This Is the Seriously Disgusting Reason Your Air Conditioner Smells Bad

Updated: Mar. 29, 2021

That funky odor in your home or office may actually be coming from your air conditioner. Here's why, and what you can do about it.

Air conditioners remove mold-friendly moisture from your home and filter allergens before they can get inside. But if you don’t keep them clean, they can make the air in your house extremely stinky, according to a new study out of Hong Kong Baptist University (HKBU).

Close-up Of Young Woman Hand Operating Air Conditioner With Remote ControlAndrey_Popov/Shutterstock

The study, led by Lai Ka-man, PhD, associate professor in HKBU’s Department of Biology, found that the flakes of skin we naturally shed (scientific name: “skin squames”) get sucked into air conditioning units. Once there, the squames become “lunch” for bacteria living in the units. After the bacteria digest the skin proteins, they produce disgusting smells, including:

  • ammonia, a urine-like odor
  • volatile fatty acids, a B.O.-like smell

“One of the most notable indoor air-quality problems is odor emission,” according to the study, published in the journal, Indoor Air: The International Journal of Indoor Environment and Health. Simply stated, bad smells can have a negative impact on your quality-of-life. In previous research, the authors found that this bacterial process was higher in smelly AC units. The new study is the first to show that skin squames are to blame, although there is a twist: the study also found foul odors in seemingly clean units.

Nonetheless, the authors state that a good filter on your AC could help: “The size of skin squames is generally larger than 10 micrometers (or 0.001 centimeters). A filter that can effectively capture particles less than this size should help improve the odor problem,” Dr. Lai said in a press release. It may also be helpful to get a sealed vacuum with a HEPA filter and use it frequently, suggests Caroline Blazovsky, of My Healthy Home.

Here are 15 surprising ways to cut down on indoor air pollution.