Now that regular cost-effective home HEPA filters are widely available I mostly rely on those. However, I find that the box fan + furnace filter works really we

Now that regular cost-effective home HEPA filters are widely available I mostly rely on those. However, I find that the box fan + furnace filter works really well in the window to bring in fresh air without letting the pm2.5 in. This is nice because otherwise CO2 levels build up and it gets stuffy inside. This works on all but the worst days when you can really smell the fire/smoke outside, AQI > 200. Overall, its not the prettiest and I'd prefer a HEPA filtered energy recovery ventilation but I'm not going to install that in my rental for a couple weeks a year. Usually in Oakland climate the windows are open.