After my last tenant moved out, yellow stuff started dripping down the walls. What could it be? I tried cleaning it, but I couldn't. She didn't smoke...

Yellow streaks on bathroom walls may seem random, but they usually signal an underlying issue that shouldn’t be ignored. Instead of being simple discoloration, these marks often point to problems like moisture buildup, residue, or poor ventilation.

One common cause is trapped moisture. Bathrooms produce a lot of steam, especially during hot showers. When this moisture settles on walls and dries, it can leave behind streaks or stains over time.

Another possibility is residue from smoke or airborne particles. In some homes, especially where smoking occurs, particles can settle on walls and mix with condensation, creating yellow drips that appear after humidity rises.

Soap and cleaning product buildup can also contribute. Over time, tiny particles from sprays, shampoos, and cleaners can cling to surfaces. When combined with moisture, they form visible streaks that slowly develop.