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Any casual reader of this blog might know that I was interested in indoor air quality including airborne viruses – long before it became fashionable. That had various origins including an undergrad focus on ecology, being raised by two heavy smokers, having to manage a coal fired stoker as a kid, working in a HEPA filtered clean room as a research assistant, and routinely getting viral respiratory infections in a hospital staff setting where we were all advised that hand washing was supposed to stop the mini-epidemics. And having asthma through all of that.
The indoor air quality issue has become complicated as our outdoor environment deteriorates. As an undergrad 50 years ago, we studied air pollution scenarios that affected large cities. That included the concept of how smog was created by photochemical reactions but a lot of the specifics were not known. More recently the entire Midwest and Northeastern US has been blanketed by wildfire smoke from Canada. Wildfire smoke is chemically complex. In a lot of areas there are air quality alerts on one day due to wildfire smoke and ozone the next day. Those alerts are graduated to advise people with health conditions like asthma, emphysema, and heart disease on the lower end to limit outdoor activities or stay inside. At high levels everyone gets the same advice.
The advice to stay inside assumes that your indoor air
quality is better than the outdoor air quality that you are being warned about. But is that a valid assumption? How do you get measurements on everything and
know the critical differences? A good
place to start is the outdoor air quality. The EPA has developed a nationwide network
of sensors that detects particulates and ozone in the air and calculates the
air quality index. The AirNow app is
available for your smart phone. It gives
you the outdoor reading, particulates, and ozone, as well as the break points from
Good (0-50) to Hazardous (301-500). It will give you conservative advice about
what to do about health and activity for those break points.
The CDC has a publication on indoor air quality in airports
(1) where smoking was allowed. It provides some more intuitive markers of
indoor quality. They found that the PM 2.5 (<2.5 micron particles per cubic
meter) were 300+ in the smoking areas and 50+ in the areas adjacent to the smoking
areas. 300+ levels are considered “very
unhealthy”. Anyone who has ever been in a
smoke-filled room can probably sense that the atmosphere is not very good for
your health either immediately of after leaving. In Minnesota when the AQI was greater than
300 due to wildfire smoke – you could smell the wood fired smoke.
With an accurate assessment of the outdoor air – what about
your indoor air quality? I was fortunate
enough to have purchased an air cleaner for my office with a PM 2.5 measure
built into the machine. It usually reads in the 1-5 range but when the wildfire
smoke arrived it was suddenly reading 40+ indoors. I had to figure out why that
number was so high. I had just replaced
my furnace and it has a MERV13 filter that should provide some filtering
efficiency. The question mark was how my
air exchanger fit into the mix.
My house is about 15 years old and like most modern houses
it is considered airtight. The concern
by builders and contractors with modern homes is that they are so airtight that
it leads to indoor air pollution from a number of sources including any
combustion processes in the home and volatile compounds in the air from various
sources like cleaning products. As a result,
air exchangers are installed to vent the indoor air and bring in fresh outdoor
air. These air exchangers are designed
to reduce heat exchange and most do not have HEPA ( High Efficiency Particulate
Air [filter]) filters. They have a relatively primitive filtration
system to remove mainly insects and very large particles. They can easily bring
in outdoor smoke so it is a good idea to have it shut off on days where there
is very high particulate matter.
The problem with my new system is that I was not sure that
the air exchanger was off. When my new furnace was installed the air exchanger was integrated into a touch panel with 30 different options and several ventilation settings. I talked
with 5 technicians (3 from the HVAC contractor, 1 from the air exchanger manufacturer,
1 from the smart thermostat manufacturer).
They all agreed shut off the air exchanger was a good idea but they gave
me widely varying advice. I decided to experiment
myself over a period of 12 hours and generated the following graph (click to enlarge).
The first section shows the AQI outdoors versus indoors running the MERV 13 filter through the furnace. There is no difference over that time period. The next period I shut off the furnace filter and used a free-standing Space Gaard air cleaner with a MERV 8 (MERV = Minimum Efficiency Reporting Values) filter. Notice that during this time period the wind picked up outdoors, blew off some smoke and the PM 2.5 dropped from 160 to about 90. At that time I talked with a 6th technician and he gave me clear advice on how to shut off the air exchanger. The last section is with the air exchanger off and all air circulating through the furnace filter MERV 13. At that point the indoor AQ drops consistently despite a blip upwards in the outdoor PM 2.5 and continued to drop to 10. To me that illustrates the importance of making sure the air exchanger if off when the outdoor AQ is poor and actively managing it to turn it one when the outdoor AQ is acceptable.
A related indoor AQ related to viral transmission is the
carbon dioxide CO2 levels. Lower levels correlate with less people rebreathing
air in the room and that decreases the risk of infection from airborne viruses.
Outdoor CO2 is roughly 400-420 ppm. My indoor measure is currently 570
without the air exchanger on.
There are currently PM 2.5 and CO2 monitors available in most home stores and large online retailers. What we really need is a more comprehensive single device that measures and records all of the parameters. I would suggest PM 2.5, PM 10, CO2, Ozone, and Volatile Organic Compounds (VOC). The closest I could come to that device was a gadget that required that I purchase a separate weather station and even then the bandwidth to multiple devices was limited.
Home HVAC system design could also use some innovation. Just
based on my experience durability is a problem. Should an HVAC system last
longer than 14 years? Probably. But the design itself does not seem very
efficient. I am not a certified HVAC
tech by any means but it appears to me that the air exchanger introduces
outdoor air into the system after the air filter so that any particulate matter
in the outdoor air does not get at least one pass through the highest efficiency
filter.
Outdoor air quality is a little discussed casualty of climate
change. As the environment deteriorates, I expect that there will be increasing
amounts of wildfire smoke and it will be chemically more complex. I currently
wear an N95 mask outdoors during the alerts, but I can envision a time in the
not-too-distant future where respirators that can also remove ozone and organic
chemicals will also be necessary. Geography is no longer helpful in separating
clean air from polluted air. Monitoring your personal indoor air quality and
figuring out how to manage it will become the most critical part of home
management. I have posted a few things that you can do right now and I am
always interested in other ideas about how to address this problem. Please post any of those ideas in the comments
section.
George Dawson, MD, DFAPA
References:
1: Centers for
Disease Control and Prevention (CDC). Indoor air quality at nine large-hub
airports with and without designated smoking areas--United States,
October-November 2012. MMWR Morb Mortal Wkly Rep. 2012 Nov 23;61(46):948-51. PMID: 23169316.
2: CDC Health Alert Advisory. Wildfire Smoke Exposure Poses Threat to At-Risk Populations. Link
Update 07/06/2023:
One week after turning off my air exchanger - the PM 2.5 in my house is down to 6 or essentially normal. I talked with my air conditioning tech who also services the air exchanger and he agreed with the approach.
Image Credit:
Canadian Wildfire Smoke in Minneapolis
Chad Davis from Minneapolis, United States, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
file URL: https://upload.wikimedia.org/wikipedia/commons/c/c0/Canadian_Wildfire_Smoke_in_Minneapolis_%2852907984452%29.jpg
page URL:
https://commons.wikimedia.org/wiki/File:Canadian_Wildfire_Smoke_in_Minneapolis_(52907984452).jpg
