The building industry typically focuses on delivering higher quantities of outdoor air as a proxy for occupant wellbeing. The quality of that outdoor air claimed center attention across Washington State with unhealthy ratings for a significant part of August. Now that the smoke has settled, it’s important to examine how outdoor air quality impacts the air we breathe indoors and what can we do to improve our indoor air quality.
As mechanical engineers in the heating, ventilation and air-conditioning (HVAC) industry, we have gained substantial indoor air quality knowledge from working with clients and monitoring our own facilities. A few of the lessons learned are that more outdoor air does not guarantee better indoor air quality and that not all filters are created equal.
Pollutants are typically measured as particulate matter (PM), which quantify substances in micrometers or one millionth of a meter. The typical industry scale ranges from 0.3 to 10. To define the scale a little more as an example, PM2.5 includes all particulate matter 2.5 micrometers and smaller. Smoke is typically measured at PM2.5, but can be as large as PM10 when visible ash is present. In actuality, much of the mass of smoke in PM2.5 is often in the 0.5 to one micrometer size. This is where differences in air filters can make a big impact.
Filters are rated based on their ability to reduce different particle size ranges, measured in minimum efficiency reporting value (MERV). The scale represents performance of a filter when dealing with particulates in the range of 0.3 to 10 micrometers. Most homes use filters rated between MERV 3 and MERV 8. Commercial facilities typically use filters rated between MERV 8 and MERV 13. To remove smoke from the air effectively, a MERV 14 filter or higher is needed with a MERV 16 filter being the most effective. A MERV 14 filter is 75 percent to 85 percent effective at removing particles in this size range, while a MERV 16 filter is 95 percent effective or higher.
Composite Average Particle Size Efficiency, Percentage in Size Range, µm
|Average Arrestance %|
|MERV 1||na||na||E3 < 20||Aavg < 65|
|MERV 2||na||na||E3 < 20||65 ≤ Aavg < 70|
|MERV 3||na||na||E3 < 20||70 ≤ Aavg < 75|
|MERV 4||na||na||E3 < 20||75 ≤ Aavg|
|MERV 5||na||na||20 ≤ E3 35||na|
|MERV 6||na||na||35 ≤ E3 50||na|
|MERV 7||na||na||50 ≤ E3 75||na|
|MERV 8||na||20 ≤ E2||70 ≤ E3||na|
|MERV 9||na||35 ≤ E2||75 ≤ E3||na|
|MERV 10||na||50 ≤ E2 < 65||80 ≤ E3||na|
|MERV 11||20 ≤ E1||65 ≤ E2 < 80||85 ≤ E3||na|
|MERV 12||35 ≤ E1||80 ≤ E2||90 ≤ E3||na|
|MERV 13||50 ≤ E1||85 ≤ E2||90 ≤ E3||na|
|MERV 14||75 ≤ E1 < 85||90 ≤ E2||95 ≤ E3||na|
|MERV 15||85 ≤ E1 < 95||90 ≤ E2||95 ≤ E3||na|
|MERV 16||95 ≤ E1||95 ≤ E2||95 ≤ E3||na|
Common filter ratings based on ANSI/ASHRAE Standard 52.2
A common assumption is to simply use a higher-rated filter within your air handling system. Why not just bump up to a MERV 14 or 16 filter as a standard? The higher the filter rating, typically, the more force it takes to push air through the filter. Higher filter ratings increase your system operating pressure and increase energy consumption. This is a major cost and energy penalty, which is difficult to justify year round when the smoke event may only last for a few weeks.
McKinstry has been working on several indoor air quality dashboards for customers as well as our own facilities in Spokane and Seattle. These dashboards use data from multiple sensors to measure all things air quality related, including temperature, relative humidity, CO2, VOCs and PM. The monitoring also outlines how outdoor air quality measures against indoor air quality, which is critical to measure. While the industry is still trying to determine appropriate levels for some of these metrics, McKinstry teams are actively measuring them so we can take action as needed. The dashboard below from our Seattle campus shows how atmospheric smoke and particulates can impact indoor air quality despite active filtration efforts.
It’s not always easy finding the best solution that aligns occupant health with operating cost and efficiency, especially considering Washington State’s limited smoke events. It’s particularly problematic for homeowners without air conditioning since furnace air handling and filtration systems are typically off during the summer months.
One solution for commercial or residential applications is to keep higher MERV rating filters on hand for smoke and other outdoor air quality events. Another is to have portable air purifiers on hand for temporary use. Artificial air scrubbers and plants, the all-natural CO2 scrubber, are also effective. Minimizing outdoor air during a poor air quality event could be another solution.
It is clear that the more data we collect, the easier it becomes to find acceptable solutions. Over time, the industry will develop better and better solutions for building occupants. There is a lot to consider, and our industry will continue to develop and share best practices as we learn and evolve.
Michael Frank is McKinstry’s vice president of engineering and design.