Traditional chlorine bleach, appropriately diluted, can be used for disinfecting and sanitizing, so it's potentially very handy to have in emergencies. It can also be confusing, however, because it degrades, there are many kinds, concentrations are not emphasized, and you want different dilutions for different purposes.
The first problem is that many things are sold as "bleach". Some of them don't use chlorine, others have additives to make them smell better or have less risk of splashing. These are all potentially good properties to have in a laundry additive, but they're not what you want for sanitizing. Everything below is for regular chlorine bleach, and not oxygen bleach, scented bleach, or splash-less bleach.
The second problem is that bleach comes in a range of concentrations. We usually talk about "regular household bleach" as being 5%-6% chlorine, but the range is larger than that. My local store brand is 8.6% chlorine, and some cheap bleaches are as low as 2.6%. Somewhere, in small print, the bottle will say what the actual concentration is.
The third problem is that bleach degrades over time, though it's hard to find numbers on how quickly. The shelf life is generally given as a year from manufacture, provided that you keep it in a cool dark place. It's relatively cheap, so it's worth buying a bottle every ~6m even if you go through it slower than that. Diluted solutions degrade much faster, so mix dilutions fresh daily.
Before working with bleach, it's good to review the ways in which it can be dangerous, especially right out of the bottle:
The same things that make bleach good at killing germs make it good at killing parts of you that it comes in contact with. It's especially important not to get it in your eyes, let it into cuts, or drink it, but it also will eat through your skin. If you've ever gotten a little bleach on your hands you might remember them feeling soft afterwards: that's because it's removed the outer layer of skin. Wear gloves and googles when working with straight bleach.
When diluting bleach you want to avoid splashing the full-strength bleach onto yourself. First measure out your water, then add your measure of bleach.
Bleach releases chlorine gas, which is toxic. Working with bleach outdoors is good, and opening a window for ventilation also works.
Mixing bleach with acids such as vinegar or other household cleaners releases large amounts of chlorine gas. Mixing it with ammonia releases chlorine gas, chloramine gas, and can make hydrazine. I see people talking about the risk of exploding hydrazine, but this looks unlikely unless you're applying heat or otherwise doing something pretty weird. Mixing is usually accidental, and typically happens when people switch from one cleaner to another. If you do switch between cleaners, clean with lots of water in between.
Also keep in mind that many things are effectively more dangerous in an emergency. While getting bleach in your eyes is never good, the recommendation is to clean them with water and then get immediate medical attention. In an emergency, that medical attention may not be available, making a bad situation worse.
Ok, so let's say you have regular chlorine bleach, not too old, you know the concentration, and you're taking appropriate safety precautions. Because bleach is dangerous, we want to use the right concentration for the task at hand. There's a tradeoff between the more effective disinfection ability of higher concentrations, and the less harm to humans and materials of lower concentrations. I've collected the concentrations of the main uses I can find, and converted them to parts per million (PPM) for ease of calculation If your bleach is marked 5.0% hypochlorite, that's 50,000 PPM, so if you wanted 500 PPM you'd mix one part bleach with 100 parts water (well, technically 99 parts water, but we don't need that level of precision).
10 PPM. In an emergency, if you don't have clean water and can't boil water, you can disinfect clear water with a very small amount of bleach. Let it sit for 30min before drinking. The EPA says you normally want no more than 4 PPM, but gives 10 PPM for emergency disinfection. This is probably because in emergencies we're starting with water that's less safe, and we're not planning on drinking water that has this much chlorine daily for years.
You can also use this disinfected water, with soap, for hand washing.
200 PPM. Sanitizing dishes: after washing and rinsing, soak for two minutes in 200 PPM bleach, then air dry without rinsing. For example, two teaspoons of regular bleach per gallon.
This is also the bleach concentration recommended for cleaning indoor sewage spills, under normal circumstances.
500 PPM ("1:100"). Disinfecting or sanitizing hard surfaces, medical equipment, contaminated cloth before washing, and hard things that have been exposed to flood water.
It is possible to use 500 PPM bleach to sanitize hands but it's much better to use soap and water or an alcohol-based hand sanitizer because bleach is bad for your hands.
5,000 PPM ("1:10"). Cleaning blood, excrement, surfaces exposed to dangerous infections like ebola and c. diff, and mold growth on hard surfaces. This is very strong, and you should be nearly as cautious with it as with raw bleach.
In many emergencies authorities will distribute information about the right level of bleach to use for the circumstances. If they do, follow their advice and not the general recommendations I've collected here.
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