Wildfires, SARS-CoV-2, & Portable Room Air Cleaners
by Kristof Irwin and M. Walker
Air quality is in the news these days. If you’ve been keeping up with the news, you’re likely not surprised (although likely as saddened as we are) to hear about the massive spread of wildfires across the American West these last months. These fires are, of course, more widespread than in recent memory and are occurring much later in conventional wildfire season. If there is any silver lining to these fires occurring contemporaneously with the SARS-CoV-2 pandemic, it’s that many large reach media outlets have been covering the topic of air quality and bringing the work of many hard working scientific researchers in the field.
However, the topic is not new in the air quality research circles. In fact, Lawrence Berkeley National Laboratories has noted the shift in wildfire intensity in their wildfire indoor air quality guidelines (an excellent source of wildfire air quality information):
“Increased outdoor temperatures and heat waves are expected to lead to increased wildfires. Data suggest a large increase since 1983 in area burned per year in the U.S. [23], although the large year-to-year variability makes conclusions difficult. Climate change is also projected to increase the number and severity of droughts in some regions of the world, also contributing to increased wildfires.”
To put a finer point on it, the Union Of Concerned Scientists has also clearly established that wildfires in the western United States are getting worse for a host of reasons that will not be easily resolved in the coming years without major domestic and foreign policy initiatives.
“While fire is a natural and essential part of these ecosystems, warming temperatures and drying soils—both tied to human-caused climate change—have contributed to observed increases in wildfire activity. The earlier snowmelt and higher temperatures—and resulting drier soils from increased evaporation—in addition to greater water loss from vegetation have contributed to lengthening the Western fire seasons. Leaders at CalFire even suggest there’s not a wildfire “season” at all anymore, as California in recent years has been battling blazes year-round.
Factors unrelated to climate change affect wildfire risk as well. Past fire suppression and forest management practices have also led to a build-up of flammable fuel wood, which increases wildfire risks. The risk to people and property is also rising because of the increasing number of homes and businesses being built in and near wildfire-prone areas known as the “wildland-urban interface.”
In addition, increased tree mortality due to bark beetle infestation—which has underlying climate drivers—has also modified landscapes in ways that make them more likely to burn. Multi-year drought and precipitation patterns also contribute to the growth of low vegetation that is prone to combustion when dry, serving as kindling for larger fires.”
This is significant for a few reasons:
Beyond the typical life-safety concerns that accompany these increasing wildfire occurrences, the sheer magnitude of wildfires across California, Oregon, and Washington (in fact it’s actually more than 4.6 million acres in 10 states, according to the National Interagency Fire Center, including the more than 1.5 million acres in Oregon and Washington) has made for challenging air quality conditions in which people are able to keep themselves safe from SARS-CoV-2 spread as they flee from evacuation zones or shelter in place in their homes.
SARS-CoV-2 notwithstanding, wildfire smoke itself is incredibly dangerous and can even be lethal. Smoke is actually made up of lots of tiny particles that are much smaller than the diameter of a human hair and as we’ve discussed in previous articles, presentations, and podcast episodes, these can penetrate deep into the lungs and enter into the bloodstream depending on their size and characteristics. The highest risk groups are people who are older and those with underlying lung or heart conditions, but children are also at a greater risk given the higher volumes of air they breathe relative to the size of their bodies.
If wildfires are to be a more frequent and intensive aspect of life in the US and future pandemics are not out of the question, how do homeowners start addressing their air quality to improve the safety their homes can provide? We’ve heard from many clients, friends, and family members in wildfire affected areas asking questions like this so we thought it was worthwhile to expand our air quality focus beyond just SARS-CoV-2 and provide some meaningful content that can serve wildfire sufferers as well.
Fortunately, the strategies to control both pathogens and poor air quality caused by wildfires intersect in significant ways. So in a continuation of our previous articles on the topics of health precautions for construction job sites and designing for healthy environments while reducing pathogen spread, as well as podcast episodes on the impact of ventilation and filtration on virus transmission, we’re bringing some applied scientific guidance for you on the topic of portable room air cleaners (or PRACs).
In many American’s homes where the building enclosure and existing mechanical systems cannot be altered without accruing great cost, there is a clear need for supplementary solutions for keeping the air clean, especially with wildfires and pandemic outbreaks happening just outside the door. Based on the scientific research available on air quality in homes, evidence suggests that increasing the capture of pollutant particulates in the breathing zone is an effective way to protect yourself and your family. This is where PRACs are useful. They allow the average homeowner to avoid a long list of questions, factors, metrics and tradeoffs about the enclosure, mechanical systems, occupancy, weather and climate (all of which make good sense to us building science geeks).
