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Why Does My Air Conditioner Smell Like Pee?

Air conditioners are designed to make indoor spaces feel fresh and comfortable, so experiencing the unpleasant odor of pee when you switch them on is the last thing you need! Starting up a dormant AC can sometimes invite this unwanted stench into the home, but why is this?

An air conditioner gives off a pee-like smell when there is a build-up of mold in the unit. This odor is not from the mold fungus itself but the result of the byproducts or mycotoxins it creates. These mycotoxins generate ammonium (which is found in urine), hence the familiar unpleasant smell.

The rest of this article will explore some topics relating to this question in-depth. Such as how mold and bacteria become trapped in air conditioners, additional reasons behind a pee-like smell in your AC unit, and the steps you can take to combat odors and improve overall indoor air quality.

What Causes Mold to Form in an AC unit?

Understanding how mold and bacteria can build up inside your air conditioner helps to have a quick overview of how air conditioners work first.

As much as it may seem like lovely fresh air is blowing into your home, there is no new air being produced. An AC simply sucks the existing air from the room inside the unit compressor, cools down this air via a refrigerant and condensing process, and sends the cool recycled air back out into the room. 

Here’s a simplified look at how your home air conditioner works:

So naturally contained within the air that is initially sucked into your air conditioner is all of the pollutants that already exist within your home, the stinkiest culprits of which are the ones you produce organically – the skin flakes you shed throughout the day. 

Human pollution won’t always be the cause of mold growth in an AC unit, however. Mold and mildew can also be down to a poorly maintained air conditioner. Let’s look at both of these causes in more detail.

Dead Skin Cells

So here’s where things get a little gross. The skin cells you shed all day and every day – also known as “skin squames” to give them their scientific name – are essentially breakfast, lunch, and dinner for the tiny bacteria that already live in your air conditioner. 

And once these microbes have digested the protein in your skin cells, the breakdown of the protein Keratin (found in hair and fingernails) is what goes on to produce the foul ammonia-like smell. This was found in a study published in the International Journal of Indoor Environment and Health, led by Lai Ka-man, Ph.D., former associate professor of Biology at Hong Kong Baptist University (HKBU).

So along with the dust and other debris that finds its way into your AC unit, human skin cells create the ideal haven for mold to not only survive in your conditioner but to thrive.

Poor Maintenance

Additionally, mold can start to form in your air conditioner if you take your eye off the ball in terms of keeping it properly maintained and cleaned on a regular basis.

Neglecting to Clean the Filter Regularly

A long-neglected and jammed air filter will noticeably reduce the airflow in your home, so you may even spot this change before you notice mold and the resulting unpleasant smells. Either way, it’s imperative that your air filter is changed regularly.

Air Con experts suggest that replacing filters within a 45 to 90-day window is a good rule of thumb, though replacing every 45 days is recommended for maximum efficiency. 

How often you should change your air filter can depend a lot on your home environment. This is why the Product Manager for Indoor Air Quality at Trane technologies, Patrick Van Deventer, suggests changing the filter seasonally to combat extra pollutants such as pollen and pet shedding. And also, after doing any kind of home renovation that could kick up dust.

Mold on the Evaporator Coils

The evaporator coils or ‘evaporator core’ in your AC is close to where the main action is – nestled inside the blower fan and a part of the unit where the refrigerant absorbs heat, so when this accumulates mold, you certainly know about it. 

Dust and dirt can, unfortunately, cling to the evaporator coils like there’s no tomorrow. If mold takes hold for a significant period, the coils will become rusted and need replacing altogether.

To avoid this costly mold build-up, I’d recommend checking out this easy step-by-step guide to cleaning your coils by HVAC technician Ron Walker.

Could Something Besides Mold Cause Pee Smell?

It’s not a pleasant thought, but the smell of urine coming from your air conditioner may point to something more disturbing than the byproduct of mold spores.

Rodent Problem

It’s not uncommon for mice and other pests to scurry into pipes and find their way into your air-con system since it makes a cozy, warm environment for them. And a pee-like smell could be exactly that – a sign that they have relieved themselves in the air ducts.

Alarmingly, a urine smell could also be an indication of a dead rodent. As a mouse decomposes, its carcass releases a series of foul-smelling gases, one of which includes ammonia. If you’re concerned that this is the case, it’s important to get your AC unit checked over and arrange for a professional duct cleaning.

How Can You Keep an AC Unit Odor-Free?

As well as ensuring your air conditioner functions properly with regular filter changes and maintenance checks, there are steps you could be taking in your everyday life to keep odors at bay and therefore increase your AC’s efficiency.

Take Active Steps to Reduce Indoor Smells

It’s important to be mindful of the indoor air pollutants you may be creating on a regular basis. Try making the following changes:

  • Avoid cleaning sprays where possible, and use wipes and liquid solutions instead
  • Switch to fragrance-free laundry products to reduce ambient air pollution
  • Towel off pets’ coats and paws to prevent spreading pollen and dander indoors
  • Set your thermostat above 65°F (18°C) in winter. According to the American Lung Association, high levels of indoor humidity increase the growth of bacteria, so this strikes a good balance

Invest in a High-Quality Filter

Your ‘skin squames’, as previously mentioned, easily become stuck in older or low-quality AC filters because they are usually too big for the unit to cope with. Most skin particles measure over 10 micrometers, which may not sound big to you and me, but this is large enough to become wedged in most air-cons, thus emitting those not-so-inviting fragrances.

Former HKBU professor and ‘skin squames’ study co-author Dr. Lai recommends installing a filter that can effectively catch these skin flakes in the air before they have a chance to clog the system. These are currently the top-rated AC filters for efficiency and allergies, according to Air Conditioner Lab.

Final Thoughts

A pee-like smell from an air conditioner is normally the result of the ammonium in the mycotoxins released by mold spores inside the unit. These mold growths are often discovered on the metal evaporator coils and are commonly exacerbated by:

  • An accumulation of human skin cells,
  • And, an unclean, poorly-maintained unit

In rare cases, a dead rodent may be the cause of a urine-like odor, as their bodies release ammonia during decomposition. Minimizing the source of indoor air pollutants in your home and carrying out regular maintenance can contribute to a more efficient, less smelly air conditioner.


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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