Vol 6. Issue 5 / February 13, 2006

Everything You Always Wanted to Know About Bleach But Were Afraid to Ask

By Mika Ono

Bleach is so common in the home and in the lab, it's easy to assume that using it is a no-brainer. You pour some out and it kills germs, right? Well, yes and no.

"Bleach is the cheapest and most common disinfectant," says Scott Curriden of The Scripps Research Institute's Environmental Health and Safety Department. "It has been around for centuries and can be remarkably effective at killing bacteria and viruses. There are, however, some things to keep in mind."

Here are some facts about bleach that may surprise you:

  • Bleach is more effective at killing germs when diluted than when used straight out of the bottle. For most uses, a ratio of nine parts water to one part bleach is recommended.
  • Bleach can expire. After a shelf life of six months, bleach starts to degrade. Even in its original bottle, bleach becomes 20 percent less effective as each year goes by.
  • Bleach mixed with water at a 1:9 ratio (i.e. 10 percent bleach) is potent for about a day (it's more unstable in its diluted form). If you plan to use a bleach solution over the span of a week for repeated disinfection, Curriden recommends mixing it at a 1:4 ratio (20 percent bleach) to make it last. His take-home message for those in the lab—if you find an undated bleach solution in a spray bottle, don't use it. Make a new batch (and date it).
  • Your mother was right when she warned you not to mix bleach and ammonia-based cleansers. Those with a chemistry background will tell you that mixing bleach and ammonia can create toxic chloramine gases and an explosive called nitrogen trichloride.
  • Bleach is basically the same as sodium hypochlorite—but not when you calculate dilutions. Confusion can arise because what is labeled 100 percent bleach is only three to six percent sodium hypochlorite. In other words, if a lab protocol calls for 1 percent sodium hypochlorite, use 20 percent bleach.
  • You need a higher percentage of bleach, about 20 percent, if you are using it with large amounts of organic materials, such as proteins and serums, as these tend to neutralize the bleach.
  • Bleach is corrosive. "Bleach can drill a hole through stainless steel," says Curriden, "that's why it's important to wipe down metal surfaces with water or ethanol after treating them with bleach." For delicate metal instruments, consider avoiding bleach altogether and using a different kind of disinfectant, such as ethanol.

While bleach can be powerful, Curriden notes it has a kinder, gentler side. "What do bleach and the beach have in common?" he asks. "Salt water—liquid bleach starts as salt water and degrades into salt water. When using bleach for disinfection, you want to make sure you're not using salt water."

For more information, Scripps Research employees are welcome to contact Environmental Health and Safety, at x4-8240.


Send comments to: mikaono[at]scripps.edu





Bleach is a cheap and common disinfectant but it helps to know how to use it, according to Scott Curriden of Scripps Research Environmental Health and Safety. Photo by Kevin Fung.