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Wednesday, November 25, 2015


Is there any chemical that has a killing contact time of 3 minutes or less?


Certainly, but it depends on what you are killing. Here are some recommended dwell times for common disinfectants.
Sodium hypochlorite (bleach): Dwell time – 5-10 minutes
Phenols/chlorinated phenols: Dwell time – Generally 10 minutes
Quaternary ammonium compounds (Quats): Dwell time – Generally 10 minutes
Silver dihydrogen citrate (SDC): Dwell time – 30 seconds to 5 minutes
You may note that SDC, which is fairly new to the disinfectant scene, offers kill times as low as 30 seconds and that is fast. However, bacteria can take up to 2 minutes and fungus up to 5. Viruses up to a full minute.
The reason for providing dwell time is to allow the chemical to stay in contact with the microorganism long enough to do whatever it does to disrupt the life cycle. If the chemical dries out (bleach, for example), there is nothing active to harm the organism. If the disinfectant leaves a residue (quats and SDC, for examples) the dwell continues and so does the killing possibility.
Some microorganisms are tough to kill and require longer exposure to active ingredients in the proper solution. And here, exposure is the key word. Microbes hidden in a layer of grease are not exposed to any amount of chemical. The disinfectant must be placed on a physically clean surface so that microbial exposure is possible. Only then will the kill desired to produce a hygienically clean (disinfected) surface be achieved.
Most disinfectants are developed to provide a broad range of effectiveness. In other words, they should kill a lot of microorganisms and not just five or ten. Since we don’t know what is on any given surface without extensive lab work, we select a disinfectant that is effective against the maximum number of different organisms we can reasonably expect to encounter on the surfaces with which we are concerned. We first choose the surfaces where disinfection seems necessary, and then we select a disinfectant that can eliminate those microorganisms we can expect to find on those surfaces. And we leave it active on the surface for the time necessary to kill the most resistant organism, which will probably approach ten minutes, no matter what we use.
This would rule against disinfecting hallway and office floors and entry mats, and possibly even phones and computer keyboards that are loaded with bacteria belonging to the daily user, hence little threat to that person. There is little risk of cross infection when the only user is the person who contributed the microorganisms to begin with. He needs only to wash his hands regularly to keep the germs from getting where they ought not.
This solves the difficult problem of how to provide adequate dwell time for disinfecting a telephone or sensitive keyboard. Wipe them with a cloth dampened with your choice of disinfectant solution and leave damp. There will be some kill, some residue, and the surface will be physically clean. True, the contact time will be less than ideal, but what does it matter? Once the surface is touched again, the recontamination has started.

Lynn E Krafft, ICAN/ATEX Editor