Its other smoke-impurities, dirts and dusts, escape, or are blown away so rapidly that they are seldom thick enough to be injurious to health, except in the narrowest and darkest streets; so that it is always safe to open your windows wide for air, wherever you may live.

The principal danger from smoke is that it cuts off the sunlight.
The Necessity for Ventilation--Impurities of Indoor Air.

The worst impurities in air are those that come from our own breaths and our own bodies; and, unexpectedly enough, carbon dioxid is not one of them.

In spite of hundreds of experiments, we do not yet know exactly what these impurities are, though they are doubtless given off from our lungs, our skins, our mouths, and teeth, especially if the latter are not kept clean and sweet, but left dirty and decaying.
We do know, however, to a certainty that air shut up in a room, or house, with people, rapidly becomes poisonous and unwholesome.

As we breathe on an average about eighteen or twenty times to the minute when we are grown up, and twenty-five to thirty times a minute when we are children, you can readily see how quickly the air in an ordinary-sized room will be used up, and how foul and unfit for further breathing it will become from being loaded with these bad-smelling lighter gases, with the carbon "smoke," with heat, and with moisture.