First, I’m not questioning the recommendation to stay indoors, or that poor air quality is bad for your health.
I don’t understand how staying inside is better, especially in homes (like mine) that don’t have central ventilation or air filters. My home isn’t air-tight, and I can’t recycle the air I’m breathing out – so there must be new air coming in from outside.
Can anyone explain *why* it’s safer to stay indoors? How is the air inside safer to breathe?
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If you don’t have any central AC or filters or anything, it’s not safer, it’s the same air. You can wear a mask though.
Even without specific air filters there is some level of filtration. The air in your house will have to pass through narrow vents and slow down. And this cause the particles in the air to settle out and fall down as dust. It is at least better then nothing.
The air in your house will be a little cleaner, because it was trapped inside before the smoke arrived. It’s a clean bubble. However, because it’s fluid, it will slowly be mixed with outside, polluted air, as the doors get opened, air exchanges through gaps in weatherstripping, etc. An indoor air filter will help keep the bubble clean.
Pollutants behave differently than the air itself.
Firstly, obviously, if you have AC or air filters, you’ll have cleaner air.
But the air that comes in through windows is also slightly cleaner than the air outside. The pollution is usually heavier than regular air. It goes up first because it’s warm coming out of car exhausts and factories and the like, but once it cools down it sinks back to the ground. It’s more likely to go ‘straight’ down (or at least with the wind currents) than it is to take a turn and go through a window. Regular air is lighter and will drift into your home much easier.
It doesn’t make the air inside clean, but it makes it cleaner than outside. Slightly. It also means that if you have your windows/doors opened downstairs, the air will be a bit cleaner upstairs because the pollutants don’t tend to rise without more heat.
Again, it doesn’t make the air clean. Just a little cleaner compared to the air outside where you live.
The degree of egress into your home depends on a lot of things, namely the nature of the openings leading outside and the difference in air characteristics (pressure, temperature, humidity). Can air blow directly in with real volume or do you mean seeping through tiny cracks here and there? With windows closed, even with many cracks, the air exchange is likely minimal.
Additionally…
Let’s say you have a window mounted AC unit. It will typically have a filter which will block large particulate matter (what you can see and smell), and the pressure from the air blowing in will push air out everywhere else that it can.
Also, air inside tends to be quite still, allowing particulate matter that is merely suspended to settle out.
However, the most dangerous pollution is the kind you often cannot see or smell – in the PM2.5 and lower range. A window AC and your nose cannot protect against this. You cannot tell it’s there, and it goes deep into your lungs. In this case, it’s better to remove window AC entirely to minimize air exchange.
If possible, I’d recommend getting yourself an indoor HEPA filter. They aren’t terribly expensive, especially if you just want to filter 1-2 rooms. I don’t know about supply at the moment… might be tight.
EDIT: I’d also add, don’t cook anything on the stove top, especially if it’s a gas stove. Indoor air quality gets very bad when you do, and in this situation you cannot vent it outside.
Buildings are usually well isolated and you breathe the inside air. That’s why you need to open the windows occasionally. But in fact you can breathe inside air longer than in the movies.
This isolation is also why there can be cold outside and warm inside: the cold outside air cannot exchange with the warm inside air.
If you have AC, it may take air from outside but filters it.
So the idea with staying inside is basically that you breathe the reserve of inside air that’s trapped in the building, until the bad outside air goes away, and only then you open the windows and get fresh air. Obviously it works only if the bad air goes away faster than you need to vent.
In older homes, if things get bad enough it *doesn’t* help. Look into the [Great Smog of London](https://en.wikipedia.org/wiki/Great_Smog_of_London), for example — **thousands** of people died in just four days.
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