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A large wildfire has been burning for four days some distance from my town. The air is smoky, but not smokier than yesterday or the day before. General wisdom holds that we should all stay indoors.

As I enjoy my relative safety and indoor accommodations, I wonder why the indoor air is safer than the outdoor air. Even buildings that had 12-foot-wide (4-metre-wide) cargo doors open yesterday contain air less smoky than the outdoor air, or at least it smells that way.

Are still there fewer smoke particles per unit volume indoors? Do indoor environments trap the particles better? Why is the air safer indoors?

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If I understand your question correctly, you are asking why air inside is usually cleaner than air outside, especially if it is smokey.

Smoke is actually not a gas, but solid particles that are suspended. Because of this, the smoke particles fall down, but at a very slow rate due to resistance. When the air is blown against the wall, the smoke can either blow around the wall, or it can be deposited into the wall (think of dust build-up). Of course, smaller particles can negotiate the curve easier, so it isn't perfect.

A house is (generally speaking) four walls, so if smoke was blowing against your house, even the small particles that could go around a solitary wall would be blocked off by the walls on the side of the house.

As the other answers suggest, a house generally isn't perfectly cut off (hermetically sealed). This means that small openings in your house can let air and let the smallest particles of smoke in. For other reasons, that is good and bad. If too much smoke is brought in, it is hard to get out (consider a cigarette smoker inside). For example, furniture acts as a capacitor of smoke, gradually releasing it in time.

In your case, consider you open your doors a bit and some smoke got inside. The smoke will diffuse throughout your house and settle into the floor. Assuming the mass of the puff of smoke is much less than the mass of air inside your house, the concentration of smoke inside your house will likely be negligible.

There are a number of different ways you can slow smoke down from entering into your house. Besides the ways listed above, building an anteroom may also prevent smoke from entering and make your house a bit more energy efficient.

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If you're in the open air you will be exposed to the full concentration of the pollutants in the air. By being inside, there's a reasonable assumption that the rate of interchange of air between inside a house and outside will be low and that inside there will be a lower concentration of pollutants.

Houses and buildings are not hermetically sealed. The gaps around windows and doors are small which results in low rate of airflow between the outside and inside of the houses and buildings. The less windy the weather the lower the flow rates.

The other way to consider it is when a room has been closed for a lengthy period while someone has been in the room, the air in the room becomes stuffy. This is because the air in the room is not being replaced by fresh air quickly enough to remove stuffy air. To remove to stuffy air the windows and doors need to be left open for a period of time for fresh air to replace the stuffy air.

The same thing happens when the air outside is heavily polluted. The air inside is cleaner and remains cleaner for a longer period if the doors and windows remain shut because the rate of transfer between air inside the building and outside is small.

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