Exploring the Potential Spread of Pathogens Through Central AC Systems

Central air conditioning (AC) systems are a staple in many homes, offices, and commercial buildings. They offer comfort, control, and energy efficiency—but they also come with hidden risks. One concern that’s increasingly gaining attention is the potential for central AC systems to contribute to the spread of pathogens, including viruses, bacteria, and mold spores.

This article dives into how pathogens can circulate through HVAC systems, the science behind airborne transmission, and what you can do to reduce the risk.

How Central AC Systems Work

Before discussing pathogen spread, it’s important to understand the basic function of central AC systems. These systems work by drawing in warm air, cooling it via a refrigerant-cooled coil, and then redistributing it through a network of ducts.

While this process cools and circulates air efficiently, it also means that any airborne contaminants present in one part of the building can potentially be distributed elsewhere.

How Pathogens Can Spread Through AC Systems

Airborne Transmission

Many pathogens—like the influenza virus or SARS-CoV-2—can spread through tiny droplets or aerosols suspended in the air. When a central AC system circulates air without proper filtration, these particles can be carried from room to room.

Lack of Ventilation

Modern buildings often prioritize energy efficiency over ventilation. Without fresh air exchange, pathogens can build up and recirculate. Poor ventilation exacerbates indoor transmission, especially in tightly sealed environments.

Moisture and Mold

HVAC systems can accumulate condensation, especially in humid climates. If not properly drained and maintained, this moisture can become a breeding ground for mold and bacteria, which can then be blown through the ducts.

Dirty or Inadequate Filters

Low-efficiency or clogged filters can’t effectively trap airborne microbes. If filters are not changed regularly, they may allow contaminants to pass freely or even become sources of contamination themselves.

Studies Supporting the Risk

A study published in Emerging Infectious Diseases (CDC) linked central air systems to a COVID-19 outbreak in a restaurant, suggesting that poor ventilation and airflow helped the virus travel between tables.

The World Health Organization has also acknowledged that airborne transmission in indoor spaces is possible, especially in poorly ventilated or crowded environments.

When Should You Be Concerned?

Pathogen transmission via AC systems is most likely in the following scenarios:

• In large buildings with shared air circulation (e.g., hospitals, office towers, schools)
• In enclosed environments with little or no fresh air exchange
• In systems that are poorly maintained or using outdated filtration

For homes, the risk is significantly lower, but still present, especially in multi-resident buildings or poorly ventilated rooms.

Minimizing the Risk of Pathogen Spread

Upgrade to HEPA or MERV-13+ Filters

High-efficiency particulate air (HEPA) filters and MERV-13 or higher filters can capture microscopic particles, including many viruses and bacteria.

Improve Ventilation

Introduce fresh outdoor air into the system using air exchangers or by simply opening windows periodically, if possible. Better ventilation dilutes indoor contaminants and lowers the risk of transmission.

UV Light Technology

Some HVAC systems can be retrofitted with ultraviolet germicidal irradiation (UVGI), which uses UV-C light to destroy microbial DNA and neutralize pathogens before they spread.

Regular Maintenance and Cleaning

Ensure that ducts, vents, drip pans, and filters are regularly cleaned and maintained. Mold growth in neglected components can become a major indoor air hazard.

Humidity Control

Maintain indoor humidity between 40%–60%, as overly dry or overly humid conditions can encourage pathogen survival and growth.

Final Thoughts

While central AC systems aren’t inherently dangerous, they can become vectors for pathogen spread if not properly managed. Understanding how these systems work—and taking proactive steps like upgrading filters and improving ventilation—can significantly reduce health risks.

As public awareness grows around indoor air quality, property owners and facility managers should view HVAC not just as a comfort system, but as a critical component of infection control.