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Key takeaways
A dirty AC can make you unwell. Its filter, evaporator coil, drain pan, blower fan and ducts trap dust, pollen, mold and bacteria, and once these parts are clogged or damp, the system blows those contaminants back into your rooms. The result can be coughing and sneezing, asthma and allergy flare-ups, mold exposure, itchy eyes and skin, and headaches or tiredness. Cleaning each part on schedule removes the source.
Every time your air conditioner runs, it pulls room air through a series of parts, cools it and pushes it back out. Each part also collects dust, dander, pollen or moisture along the way. Kept clean, your AC helps keep indoor air fresh. Neglected, the same parts become the source of the problem.
Below, our AC technicians break down each part of your system, what collects inside it, and the health effects that follow:
A split or central AC has five parts that directly affect the air you breathe. Here is what each one does and what builds up inside it.
The air filter is the first part room air passes through, trapping dust, pollen, pet dander and other airborne debris before the air reaches the cooling parts. A clogged filter restricts airflow, and according to the US Department of Energy, dirt can then bypass the filter and settle on the evaporator coil. In dusty conditions, most households need to clean the AC filter every two to four weeks during peak summer use.
The evaporator coil sits behind the filter and does the actual cooling. As warm air passes over it, the coil absorbs heat and pulls moisture out of the air, so it stays wet whenever the AC runs. Dust sticking to this damp surface forms a film where mold and bacteria grow, which is why regular AC coil cleaning matters for air quality.
The moisture removed from the air drips off the coil into a drain pan, then flows out through the condensate line. If the line clogs with dust, sludge or algae, water sits in the pan. Standing water in a dark, cool space breeds mold, bacteria and slime. Signs of a blocked line include a musty smell when the unit starts and AC dripping water from the indoor unit.
The blower fan pulls warm air into the unit and pushes cooled air back into your rooms. Dust and grime build up on the fan wheel over time. Because the fan sits after the coil, anything collecting or growing on it is blown straight into the room every time the AC switches on.
In central and ducted systems, ducts carry conditioned air from the unit to each room. They collect dust, debris and, where condensation forms, mold. Air moving through dirty air ducts picks up these particles and delivers them through every vent in the house.
The health effects of a dirty AC come from one mechanism: the system recirculates whatever its parts have collected. According to the US Environmental Protection Agency, indoor concentrations of some pollutants are often 2 to 5 times higher than typical outdoor levels. A neglected AC is one of the main ways those pollutants move around a home.
Clogged filters and dusty coils release fine particles into the airflow. Breathing these in irritates the nose, throat and airways, causing coughing, sneezing, a scratchy throat and congestion. Symptoms that ease when you leave the house and return indoors are a useful clue the AC is involved.
Dust mites, pollen and pet dander trapped in the system are recirculated with every cooling cycle. For people with asthma or allergies, this repeated exposure can trigger wheezing, chest tightness, shortness of breath and persistent allergy flare-ups.
Wet coils and damp drain pans are where mold in your AC usually starts. Running the unit carries spores into the room. The US CDC's National Institute for Occupational Safety and Health reports that people who spend time in damp buildings experience respiratory symptoms and infections, new or worsening asthma, allergic rhinitis and eczema. A musty or sour smell when the AC starts is the most common early warning sign.
Airborne dust and mold spores can cause red, itchy or watery eyes and skin rashes, both listed by the CDC among common reactions to mold. A dirty system also runs longer to reach the set temperature, drying the air further, which can leave skin feeling tight and eyes gritty, especially overnight in bedrooms.
The US EPA links poor indoor air quality with headaches, dizziness and fatigue. When a dirty AC recirculates stale air with too little fresh air coming in, carbon dioxide and indoor odors build up, and people often feel tired or foggy without an obvious cause. When these symptoms clear soon after leaving a building, the pattern is often described as sick building syndrome.
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