Air conditioners that keep summer heat at bay can become channels for spreading mold, bacteria and dust indoors if not properly maintained, experts have warned. Specialists advised receiving professional cleaning at least once a year, and preferably twice a year, noting that mold typically begins to grow in earnest inside air conditioners about five to six months after cleaning.
The South China Morning Post (SCMP) recently reported a case in which a Hong Kong influencer, cleaning a home air conditioner for the first time in a year, discovered an ant nest, dead insects and even a live gecko. Thick layers of dust, grime and mold had also accumulated inside the unit.
In Hong Kong, where intense heat and high humidity persist, air conditioners are an essential appliance. But without proper maintenance, their interiors can turn into environments where mold, bacteria and foreign matter thrive.
Philip Li, chairman of the Hong Kong Institute of Allergy and a clinical immunology professor at the University of Hong Kong, explained that "a dirty air conditioner acts to breed and spread allergens." Each time a contaminated unit is turned on, mold spores and dust particles that have built up inside can spread into indoor air, sometimes accompanied by a musty odor.
Mold and dust mite allergens in particular can trigger or worsen allergic rhinitis and asthma. Children, the elderly and people with respiratory conditions need to be especially cautious. Inhaling mold spores can cause airways to swell and narrow, leading to nasal congestion, coughing and wheezing.
A dirty air conditioner affects not only health but also cooling efficiency. When dust and contaminants accumulate on the evaporator coil that produces cold air, the device must use more energy, raising electricity bills and lengthening cooling times. The risk of an air conditioner breaking down in the middle of summer can also increase.
Jeff Lam, a director at Breathe Easy, a Hong Kong air conditioner cleaning company, recommended professional cleaning at least once a year, and twice a year to prevent mold growth. "About five to six months after cleaning, mold growth can become noticeable again," he said.
Filters should be cleaned at least once a month during summer. Those who keep pets or live in low-floor housing with heavy vehicle traffic may need to clean them weekly. Air conditioners installed near kitchens require more frequent maintenance, as oil and smoke generated during cooking can be drawn inside.
Managing indoor humidity is also important. Breathe Easy recommended keeping indoor relative humidity at around 55 to 60 percent to reduce mold growth. Running the fan mode for a few minutes before turning off the air conditioner to dry any remaining moisture inside also helps prevent mold and odors.
However, it is best to avoid indiscriminate use of commercial air conditioner cleaning products. Some chemical components can corrode or damage the sensitive coils inside. If mold is clearly visible or odors persist, it is safer to have a professional cleaning company inspect the unit rather than disassembling it yourself.
