Most of the time, that rotten egg smell comes from a natural gas called hydrogen sulfide (H₂S). It’s made by tiny, hard-working bacteria that live in groundwater, especially in wells. These little guys break down organic matter—like leaves and minerals—and produce that potent, eggy odor as a byproduct.
Think of it this way: it’s like a bacteria-powered kitchen compost bin, but happening deep underground. The smell isn’t usually dangerous at low levels, but it’s your water system waving a weird flag. So, why does it only pop up sometimes? It depends on where your water comes from.
Well Water vs. City Water: A Tale of Two Pipes
If you have a private well, you’re more likely to meet this stink because groundwater sits longer in contact with rocks and minerals. Hot water often makes the smell worse because heat releases the gas faster, like opening a soda can. Ever noticed how your hot water smells stronger than cold? That’s hydrogen sulfide escaping into the air more quickly.
City water users aren’t off the hook, though. Your water treatment plant adds chlorine to kill bacteria, which normally neutralizes H₂S. But if the smell sneaks through, it’s often because of a reaction inside your own water heater. That’s right—the magic can happen in your basement, not the reservoir.
