Buildings, Thermal Comfort, and Flu Season

Why Flu Season Is a Building Problem

Seasonal influenza affects up to a billion people globally each year, with hundreds of thousands of deaths. While flu is a biological phenomenon, it is also influenced by the conditions inside buildings, where most transmission occurs during colder months. As outdoor temperatures drop, people spend more time indoors with windows closed and heating systems running. These winter building conditions—cooler surfaces, dry air, and restricted airflow—create an indoor environment highly conducive to the spread of respiratory viruses.

Cold, Dry Air and Influenza Transmission

Influenza viruses survive and spread more efficiently in low-temperature, low-humidity conditions. When indoor air is heated in winter without humidification, relative humidity often falls below 20–30%. Such dry air dries out the nasal passages, weakens the body’s first line of immune defense, and allows viruses to remain airborne longer.

Health organizations consistently highlight a mid-range indoor humidity of 40–60% as optimal for reducing viral survival while maintaining comfort and preventing mold growth. Maintaining this range during heating season is particularly important in offices, schools, childcare centers, and multifamily residences where people share air for extended periods.

Thermal Comfort, Behavior, and Infection Risk

Thermal comfort affects more than comfort—it shapes behavior that influences disease transmission. Occupants who feel cold may use space heaters, block vents, or adjust thermostats in ways that disrupt airflow and reduce ventilation effectiveness. In poorly ventilated rooms, airborne particles accumulate more easily, increasing the chance of viral spread.

Ventilation remains one of the most effective building-level tools against respiratory infections. Increasing outdoor air intake, improving filtration levels, and ensuring consistent temperature control help buildings dilute and remove airborne viruses more effectively. This is why many health and engineering organizations emphasize maintaining stable indoor temperatures, adequate humidity, and high-efficiency filtration, especially during the winter months.

Building Operations That Support Health

Instead of focusing solely on flu-season interventions, buildings can be operated year-round in ways that support stronger indoor health resilience, including:

• Maintaining temperature stability through well-designed envelopes, reducing drafts, cold floors, and temperature swings that affect comfort and behavior.

• Actively managing humidity, particularly in cold climates, to prevent excessively dry conditions during the heating season.

• Ensuring ventilation systems run as designed, with adequate outdoor air and clean filters that meet or exceed recommended efficiency levels.

• Monitoring indoor conditions (CO₂, humidity, temperature) to ensure the building supports both comfort and infection control.

• Designing with future flu seasons in mind, integrating passive and active systems that maintain healthy indoor conditions without excessive energy use.

Conclusion

Flu season is influenced not just by viruses, but by the thermal and air-quality conditions inside our buildings. When temperature, humidity, and ventilation are well managed, buildings become powerful tools for reducing infection risk. In this sense, thermal comfort is not merely a matter of well-being—it is a practical, science-backed component of public health.

References

• World Health Organization (WHO) – Seasonal influenza fact sheets

• Centers for Disease Control and Prevention (CDC) – Influenza seasonality and environmental factors

• ASHRAE – Humidity control and ventilation guidelines

• Harvard T.H. Chan School of Public Health – Healthy Buildings Program – Indoor environmental quality and infection control

• U.S. Environmental Protection Agency (EPA) – Ventilation and indoor air guidance