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I'll first provide lots of background information. My actual question is at the very end.

Background

The spread of athlete's foot

Athlete's foot is a fungal infection which is mildly contagious. The infection can be spread by skin particles left on towels. (Source.)

If one person in a family has a fungal infection (e.g. athlete's foot or jock itch), their laundry may also contain fungi. I suspect that towels and laundry machines might make it easier for these fungi to spread: e.g. from the patient's feet to their groin. See, for example, here.

Water at 140 °F

You can kill fungi using water at 140 °F (60 °C). Unfortunately, your water heater probably doesn't produce such hot water, because such hot water is dangerous and can kill unsuspecting individuals. (Sources.) Still, you can make such hot water in other ways. (Source.)

Antifungal rinses

Another option is to use a suitable antifungal rinse.

The most common cause of athlete's foot is T. rubrum. (Source.)

T. rubrum is a dermatophyte-type fungus. Dermatophytes spread through spores. These spores can sometimes live for up to 20 months. (Source.)

Canesten Laundry Hygiene Rinse is 7% benzalkonium chloride. (Source.) A competing product is Bluo Laundry Sanitiser, which is also 7% benzalkonium chloride. (Source.) These products are sold in Australia but not in North America. (How safe are they? I dunno; I'm not a toxicologist.)

The Canesten rinse label makes an impressive claim: "eliminates ... fungi from your washing". Still, it looks like these rinses' fungistatic effects are stronger than their fungicidal effects. In reality, they're not perfect at killing T. rubrum, but they're still better than nothing. An Australian lab found that, in 20 minutes, these types of rinses reduced T. rubrum by between 90% and 99%. (Source.)

(Each rinse is marketed as both antifungal and antibacterial, though there's limited evidence that people can catch bacterial infections from clothing. I'm not convinced that their antibacterial effects are really necessary.)

Use during the rinse cycle, not the wash cycle

Each manufacturer says that its benzalkonium chloride rinse should be added during your washing machine's final rinse cycle, not its initial wash cycle.

You can do this using your clothes washer's fabric-softener dispenser, if present. On a top-loader, this is usually a cup-like device on top of the machine's central agitator post. (Source.) Or you can buy a softener dispenser ball, though they're not always 100% reliable. Or you can add the rinse manually at the correct moment.

A sample washer program

Washing machines can vary. Here's a description of the normal program of certain inexpensive top-loading Whirlpool agitator-type washing machines:

  • Fill - agitate: 16 minutes.
  • Drain: 2 minutes.
  • Spin: 2 minutes.
  • Rinse, fill, & agitate: 4 minutes.
  • Drain: 2 minutes.
  • Spray & spin: 2 minutes.
  • Spin only: 4 minutes.

Total: 32 minutes.

(Based on: wiring diagram W10118358, rev. A.)

The wash cycle is longer than the rinse cycle.

My question

To summarize the above background information: Canesten laundry rinse (7% benzalkonium chloride) has both antifungal and antibacterial effects.

And so I wonder: Why does the manufacturer recommend that it be used during your laundry machine's rinse cycle instead of its (longer) wash cycle?

Any answers or guesses are welcome.

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    $\begingroup$ I think your question has been answered below (with the caveat that you should stop the washer in the rinse cycle to allow more time to disinfect.) But please note that a contaminated towel usually means an unwashed one. I strongly suggest you do not use benzalkonium chloride in the wash. Tines pedis is not highly infectious in the home. It is rather much more related to your individual susceptibility to the fungus. People live together for decades without ever passing it on to members in close quarters. The potential to harm the environment far outweighs the benefits here. $\endgroup$ – anongoodnurse Apr 21 '17 at 13:44
  • $\begingroup$ I've since done some more research. Indeed, it turns out that an expert body doesn't recommend to use benzalkonium chloride to kill fungi in the wash. Instead, it recommends the use of a washing powder containing activated oxygen bleach, as well as various other techniques. See this post for more details. $\endgroup$ – tealhill Jul 18 '18 at 19:12
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The label of the veterinary product Fung-A-Way, which is Benzalkonium Chloride 0.15%, states that "Efficiency is neutralized by soap or detergent residues."

The website http://www.mpbio.com has the following under its description for Benzalkonium Chloride: "Incompatible with anionic detergents, such as soap, and with nitrates."

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Benzalkonium chloride is what's known as a quaternary ammonium cation, or quat. These are disinfectants commonly used in medical settings, for example. The mechanism of action is thought to be due to the long alkyl chains, causing membrane leakage (ref1, ref2).

Like any disinfectant, however, two things are required for effective disinfection: The appropriate concentration, and the minimum required contact time. The following report goes over the importance of contact time but also makes a significant discovery: many disinfectants dry before their required contact time while failing to marginally kill anything except bleach.

The required contact time for quats happens to be 10 minutes. So on a hard non-porous surface youd spray it wet, come back in ~5 minutes and spray it wet again to ensure disinfection. The reason you would want to introduce a quat on the final rinse is because if you immediately wash it off or wash it off within minutes, your disinfectant is most likely to do nothing.

Edit: You said laundry but when I think washer in the US, I think dishes, which made no sense anyways.

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  • $\begingroup$ My question describes a sample laundry-machine program. It looks like the "wash" cycle is about 15 minutes long. Wouldn't this be enough time for the disinfectant to kill enough fungi? $\endgroup$ – tealhill Apr 21 '17 at 5:57
  • $\begingroup$ A) Find a top-loader agitator laundry machine. Use your thumb (or a scrap of cardboard) to defeat the lid switch. (Also watch the drain hose, if the machine drains into a sink.) You'll quickly learn that the wash cycle does not provide a constant clean water flow. The machine fills itself, agitates for 15 minutes, then drains itself to prepare for the rinse cycle. B) OK; good to know. On second thought, I'm thinking I'll indeed avoid the chemical. $\endgroup$ – tealhill Apr 21 '17 at 18:52
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    $\begingroup$ Interesting! I redacted that last comment because I obviously know nothing about my own appliances. So some extra thoughts: I don't know how the quat reacts with your clothes over time, if it'll ruin them, or if the detergent may block the mechanism of action. The manufacturer might be able to tell you if you call them. $\endgroup$ – CKM Apr 21 '17 at 21:17
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Most disinfectants are rendered ineffective or much less effective in the presence of organic materials. Washing your clothes prior to adding Canesten in the rinse allows removal of organic matter and therefore more effective action.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ Welcome to Biology.SE! Your answer is much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. ——— Please take the tour and then consult the help pages for additional advice on How to Answer effectively on this site and then edit your answer accordingly. Thanks! 😊 $\endgroup$ – tyersome Oct 23 at 17:27

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