Edward Smith found that an adult asleep exhaled about nineteen grains of carbonic acid per hour, and, when he walked three miles an hour, the amount was increased to 100.6 grains.

W. R. Nichols, of Boston, found in passenger-cars 23.2 parts of carbonic acid to 10,000 parts of air, and in the Berkeley Street sewer 10.4 parts per 10,000. Wilson found in Portsmouth Prison, in cells containing six hundred and fourteen cubic feet of air, always occupied, 7.20 parts per 10,000, and in cells containing two hundred and ten cubic feet, occupied only at night, 10.44 per 10,000.

Besides the carbonic acid, there is exhaled from the lungs a small amount of organic matter, of unknown composition. It forms a glutinous coating on the furniture, walls, and windows of closed rooms, decomposes rapidly, imparts a peculiarly offensive odor to the air of a badly-ventilated room, and poisons those who inhale it. