I'm confronting once again the California wildfire season. The outdoor air quality is poor right now and it's spreading all over. If we bunker down in place I'm worried about the air conditions inside our living space getting too unbearable. We have a couple of portable HEPA filters and they do a great job filtering and recirculating the air, however we still need fresh air in the building. The trapped heat, humidity, increasing carbon dioxide concentration, major stuffiness, etc is a lot to deal with during the summer heat where normally opening a window to the outside would mitigate all of the above. However in these wildfire conditions ventilating with outside air subjects you to wildfire smoke which immediately and overwhelmingly infiltrates the house.

**How can one exchange air with the outside while filtering the incoming air sufficiently to ideally eliminate wildfire smoke?**

[![map of California with associated Air Quality Index][1]][1]

To be concrete we're talking about taking outside air with a [Fine Particulate Matter (PM2.5)](https://www.baaqmd.gov/about-air-quality/current-air-quality/air-monitoring-data/#/aqi?id=316&date=2020-08-19&view=hourly
) reading greater than `150` and dropping it down to the healthy range of at most `50`.

    PM2.5 > 150 ---> PM2.5 <= 50

Here's some background on wildfire smoke that the filters will need to contend with:

>Particles from smoke tend to be very small - less than one micrometer in diameter. For purposes of comparison, a human hair is about 60 micrometers in diameter. Particulate matter in wood smoke has a size range near the wavelength of visible light (0.4 – 0.7 micrometers).
>
>https://www3.epa.gov/ttn/amtic/files/ambient/smoke/wildgd.pdf


> Particulate Matter, or “soot,” is made of microscopically small particles, either solid or liquid. The smaller the particles, the deeper they can penetrate into the respiratory system, which causes more significant health problems.
>
> Particles less than 10 micrometers in diameter can cause significant health problems. **Particles under 2.5 micrometers, called “fine” particles, are considered the most dangerous. Fine particles can easily bypass filters in the nose and throat and penetrate deep into the lungs.** Wood smoke from woodstoves and fireplaces is the major source of fine particulate matter in the wintertime in the Bay Area.
>
> https://www.sparetheair.org/understanding-air-quality/air-pollutants-and-health-effects/particulate-matter



  [1]: https://i.sstatic.net/2i9De.jpg