Air conditioning units offer comfort and can save lives during extreme heat events. However, under specific circumstances, these systems can contribute to various health issues. Concerns range from the circulation of harmful biological agents that flourish in moist environments to physical discomfort caused by changes in air quality and temperature extremes. The risk of getting sick from an air conditioner is often directly related to the cleanliness and maintenance of the system itself.
Pathogens That Thrive in AC Systems
Certain pathogens find ideal conditions for growth within the dark, moist components of air conditioning infrastructure, especially those involving standing water. The most serious risk involves the bacterium Legionella pneumophila, which causes Legionnaires’ disease, a severe form of pneumonia. This bacteria thrives in warm, stagnant water typically found in large, complex systems like cooling towers, evaporative condensers, and large-scale humidifiers in commercial buildings. When this contaminated water is aerosolized and inhaled, it can lead to infection.
Residential AC units rarely pose a risk for Legionnaires’ disease because they do not use cooling towers, but they can easily become breeding grounds for mold and fungi. The cooling process creates condensation on the evaporator coil, which collects in a drip or condensate pan. If this moisture is not drained properly, the standing water and damp surfaces offer the necessary environment for various molds, such as Aspergillus or Stachybotrys chartarum (black mold), to develop. The fungi release microscopic spores into the air circulating through the ductwork and into the living space. Exposure to these spores can lead to respiratory irritation, allergic reactions, and the exacerbation of asthma symptoms. Individuals with compromised immune systems are vulnerable to serious lung infections from inhaling these airborne agents. A persistent musty odor near the vents often signals active microbial growth within the system.
Physical Symptoms Caused by Air Quality and Temperature
Beyond biological contaminants, the physical process of air conditioning can cause uncomfortable physical symptoms. AC systems strip moisture from the air to lower humidity, which can result in excessively dry indoor environments. When the air lacks humidity, it quickly dries out mucous membranes in the nose and throat, leading to irritation, sore throats, and nasal congestion.
This dryness impairs the body’s natural defense mechanisms, making it easier for common cold or flu viruses to take hold. Low humidity also accelerates the evaporation of moisture from the skin and eyes, frequently causing dry, itchy skin and eye irritation.
Another common complaint is the physiological stress caused by moving rapidly between a cool, air-conditioned space and high outdoor heat, sometimes called thermal shock. When the temperature difference is severe—often greater than 10 degrees Celsius—the body’s temperature regulation system struggles to adjust quickly. This sudden change can cause blood vessels to constrict rapidly, potentially leading to headaches, dizziness, and general malaise.
If filtration is inadequate, air conditioners circulate irritants already present in the building. Dust, pollen, pet dander, and other particulate matter are easily picked up and blown back into the air stream. This circulation of existing allergens can trigger or worsen allergic rhinitis and asthma attacks.
Essential Maintenance to Prevent Illness
Preventing health risks associated with air conditioning systems relies on consistent and proper maintenance. The most direct action a homeowner can take is regularly replacing the air filter, which traps particulate matter and allergens that could otherwise be circulated. For residential systems, a filter with a Minimum Efficiency Reporting Value (MERV) rating between 8 and 13 is recommended to effectively capture fine particles without restricting airflow.
Filters should be replaced every one to three months during heavy use, and more frequently if there are pets, smokers, or allergy sufferers in the home. It is also important to address the moisture component by inspecting and cleaning the condensate drip pan and drain line. Stagnant water in the pan creates an environment for mold and bacterial growth, so the pan should be kept dry and periodically cleaned with a solution like diluted bleach or vinegar.
Ensuring the drain line remains clear prevents water from backing up into the system, which commonly causes mold growth and excess indoor humidity. Beyond routine filter changes and cleaning condensate components, professionals should inspect the entire system, including the evaporator coils, at least once or twice per year. This professional cleaning ensures that coils remain free of accumulated dirt and biofilm.
Finally, managing the relative humidity level within the home is a proactive measure against microbial growth and physical discomfort. The ideal range for indoor relative humidity is between 30% and 60%. This balance discourages the growth of mold and dust mites while preventing the excessive drying of the respiratory system. In humid climates, supplementary dehumidification may be necessary to maintain this balance.