Sink cabinets and chemical storage
Sink cabinets are a significant area where water problems occur, and where mold and mildew are likely to be found. Think about what's located in your sink cabinets: the underside of metal sinks, garbage disposals with metal casings, copper water supply pipes, and plastic or metal drainage pipes. Both plastic and metal are affected by corrosive chemicals, and continued corrosion and rusting can eventually result in leaks. Many people (like 99% of people, probably!) store household cleaning products and supplies in the sink cabinets (see Figure 12) without realizing the possible consequences of doing so.

Figure 1. Chemicals stored in the kitchen sink cabinet.
Sink cabinets are absolutely the wrong place to store cleaning chemicals because, by their very nature, they are corrosive, so do not store chemicals in them! Additionally, children won't be able to gain access to harmful and dangerous chemicals if they are not in those low sink cabinets. And no one ever takes all those chemicals out of the sink cabinets to inspect the cabinet floor and the water and drainage pipes—unless they're moving, a significant leak is noticed, or a young child is in the hospital after gaining access to the chemicals. Regardless of what you store in your sink cabinets, remember to childproof your cabinets and drawers if young children are around.
Water supply lines and drain pipes should be checked regularly. Here's how to do it on a daily basis with little effort on your part: Store dry materials (towels, bathroom tissue, boxes, etc.) in sink cabinets (see Figure 2 and Figure 3). If normally dry materials are wet when you remove them, you know you've got a leak of some type somewhere, so check for leaks in the water pipes or drain pipes, and check for deteriorated caulking/grouting around the sink and countertop. Have a licensed plumber repair or replace any plumbing components, and have the deteriorated caulking/grouting repaired.

Figure 2. Good sink cabinet storage. Note that the few chemicals stored there have not yet been opened.

Figure 3. Good sink cabinet storage. Again, the few chemicals stored have not yet been opened.
So where should you store common household cleaning chemicals? A cabinet out of the reach of young children in the garage or at an exterior location is great, but if you must keep them inside, an upper hallway closet, the cabinet above the microwave oven, or the cabinet above the refrigerator make good interior locations. If it means that you have to go buy a step ladder to get the chemicals each time you need them, I think that small inconvenience is far better than the "inconvenience" of going to a funeral for a dead child or visiting an injured child in the hospital for several days. I hope you agree.
Regardless of where you store the chemicals, make sure the cover is tightly closed and secured so that it doesn't spill if you accidentally knock it over or drop it. If you do have to keep chemicals in lower cabinets or drawers—and you shouldn't—again, make sure those cabinets and drawers have child-proof latches on them if you have young children in the house.
Recommendations
- Recommend storing chemicals in upper cabinets, in locked cabinets, and/or in exterior cabinets.
- Recommend storing dry materials such as towels, boxes, and tissue in sink cabinets to facilitate daily monitoring of the water supply lines and drain pipes.
- Recommend childproofing all lower cabinets and drawers.
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You have brought up some excellent points, however, where do you suggest people put these products? They do so under the cabinets for easy access.