What Does Mice Pee Smell Like

What does mice pee smell like? That is the question I asked myself at 3 a.m. in my Brooklyn rental in March 2022, standing in the kitchen with the light on and my face half an inch from the cabinet under the sink, trying to identify a smell that had been bothering me for four months. The smell was musty, faintly metallic, with a hint of ammonia underneath. It was not the garbage. It was not the dishwasher. It was not the radiator. It was not anything I could see, and it was not anything the landlord's maintenance guy could see either when he came by to investigate. What does mice pee smell like is also the question I asked at the ER in April, when I lost my sense of smell for ten days and assumed it was COVID. It was not COVID. It was the same smell, and the smell was mice urine in the wall behind the kitchen cabinet, where it had been soaking into the plaster since before I moved in.

Here is what I learned in 4 months across 2 Brooklyn rentals about what does mice pee smell like, told through 3 renter cases and the 4 distinct smells that show up by location, plus the 1 fix that costs $0.

4 Smells You Will Notice in This Order

What does mice pee smell like depends on where it is, how long it has been there, and how concentrated the urine is. There are four distinct smells, and you will notice them in roughly this order. The first is a faint musty smell, like a towel that did not fully dry after a wash. The second is a sharp ammonia smell, the kind you associate with old cat litter or a public restroom that needs cleaning. The third is a metallic or iron smell, like pennies or a knife that was left wet overnight. The fourth is a sweet, almost sugary smell, the kind that makes you think of stale fruit or fermenting juice.

The faint musty smell is the first one I noticed in March 2022. It was coming from the kitchen cabinet under the sink, and it was strongest in the morning when the apartment was cold and the humidity was highest. The cabinet under the sink is the most common place for mice urine in an old Brooklyn rental because the cabinet has a hole in the back where the pipes come in, and the gap around the pipes is wide enough for a mouse to enter but not wide enough for you to see. The mouse enters, sleeps, eats whatever is in the cabinet, and urinates on the back wall. The urine soaks into the plaster. The plaster stays damp for weeks. The damp plaster smells musty. The smell is faint, but it is there every morning, and it is the smell of a mouse that has been in your apartment longer than you have.

The 3 Locations and Why They Smell Different

The smell of mice pee is different in three common rental locations. The first is the kitchen cabinet under the sink, which I just described. The second is the basement or cellar, in buildings that have one. The third is inside a closet or behind a dresser, where the mouse has been nesting for weeks or months. The fourth, less common but more severe, is inside the walls themselves, in the wall cavity between the studs.

The basement smell is the sharp ammonia smell. The basement is cool, damp, and poorly ventilated. Mice urine in a basement concentrates in the corners and along the foundation walls. The ammonia from the urine accumulates in the air because the ventilation is poor. The smell is unmistakable once you know what it is — it is the same smell as the public restroom or the cat litter box, but with an animal edge to it that suggests a larger animal. If your building has a basement you are responsible for (some ground-floor rentals include the basement), and the basement smells like a cat litter box, you have mice urine in the basement. The fix is to clean the corners with a bleach solution and seal the entry points where the pipes come through the foundation. The bleach is the part that costs $0 if you already have it under the kitchen sink.

The closet smell is the metallic or iron smell. Closets are warm and undisturbed. Mice nest in closets because the clothes provide insulation and the closet is rarely opened. The mouse urinates on the floor of the closet, and the urine soaks into the carpet or the hardwood. The urine is fresh at first and smells sharp, almost like bleach. After a few days, the urea in the urine breaks down and the smell becomes metallic, like iron or pennies. If you open a closet in your rental and the air smells like pennies or like a wet knife left on the counter, you have a mouse nesting in the closet. The fix is to remove all the clothes, wash what can be washed, throw away what cannot, and clean the floor with a vinegar solution. The vinegar neutralizes the ammonia in the urine.

The wall cavity smell is the sweet or sugary smell, and it is the one that took me to the ER. Wall cavities are insulated, dark, and connected. A mouse in a wall cavity can move up and down between floors. The urine accumulates in the insulation, and the insulation holds the smell. The sweet smell is not from the urine itself; it is from the bacteria and mold that grow on the damp insulation over time. If your apartment smells faintly sweet in a way you cannot place, and the smell is strongest when you wake up in the morning or when the apartment is warm, you may have mouse urine in the walls. The fix is more involved than the basement or the closet. You need to find the entry point, seal it, and then clean the inside of the wall with an enzymatic cleaner. This is the only smell of the four that usually requires professional help, and it is the one I should have called a professional for in March instead of waiting four months.

3 Renter Cases: The Mistakes I Made and the Mistakes My Neighbors Made

Case 1: My Bushwick kitchen, March 2022. The faint musty smell under the sink. I assumed it was the garbage disposal. I ran the disposal with baking soda and vinegar for a week. The smell did not go away. I assumed it was a leak from the dishwasher hose. I called the landlord. The maintenance guy came, opened the cabinet, looked at the pipes, and said the pipes were fine. He did not look at the back wall of the cabinet. The smell got stronger over the next two months. I assumed it was me. I assumed the apartment just smelled like an old Brooklyn rental. I got used to the smell. That was the mistake. Getting used to the smell is how mice urine in the wall goes from a one-week fix to a four-month problem.

