When it comes to Indoor Air Quality (IAQ), the traditional engineering mindset has often relied on a simple mantra: the solution to pollution is dilution. For decades, the default response to poor indoor air has been to open the dampers, increase the air exchange rates, and pump in more outside air.
But as we face new challenges with airborne pathogens, IAQ decisions must hold up beyond “this is what we’ve always used.” A recent comparative study of ASHRAE Ventilation Standards reveals that relying solely on passive dilution may leave building occupants vulnerable.
The Limits of Passive Dilution
To evaluate the true efficacy of current ventilation methods, a study compared the pathogen-reduction rates of ASHRAE Standards 62.1, 170, and 241 against those of a continuous active in-room air-cleaning device (CA-IRAC), specifically a CASPR unit.
The CASPR unit used in the study actively cleans the air in the space, using oxidizers. To create a true comparison, the testing chamber utilized different percentages of outside air based on the specific standards:
- Standard 62.1 utilized 15% outside air and no filtration.
- Standard 170 utilized 33% outside air and no filtration.
- Standard 241 utilized 41% outside air and no filtration.
- The CASPR control utilized 0% outside air and no filtration.
Using the bacteriophage MS2 as a safe surrogate for testing respiratory pathogens, researchers measured the reduction in the pathogen over a 60-minute period.
The Critical 4-Minute Mark
When an infected individual introduces infectious respiratory particles (IRPs) into an occupied space, the time required to neutralize them is critical. The study showed staggering differences in the first four minutes of continuous operation:
- Standard 62.1 achieved a meager 1.52% reduction.
- Standard 241 achieved a 32.39% reduction.
- Standard 170 achieved a 49.50% reduction.
- The CASPR unit achieved a massive 70.26% reduction.
Furthermore, at the end of the one-hour testing period, CASPR reached a 99.5% reduction of the pathogen.
Time to Rethink the Standard
The data points to an uncomfortable truth for traditional HVAC design: current ventilation guidance may be insufficient to adequately reduce the risk of infection from IRPs for occupants across various building types.
The study concludes that dilution of indoor space with varying percentages of outside air is not as reliable nor as effective, compared to results from a continuous active IRAC like CASPR. In other words, simply replacing indoor air with outside air does not neutralize threats as rapidly or completely as active, continuous air cleaning.
As we design the next generation of safe, efficient buildings, we can no longer rely entirely on outside air to solve our indoor problems. Ventilation is necessary for the health and well-being of facility occupants, and it should be paired with technologies that more reliably and effectively reduce the possibility of transmission of respiratory infections from one occupant to another.
Read the full study: CLICK HERE