We often worry if our indoor PM2.5 readings are above 5 µg/m³, since the WHO guideline for long-term exposure is 5. But the outdoor air, which we generally think of as “cleaner” or “healthier,” often has PM2.5 levels above that—sometimes between 10–20 µg/m³ in my area. So if my indoor air has PM2.5 levels higher than 5, but still lower than outside, is that considered a good or acceptable reading?
Is your indoor consistently above 5?
I also have outdoor that is usually 5-15 while my indoor is consistently 0-5
I’ve personally been more concerned with consistently 1000+ CO2 values due to a tight house, so have to balance opening windows to manage both.
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Generally speaking, you want to decrease your exposure as much as possible. While the WHO have given the long-term exposure guideline of 5 µg/m³, it’s also known that there is no safe level of PM2.5.
Since we spend so much time indoors, you will likely have more exposure to PM2.5 indoors (even if the concentration is a bit lower).
Personally, I would aim to always have my indoor PM2.5 < 5 µg/m³, if possible.
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If you had an air sealed home and the PM2.5 readings are consistently higher than 5, what would that indicate? And would those particles be different than those that come from the outside?
It would indicate that there is a source of PM2.5 indoors. The particles would likely be different, yes, but it would depend on what source the particles are coming from. While we consider PM2.5 as a uniform category, it’s actually much more complex: Beyond the numbers: PM2.5 is not PM2.5
For example, if you indoor PM2.5 concentration is largely made up of dust and pet dander particles, these are much likely much less harmful than the same PM2.5 concentration emitted by a gas stove.
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There has been a lot of wildfire smoke around Toronto, ON, Canada this year so the outside air quality has not been good. My apartment is relatively new so it is pretty air tight with an Energy Recovery Ventilator (ERV) that continuously exchanges a small amount of inside air to keep things “fresh” and the CO2 levels in check - not great when the outside air is not great.
I have upgraded the external air filter in the ERV to the best supported which is still not really good enough for filtering out much PM2.5. It was possible to upgrade the air filter on the heat pump air handler from the default 1" “dust” filter to a much better 2" filter that does filter PM2.5 quite well. The last piece was to change the default thermostat to a better one that has a “minimum fan time” option so that the fan will run even when the AC is not required for cooling so the indoor air is filtered. This has kept the indoor air quite good so far.