We all know that it's possible to purify water by boiling it, but this can be difficult in certain circumstances. For example, if you're stuck in a small apartment during a disaster, the utilities may be shut off and the running water may be contaminated. You probably won't want to start a smoky wood-burning fire in the kitchen, so it's important to have a backup plan for purifying drinking water. Fortunately, this is easy to do with a cleaning chemical most people already have in their homes: bleach.

Clean drinking water is a vital resource that may become difficult to obtain in an emergency.

It's extremely easy to purify water with bleach, but you'll need to know the right ratio of bleach to water. Too much bleach, and the chemical itself will harm your digestive system—too little, and bacteria in the water will make you sick. You've got to get the balance just right.

If you're not at home, look for pure bleach in a kitchen or janitorial closet.

Most household bleach contains 8.25% sodium hypochlorite, which is the active ingredient you'll want to look for on the label. If your label indicates that the sodium hypochlorite levels are lower than 8%, you may want to carefully add a few extra drops of bleach to your water to reach the correct concentration. Also, be sure your bleach is of the regular unscented variety, without any added ingredients that may not be safe to consume.

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The United States Environmental Protection Agency (EPA) recommends the following amounts  of 8.25% sodium hypochlorite bleach to add to water:

Here's an easy way to remember the ratio: 2 drops per 1 liter of water gives us “21”, just like the legal drinking age in the US. So, just remember “drinking starts at 21”.

Here are some other tips:

• Find a brand-new bottle of bleach, if possible. If not, get a clean used bottle that has been stored at room temperatures for less than a year.
• Use a clean eyedropper to portion out the bleach carefully, and be sure to use a clean container for your water.
• If the water is cloudy, colored, or very cold, double the amount of bleach used. You may also want to filter the water through a cloth or paper filter to remove debris.
• Stir the bleach into the water, and let it stand for at least 30 minutes.
• When it's ready to drink, the water should have a slight chlorine odor. If it doesn't, add a repeat dosage, stir, and wait another 15 minutes before drinking.

Written by Patrick McCarthy