Improving winter IAQ: Key steps for a healthier commercial building

Author: Bryan Cummings, PhD, Sr. Building Environmental Sciences Engineer

A good mechanical system balances the needs of thermal comfort, indoor air quality and energy efficiency. As temperatures plummet during the winter months in cold regions, we spend more of our time indoors and seal up our buildings to stay warm. This carries implications for mechanical system operation and the qualities they affect. This blog post focuses on the indoor air quality (IAQ) implications of wintertime climates and building operation.

In public buildings, employees, clients, patrons and visitors spend extended periods in shared spaces breathing the same communal air. Poor IAQ carries health concerns from respiratory irritation to the spread of viruses and also reduces focus and productivity.

Heating, Ventilation and Air Conditioning (HVAC) systems play a critical role in managing and improving IAQ. There are multiple solutions for a variety of budgets that can help address IAQ concerns in the winter with positive returns on investment.

Key Challenges in the Winter

During the winter, we tend to keep our windows and doors shut, reducing natural ventilation in our homes. Likewise, larger mechanically ventilated buildings are often operated to provide less outdoor air as the temperature falls. This creates an environment where indoor-sourced contaminants, such as viruses, bacteria and VOCs can accumulate.

Less ventilation is a primary reason why respiratory infections—from the common cold to COVID-19—are more widespread in the wintertime. We also tend to spend more time indoors when the weather is bad, which compounds infection risks, especially when large groups of people gather in close proximity.

Very cold air does not hold much water vapor. When cold outdoor air is brought indoors and conditioned to warmer temperatures, the mass of water in the air remains constant but its capacity for holding moisture rises. This elevates the sensation of the dryness of air, quantified by the relative humidity (RH) metric. Indoor RH can often drop below 10% under certain wintertime conditions.

Low-RH air causes some benign respiratory irritation and can dry out skin. But it also reduces our bodies’ natural first line of defense against respiratory infections. Healthy mucous layers in our nose and throat help to capture microbes and other small particles before they cause infection. Their ability to do so is weakened when they are dried out.

Many viruses also survive longer in dry air for both physical and virological reasons. First, dry air causes more moisture from bioaerosols to evaporate, creating a lighter particle that can stay airborne for longer. Second, aerosol liquid water can facilitate chemical reactions that can inactivate viruses, a phenomenon that is reduced after evaporation of aerosol water in dry air.

Key Strategies to Improve IAQ

1. IAQ Monitoring

An easy first step is to deploy CO2 monitors and to display levels to indicate ventilation quality, spot potential issues when they arise and bolster the confidence of building occupants.

RH, particle concentrations and VOC concentrations can also be easily monitored.

For buildings with larger budgets, sensors, continuous monitoring and displays can be integrated into advanced building automation systems (BAS).

2. Ventilation

To improve ventilation, buildings can consider providing more outdoor air than is prescribed by code minimums. For example, LEED and WELL standards give credits for providing 30% more outdoor air than is required by ASHRAE Standard 62.1.

Since cold outdoor air must usually be heated before being distributed to rooms, an energy efficient approach is to increase outside air flow rates only when infection risks are particularly high. ASHRAE Standard 241, Control of Infectious Aerosols, introduces the concept of Infection Risk Management Mode (IRMM), to be turned on during high-risk periods like flu season. The standard allows for either ventilation or filtration to be used to increase the equivalent air exchange rates in a space.

In the right climates, economizers should be installed to ensure that free cooling is used whenever possible, which provides additional ventilation as a co-benefit.

3. Filtration

Filtration removes microbes, respiratory aerosols and other particulates from indoor air. Since air must be pushed through a filter medium using a fan, filtration increases power costs but does not affect the heating and cooling loads.

Portable air filters can be purchased off-the-shelf and deployed in occupied spaces as a low-cost supplemental solution during the wintertime. But for more control, filtration systems should ultimately be incorporated into the central HVAC system in commercial buildings. Especially in high-risk spaces, standalone fan filter units can be used to avoid over-sizing the ducted system.

To ensure their effectiveness, filters must be regularly replaced, and a well-sealed installment within their housing must be ensured. For example, one study (Ward & Siegel, 2005) found that just a half-inch gap between a highly effective MERV 15 filter and its housing allows enough bypass that reduces the overall effectiveness to that of a low-level MERV 8.*

4. Humidification

Adding dedicated humidifying equipment can help improve comfort and reduce our susceptibility to respiratory infection. Like filtration equipment, portable humidifiers can be deployed as a short term or supplemental solution, while a centralized approach may provide more control.

Steam or evaporative humidifiers can use untreated municipal water, while ultrasonic humidifiers should be avoided unless distilled water is used, as they can aerosolize water impurities and worsen overall IAQ. Maintaining RH above 40% is ideal to optimize our defenses against infection.

5. Temperature control

Wintertime temperature setpoints often exceed the minimum threshold required for thermal comfort, which wastes energy and exacerbates the dry out sensation.

Controls should be updated to only provide the necessary amount of heating.

A Worthy Investment

Every day, millions of people are subjected to poor air quality in their workplaces, schools, retail environments and other commercial spaces. This can be addressed by prioritizing ventilation, upgrading and/or installing air filtration systems, and regulating humidity levels.

While these measures are an added cost, investments are worthwhile for commercial applications. Reducing transmission of diseases within the workplace will reduce employee absenteeism, and employees are made more productive by good thermal comfort and low CO2 levels. These investments bolster the bottom line of an employer by paying for fewer sick days, improving overall productivity and even lowering healthcare premiums. Commercial landlords may also be able to justify a rent premium for healthy buildings.

Improving IAQ—especially in the cold winter months—is not just a health necessity but also a smart financial decision.

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Bryan Cummings, Ph.D., spent seven years as an indoor air quality researcher at Drexel University prior to joining Harris as a senior building environmental science engineer. He currently leads computational fluid dynamics simulations and analysis as a member of the Harris Design Studio.

*Ward, M., & Siegel, J. A. (2005). Modeling filter bypass: Impact on filter efficiency. ASHRAE Transactions, 111(1), 1091-1100.