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