Michelle Lai had no idea the air conditioners in her home could have been contributing to the respiratory problems of her two children who suffered mild allergies. Lai moved from Canada to Hong Kong in 2017 with her two girls, who are now aged five and seven. Both developed wheezing and shortness of breath a year later. “We took measures such as not having a lot soft furnishings and cushions lying around and we kept the flat well ventilated,” she says. “My girls also wanted a cat but we decided against it – that was a difficult decision.” (Proteins found in an animal’s skin cells or saliva can trigger allergies, causing sneezing, a runny nose, wheezing and breathing difficulties.) Lai says it was only when she went to an allergy expert that she learned poorly maintained air conditioners could aggravate existing respiratory problems. Each year, the sound of buzzing air conditioners heralds the start of Hong Kong’s hot and humid summers. With seasonal consistency, cold wars erupt in offices citywide as workers fight over control of the temperature gauge, and for good reason: research shows working in over-air-conditioned environments can trigger chronic headaches and fatigue. Air-conditioning units might often be positioned out of sight but their maintenance should not be out of mind, in particular for those suffering allergies or other respiratory conditions. “It’s important to keep air conditioners clean as they can harbour dust and allergens that are deposited in the unit’s dust filter, and those allergens can potentially trigger asthma and allergy symptoms,” says Chan Wing-kai of the Hong Kong Asthma Society. Adrian Wu Young-yuen, a specialist in immunology and allergy at Hong Kong’s Centre for Allergy and Asthma Care, recommends installing a HEPA (high efficiency particulate air) filter that works by forcing air through a fine mesh that traps harmful particles such as pollen, dust mites and tobacco smoke. “I usually advise my patients to install an additional HEPA filter into the air-conditoning system,” he says. The Hong Kong Consumer Council recommends cleaning dust filters once or twice a month and setting the temperature at not lower than 25.5 degrees Celsius (78 degree Fahrenheit) to save energy. Water leaks from neglected air-conditioning systems can be problematic, creating damp conditions in which mould and mildew can thrive. Many types of mould can trigger allergic reactions. Meanwhile, air-conditioning units that drip water outside are common in Hong Kong, a particular problem given the density of people in the city. Faulty cooling units generate pools of water contaminated with bacteria and fungi that can trigger allergic reactions. To spur people to repair their units, the Food and Environmental Hygiene Department introduced hefty penalties, up to a maximum of HK$10,000 (US$1,300), for those whose dripping units cause a nuisance to others. Should kids be allowed to play in puddles and dirt? As well as health costs, there’s also the environmental cost. Air conditioners account for 30 per cent of Hong Kong’s annual energy use, the largest of any type of electricity use in buildings.