• There is a higher level of PM2.5 pollution outside of the house. Readings are in the 80-150 range due to wildfires. (PM2.5 levels determined by sensors in PurpleAir.com's sensor network)
  • Using a handheld air monitor that can measure PM2.5 readings were taken indoors.
  • Most rooms in the house are reading +/- 40, but one room read 450
  • smokey haze could be seen in the room during the highest readings (what alerted us to the problem)
  • relative humidity outside is typically low (30-40%)
  • the room with the high reading (450 PM2.5) has
    • high humidity (80-90%) to support tropical plants
    • windows from the 1990s (older, don't seal well)
    • an attic access panel that allows some air movement
    • no fans that would draw in air from outside or push air outside
    • room is long and skinny (40'x15') with windows along the long axis and has a door halfway down the 40' axis
    • one end has higher humidity (where the humidifier is)
  • putting 3 small HEPA filters in the room can reduce PM2.5 to 5 in about an hour. After about 10 hours, readings of PM2.5 are much higher 80-400
  • if we reduce the PM2.5 to under 10 and close the door halfway along the 40' axis of the room, the lower humidity side of the door stays free of PM2.5 whereas the PM2.5 on the higher humidity side of the door continues to increase.
  • there is no internal source of PM2.5 - this only started this week as smoke has blown into the area

Q: what's the likelihood that the humidity differential between indoors and outdoors is "pumping" air and effectively pulling smoke indoors?

Q. What's the likelihood of the humidity differential being able to concentrate the smoke indoors (there have been no readings outside that come close to the 450 PM2.5 that was seen indoors, could the humidity differential be "pumping" pollution indoors and concentrating it)?

Q. what else might be contributing or causing this?

  • 2
    $\begingroup$ High PM2.5 in the high humidity room might be an issue for your instrument, unless it is a high-priced one (check notice for RH correction). These two articles suggest this could indeed be the issue: 1 (behind paywall unfortunately) and 2 (on PM10). The first tested twelve PM2.5 instruments and indicate increasing bias with RH for almost all of them. $\endgroup$ – Luc M Oct 5 '20 at 21:24
  • $\begingroup$ How does the temperature in the high humidity room compare to the temperature outside? I am highly suspect of your numbers. Maybe its instrument error? Maybe there is a source that you aren't fully aware of, like asbestos? $\endgroup$ – BarocliniCplusplus Oct 9 '20 at 19:45
  • $\begingroup$ @LucM If they are using optical scatterometry (measuring either scattered light positive-going pulses or negative-going dips in transmitted light) it's hard to understand how even the cheap ones could be affected much by non-condensing levels of humidity. Instead I wonder if some of the porous particles in the population are absorbing water and either becoming larger or their index of refraction is changing thereby increasing their interaction with the laser beam. That aspect might be best asked as a new question. $\endgroup$ – uhoh Nov 7 '20 at 0:54
  • 1
    $\begingroup$ @uhoh the origin of the increasing bias with RH was not the focus of the articles I mentioned above. I can imagine though that if we assume non-condensing levels of humidity your explanation makes sense. The instrument would be counting smaller porous particles (enlarged by high humidity) which would not be detected under dryer conditions. However for comparability of measurements, that is not ideal (that's why as stated in 2 "monitoring instruments are fitted with a heater or dryer at the inlet"). $\endgroup$ – Luc M Nov 8 '20 at 0:29
  • $\begingroup$ @LucM or crank up the laser intensity and bake them in situ. That's mostly humor but I somehow remember once reading about laser-heating up to the point where particles incandesce (thermally radiate) and measuring something else as well, maybe cool-down time? $\endgroup$ – uhoh Nov 8 '20 at 2:16

Humidity or recent rain plays a huge difference in air pollution. Seattle has less air pollution, because the effect of heavy rain and water accumulating on particles 'wash" the air. In hotter, drier Los Angeles the pollution stays in the air longer because there's less moisture to weigh it down. Mold, spores, dust and allergens do thrive in humidity but those aren't Pollutants in the classical industrial sense.


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