If you smell dampness coming from underneath your kitchen sink, you likely are smelling mildew caused by water that has accumulated somewhere under your sink, either within your pipes or in the cabinetry. And it's likely making your entire kitchen smell like a basement. To fix the problem, you must first discover the reason for the smell. Then you can take steps to eliminate the reason for the lingering odour.
Sourcing the Problem
Open the drawers or kickboard under your sink to reveal the sink pipes and garbage disposal, if you have one. Inspect the pipes for small holes that might be leaking water. Wet spots in the wood underneath the pipes are a telltale sign. Run your tap so that water runs through the pipes while you look underneath the sink for signs of leakage. Your leak also might be coming from the rubber tubing connecting your garbage disposal to your sink, so inspect that as well. If you still don't see leaks, then the problem might be a blockage in your pipes. Stuck food or other debris might be causing the pipes to decompose.
Repairing the Problem
If you find a leak, fix it immediately. Otherwise, each time you turn on your faucet, water will end up leaking down into your cabinet, which will only make the smell worse and damage your cabinet. If you're not handy, call a plumber to fix the leak. In the meantime, avoid using the sink as much as possible. If you suspect the problem is a blockage in your pipes, pour a heavy-duty solution for clogged drains down your sink, wait the recommended amount of time, then flush with hot water.
Eliminating the Smell
Clean the area under your sink with a detergent or disinfectant. This will kill any mould that has been growing under the sink. Combine dampness with the dark, unventilated area underneath the sink, and you've got perfect mould-growing conditions. Keep the cabinet doors or kickboard open to let the area air dry fully. Place a moisture absorbent under the sink to absorb leftover moisture before you close the area.
Freshening Up the Kitchen
To eliminate any lingering odour of mildew caused by dampness, occasionally cut up a lemon into thick slices and put it down your waste disposal. As the disposal's blades cut up the lemon, it will release a citrus smell. Don't flush water down the disposal for an hour or more to allow the lemon smell to really soak in.