So it is well to remember that we are fearfully poisonous to ourselves, unless we give nature full chance to ventilate us.
There are also other ways in which the air in houses may be made impure besides by our own bodies, but none of them is half so serious or important.

All the lights that we burn in a house, except electric ones, are eating up oxygen and giving off carbon dioxid.

In fact, a burning gas jet will do almost as much toward fouling the air of a room as a grown man or woman, and should be counted as a person when arranging for ventilation.
If gas pipes should leak, so that the gas escapes into a room, it is very injurious and unwholesome--indeed, in sufficient amounts, it will suffocate.

Or, if the sewer pipes in the walls of the house, or in the ground under the cellar, are not properly trapped and guarded, _sewer gas_ may escape into the house from them, and this also is most unwholesome, and even dangerous.
Cellar and Kitchen Air.

Houses in which fruit and vegetables are stored in the cellar become filled with very unpleasant odors from the decay of these.