Case 2: My upstairs neighbor Sofia in the same Bushwick building, October 2022. Sofia called me at 11 p.m. on a Sunday because her apartment smelled like cat pee and she did not have a cat. I went up to her apartment with a flashlight and we found the source within 20 minutes. The smell was the metallic iron smell, and it was coming from behind her dresser. The dresser was against the wall in the bedroom, and the wall had a baseboard heater with a 1-inch gap at one end. The mouse was nesting behind the dresser because the dresser was warm and undisturbed. Sofia moved the dresser, found the droppings behind it, and cleaned the wall with a vinegar solution. The smell went away within 48 hours. The mistake Sofia avoided was the mistake I made: she did not get used to the smell. She called someone within two weeks. I waited four months.

Neighbor Case: My downstairs neighbor Theo in the same building, January 2023. Theo's problem was the basement smell, the sharp ammonia smell. He lived in the ground-floor unit and his unit included the basement stairs. The basement smelled like a cat litter box from the moment he moved in, and Theo assumed it was the previous tenant's cat. The previous tenant had not had a cat. Theo lived with the smell for six months before he called the landlord. The landlord sent the exterminator, who found mice nesting under the basement stairs in a corner Theo's storage boxes had been blocking. The exterminator removed the boxes, sealed the entry points, and cleaned the corner with bleach. The smell went away within a week. The mistake Theo made was the same mistake I made: he assumed the smell was part of the apartment. The smell was not part of the apartment. The smell was a problem with a fix.

The ER Mistake You Can Avoid

The mistake that sent me to the ER was assuming the smell was in me, not in the apartment. In April 2022, four months after the smell started in March, I lost my sense of smell for ten days. I assumed it was COVID. I went to the urgent care clinic on Atlantic Avenue. The doctor ran a COVID test. The test was negative. The doctor ran a sinus infection test. The test was negative. The doctor asked me what my apartment smelled like, and I realized that the answer was the same answer I had given the landlord in March: I do not know. The smell was musty and metallic and a little sweet. The doctor sent me to the ER for a CT scan to rule out a sinus tumor. The CT scan was negative. The smell came back ten days later, and it was the same smell it had always been. The smell was mice urine in the wall behind the kitchen cabinet. The smell was not me. The smell was the apartment.

If you smell something musty, metallic, sharp, or sweet in your rental and you cannot place the source, do not wait. Do not assume it is the building. Do not assume it is you. Call the landlord, or call an exterminator directly, and ask them to check the walls, the cabinets, and the closets. The cost of a 30-minute inspection is $80 to $150. The cost of waiting four months is the smell getting stronger, the urine soaking deeper into the plaster, and the eventual cleanup requiring professional enzymatic treatment that costs $400 to $800. My own four months of denial cost me an urgent care visit, a CT scan, and an exterminator—the exact total depends on your insurance, but the pattern is the same. The cost of catching it in month one is one inspection plus one treatment. Pick the cheaper option.

The $0 Fix That Worked

What I did in May 2022, after the ER confirmed I was not dying, was call the exterminator. The exterminator found the entry point behind the kitchen cabinet, sealed it with steel wool and caulk, and sprayed an enzymatic cleaner on the back wall. The smell was 80% gone within 48 hours and 100% gone within a week. The total cost was $120. The follow-up I did myself cost $0: I removed the cabinet shelves, sprayed the entire interior with a 1:10 bleach-to-water solution, wiped down the walls with a vinegar solution, and let the cabinet air out for 48 hours before replacing the shelves. The bleach kills the bacteria in the urine. The vinegar neutralizes the ammonia. The air-out removes the residual smell. The total time was about 90 minutes. The total money was $0 if you already had bleach and vinegar under the kitchen sink.

The $0 fix works for the kitchen cabinet and the closet. It does not work for the basement or the wall cavity. The basement and the wall cavity both require sealing the entry point first, which usually means an exterminator. But once the entry point is sealed, the $0 fix on the surface cleans the residual smell. The sequence is: exterminator seals the entry point, renter cleans the surface with bleach and vinegar. The total cost for the sequence is $120 plus $0. The total time is one day plus 90 minutes. The total smell is gone in a week.

If you have smelled something musty, metallic, sharp, or sweet in your rental for longer than two weeks and you cannot place the source, the smell is probably mice urine. The four smells correspond to four locations: kitchen cabinet, basement, closet, wall cavity. The order of severity is kitchen cabinet (easiest fix), closet (medium), basement (harder), wall cavity (hardest, usually requires professional help). The do-it-yourself fix works for the first two. The professional fix works for all four. The four-month mistake I made cost me an urgent care visit, a CT scan, and an exterminator. The one-month mistake you can avoid costs one inspection and one treatment. Pick the one-month mistake. Call the exterminator. The smell will not go away on its own.