Building a tight envelope and having a good air filter turns out to work well when the fires are burning.
In California right now, the air quality is terrible due to smoke from unraked burning forests. One expert with an air quality meter notes that "old homes are very leaky and in smoky conditions that is BAD. Polluted air infiltrates the house through all kinds of penetrations: older windows and doors, electrical outlets, plumbing fixtures etc." He recommended taping cracks and windows to prevent infiltration.the Berkeley Lab Indoor Environment Group noted that newer houses often have mechanical ventilation systems that should be turned off.
Many recently constructed homes and some that have undergone extensive retrofits have a mechanical ventilation system to ensure they get enough outside air under normal conditions. (In California the most common system is an exhaust fan in the laundry room that is designed for continuous operation. Elsewhere the systems may bring in outdoor air through the forced air system, with a control on the thermostat or "Air Cycler" unit.) In most cases, mechanical ventilation should be turned off during severe outdoor air pollution events.
But not in all cases;
The exception is a system with a high-efficiency filter. If you live in an ultra-tight home, such as a Passive House, you should not turn off your ventilation system. Instead, you should rely on filtration.
Air leakage is tightly controlled under the Passivhaus standard. Chie Kawahara, who lives in the Midori Haus (I reviewed her book about it here), notes that even in her house some particles are still getting through, along with a bit of skunk smell. She runs an indoor air purifier as well during these really bad days. Chie writes about managing indoor air quality:
We enjoy living in Midori Haus built to the Passive House (Passivhaus) standard. The tightly sealed enclosure, about 10 times tighter than conventionally built houses, keeps random air from coming in from random places. The heat recovery ventilator provides us with continuous filtered fresh air. Only during these extended bad air quality days do we need to pay special attention to our ventilation system to keep our indoor air clean.
For many years I argued that we should build houses like they did in Grandma's day, with big double hung windows and lots of natural ventilation. I have had to rethink that in the face of climate change and what we are learning about the air we breathe. Particulate pollution is far deadlier that we knew, whether it is from burning diesel or burning wood. New studies are linking it to cancer, diabetes, Alzheimers and obesity.
I have noted that California is changing its building code to require significantly better insulation and windows to reduce energy consumption. But they are also requiring "highly efficient filters that trap hazardous particulates from both outdoor air and cooking." It may well be time to make Passivhaus level Air tightness requirements part of that new code as well; these forest fires will not be the last, even if everybody starts raking.