Smoke, Particles, PM2.5

Maintain Indoor Air Quality Despite Wildfires and Heavy Smoke

Berkeley Lab study finds that Low-Cost Indoor Air Quality Home monitors can be helpful during wildfire season.

California WildFire Smoke Insights

In California, In general, the way Indoor Air Quality is maintained, even during wildfires, is that homes have “fresh air” brought into the breathing zone through leaks in the outside walls. This is accomplished by creating a partial vacuum in the home by running exhaust fans. The  exhaust fan-created suction draws outdoor air (smokey and toxic particle-laden) into the home to fill the breathing zone’s partial vacuum through  leaks, cracks,and  holes in the exterior walls. Building scientists call these exterior walls “the exterior pressure envelope.”. And building scientists say that the exterior walls (exterior pressure envelopes) should be as airtight as possible/reasonable..

Suction-powered ventilation is the way it has been done for decades. But maybe right now is the time to think about change.

  1. The air sucked or leaked in is only incidentally and partially “filtered” when it passes through small cracks and orifices in the exterior envelope (exterior walls), which almost all building scientists agree should be airtight and have no leaks..(or as few as possible.) It is further revealed that activated carbon filters can break down Ozone to make it less harmful, perhaps even better than small cracks, fissures and holes in a building’s “exterior pressure envelope.”
    • In other words,it appears that in most California homes there is no formal filtration of the smoke and micro particulate-loaded air before it gets into your breathing zone and into your family’s lungs.
    • However, a benefit of the exhaust ventilation theory is that as ozone, a common California contaminant, passes through the small holes and leaks, it is partially broken down and rendered less harmful before it enters the breathing zone. A definite benefit. Some scientists advise that ozone is reduced by about 50% as it passes through the holes in exterior walls
  2. Building scientists are well aware that HEPA air filters are readily available and reliably remove 99.97% of smoke and other combustion particles, even  smaller than 0.3 microns.
    • Indoor air quality scientists are also aware that high MERV number filters in the range of 14-16 are able to remove 95%+ of similar particles, and that much cleaner indoor air could be easily provided by first passing fresh air through high MERV number filters instead of cracks and holes in the building’s exterior pressure envelope.
    • The toxin/particle-loaded air can be cleaned by HEPA filters once it inside the breathing zone, but the suggestion of some is that it should be cleaned by a highly efficient filter system before it even starts to enter the home
  3. Caveat and Suggestion. This writer’s home (SW Florida) is served by a fresh air system that first of all pressurizes the breathing zone so that the indoor pressure inhibits the flow of outdoor “dirty” air into the home through exterior pressure envelope openings while it is pressurized. Secondly, this home’s fresh air system incorporates a MERV 14-15 filter through which all ventilation system-supplied air must pass before it enters any area or zone within the exterior pressure envelope. Thirdly, the home’s ventilation rate is operated by a demand control ventilation system controller that is programmed to maintain an indoor carbon dioxide level of  of about 600 parts per million which is the CO2 exposure level that is reported by recent research and more and more to provide startling improvements in cognitive function and sleep over the frequently accepted CO2 exposure of 1100 parts per million (outdoor of 400 ppm plus 700.) The 1100 ppm “standard” has been around and accepted as satisfactory for many decades. This writer has been unable to find any research supporting the 1100 ppm standard. The writer’s ventilation system operates 24/7/365 and easily maintains 600 parts per million except when is occupied by more than about eight  adults. When the home is vacant, CO2 levels drom to around the same as outdoor levels. When the system is turned off, CO2 commonly rises to 1800–2000 ppm overnight with two occupants. Recently, at about 6 am, this writer observed that the indoor CO2 level was  over 2000 ppm even though the system had been operating all night, as usual. The system was delivering fresh air at maximum rate, approximately 70 cfm. Other than the full speed operation at CO2 levels above 2000 ppm, nothing  was unusual. . . . until the front door was opened to dense smoke. Fire in the everglades, lots of smoke and elevated CO2 outdoors. The  only sensory clue indoors was the high rate of outdoor air being delivered into breathing zone. In the breathing zone, no odor of smoke whatsoever. But CO2 definitely was high.

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