But given how vast the marketplace is for such filtration systems, what is the simplest and most accessible science-based way to choose the right one for a home? It’s not as difficult as it may seem at first glance. Before we give you the basic math to work out and properly size one for your home, let’s talk about an important performance metric to consider when making a purchase.
Clean Air Delivery Rate (CADR)
The Clean Air Delivery Rate is the metric you want to look for in making the right choice for an air cleaner. The metric is a measure of how much clean air the unit can provide through its filtration system. Another way to think about this - the CADR tells you how fast a portable room air cleaner can clean the air within a given room size.
The CADR rating is measured in CFM, which you’ve likely encountered before when reading about mechanical systems or blower door leakage testing. For us non-metric literate Americans, CFM stands for “cubic feet per minute” (or m³/hour). This rating was developed by AHAM (Association of Home Appliance Manufacturers) and determined by the ANSI/AHAM AC-1 test.
While this is an effective test, it’s not without caveats (testing protocols are never perfect). CADR ratings apply to a specific category of contaminants and in this case we’re talking about particulates like dust, pollens, and smoke. There are other considerations to make when looking to filter the smaller viruses and bacteria that are NOT bound to some other particulate host. It’s also worth noting that the testing protocol for this rating is 20 minutes so while we can reasonably extrapolate performance beyond this threshold, not all cleaners are created equal and mileage may vary.
Another consideration is that of the ionizer. Ionization is fundamentally a process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons. When air cleaners use ionization, the idea is to electrically charge air molecules so that airborne particles become charged as they attract charged ions from the ioniser via electrostatic attraction (think rubbing socks on carpet). In theory, these particles in turn are attracted to any nearby earthed (grounded) conductors in plates designed within an air cleaner. Most often, they simply attach to the nearest walls and ceilings and are easily resuspended into the air. What makes ionization a point of interest with regard to the CADR is how it can bias the results of the ANSI/AHAM AC-1 test.
When Positive Energy specifies filtration systems, we use AHAM’s CADR rating as a reliable and accurate measurement benchmark. In short, the better the CADR rating, the more powerful a portable room air cleaner’s fan is and the better it can filter unwanted particles (like wildfire smoke) from the air. As you’re shopping for a PRAC, we recommend using the CADR as the primary performance metric on which to base your decision.
Show Me The Math
Let’s now consider two scenarios and calculations you can make for informed purchase and use of your portable room air cleaners.
I’m Looking To Buy
Know the room area
The area of the room or the area in the room I want to clean is A = ___ SF (ft^2)
The ceiling height is H = ___ ft
The ACH I want is 2 or 5 or 8; ACH = ___ (1/hr) oddball units, but that's what they are
Recommendations: ACH = 2 for normal use, 5 for allergies/mild asthma, 8 for smoke or sensitive asthma (For reference - ASHRAE-170, which specifies ventilation for healthcare spaces, requires 20 ACH for Operating Rooms in hospitals)
ACH is the number of air changes (exchanges) per hour
Then, you'll need a minimum CADR = (A*H*ACH)/60
I Already Have One!
Looking to know how large of a room/area it can clean
The CADR of the PRAC I'm looking at is ___ CFM (ft^3/min)
Ceiling height H = ___ ft
The ACH I want is 2 or 5 or 8; ACH = ___ (1/hr) oddball units, but that's what they are
This will serve a room/area of A = (CADR*600/(ACH*H)
In Conclusion
Now you know how to size a PRAC effectively and you know how to evaluate performance metrics across competing products. Take a look at The Wirecutter’s recent review of portable room air cleaners for a pretty comprehensive list of consumer grade pieces of equipment you can buy online today.
On a tactical level, it may be worth considering the purchase of a larger unit than you need at a minimum so that it can run on lower speed (typically the lower a fan speed, the quieter its operation). Loud fans often cause folks to operate these units less (after all, you’ve got to be able to hear your Netflix binge well enough) and the filters will last longer between changes (all things equal as far as pollutant loading in the room air). And for more information on other practical, low cost ways to protect yourself from poor air quality caused by wildfires, here’s an interview our friend Dr. Brett Singer at Lawrence Berkeley National Laboratories did for an LBNL newsletter last year.
Finally, these are challenging times our country and society are facing. Our hope is that together we can increase our collective knowledge of how to keep our homes/families safe in the midst of major disruptions. We also want to emphasize that caring for each other as human beings, colleagues, friends, and family members is so important. Listening to expertise when attempting to solve complex, technical problems - like a pandemic or wildfire safety/management - is the path forward for our society. We’re here for the ride with you and look forward to all the learning and growth we’ll do along the way.