Air conditioning (AC) is often blamed for causing a “chest cold,” a lay term usually referring to a lower respiratory infection like acute bronchitis. An AC unit cannot spontaneously generate or transmit a respiratory virus; viruses must originate from an infected person or contaminated surface. However, the environment created by AC can significantly alter the body’s natural defenses. In some cases, AC systems can also directly introduce infectious agents, making the link between AC use and respiratory symptoms complex.
The Difference Between AC Irritation and Viral Illness
Symptoms that appear after prolonged exposure to air conditioning are frequently the result of non-infectious irritation rather than a true viral infection. AC works by drawing moisture out of the air to cool it, causing a substantial drop in indoor humidity. This low-humidity environment can dry out the delicate mucous membranes lining the nose and throat. A dry mucosal layer results in temporary symptoms of irritation, such as a scratchy throat, a persistent, mild cough, and nasal congestion.
This physical discomfort is often mistaken for the beginning of a cold or a mild infection. Moving rapidly from a hot, humid outdoor environment into a cold, air-conditioned space can also shock the respiratory system. This sudden temperature change may trigger temporary reactive inflammation in sensitive airways. These symptoms resolve quickly once the person leaves the conditioned environment, distinguishing them from the sustained symptoms of a genuine viral illness.
How Cold Air Affects Respiratory Defenses
The cold and dry air produced by an AC unit can compromise the body’s primary defense system against respiratory pathogens. This system, known as the mucociliary escalator, uses hair-like structures called cilia to sweep mucus, trapping inhaled particles and microbes, out of the lungs. When the air is cold and dry, the sticky mucus layer thins and the cilia’s movement slows or temporarily halts. This disruption reduces microbial clearance efficiency, allowing viruses or bacteria to bypass this defense mechanism more easily.
Cold air also directly impacts the immune response within the nasal passages. Studies indicate that when the temperature of the nasal mucosa drops, local immune cells release fewer antiviral molecules, specifically interferons. These interferons are the body’s early warning system against viral invaders. A reduced interferon response creates a window of vulnerability, lessening the host’s ability to fight off a newly encountered respiratory virus. The combination of impaired physical clearance and a dampened local immune response increases susceptibility to infection.
Hidden Respiratory Hazards Lurking in AC Units
While cold air does not carry viruses, poorly maintained air conditioning systems can become breeding grounds for biological hazards that cause serious respiratory illness. AC units, particularly central systems with cooling coils and drip pans, create a moist environment where fungi and bacteria can thrive. These microbes are then aerosolized and distributed throughout the building when the unit operates.
One concerning hazard is the bacterium Legionella pneumophila, which grows vigorously in warm, stagnant water (77°F to 113°F). Inhaling contaminated water droplets from cooling towers or evaporative condensers can lead to Legionnaires’ disease, a severe form of pneumonia, or the milder Pontiac fever. Fungi and molds, such as Aspergillus and Penicillium, also accumulate on damp coils and filters. Exposure to these fungal spores can trigger allergic reactions, asthma exacerbations, or hypersensitivity pneumonitis, sometimes called “Air conditioner’s lung.”
Strategies for Safer Air Conditioning Use
Mitigating the risks associated with air conditioning involves balancing cooling with maintaining a healthy indoor environment. One effective strategy is controlling indoor humidity, which should ideally be kept between 40% and 50% relative humidity. Using a separate humidifier or dehumidifier can prevent excessive dryness that impairs respiratory defenses and discourage mold growth.
Regular, proactive maintenance of the AC system is equally important to prevent biological contamination. Air filters should be replaced or cleaned according to the manufacturer’s schedule, typically every one to three months, to prevent the buildup of dust, allergens, and mold spores. The temperature setting should also be moderated, avoiding extreme cold that causes a stark difference from the outdoor temperature. Directing the airflow away from where people sit or sleep can also prevent localized drying and irritation of the eyes and throat.