One dangerous item found in many American homes is the combination of bleach and ammonia. When these two substances mix, they create chloramine gas, which can cause coughing, nausea, chest pain, and even fluid in the lungs. Mixing bleach with vinegar produces chlorine gas, which has been used as a chemical weapon. The heat and humidity in a kitchen can weaken plastic containers, increasing the risk of leaks and harmful vapors. To stay safe, it is advised to store only one type of cleaner, such as dish soap, under the sink. Heavy-duty chemicals like bleach, oven cleaner, and drain openers should be kept in a locked, ventilated utility closet or garage. It is also important to never mix cleaners, even those that seem natural, such as vinegar and hydrogen peroxide.
Another item to avoid storing under the kitchen sink is paper products like towels, napkins, and grocery bags. While these may appear harmless, they are prone to mold growth in damp environments.
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Source: Kitchen, by recipes