis.” International Endodontic Journal. 37(7). 2004. PMID: 15189432

 

Krilov L and Harkness M. “Inactivation of respiratory syncytial virus by detergents and disinfectants.” Pediatric Infectious Disease Journal. 12(7). 1993. PMID: 8346001

 

This Oregon State University Research Office publication also contains some interesting information about the chemical qualities of bleach:

“Fact Sheet: Disinfection Using Chlorine Bleach.” Oregon State University Research Office. Published December, 2011. Accessed January, 2013.

http://oregonstate.edu/dept/larc//sites/default/files/pdf/chlori ne-fact-sheet.pdf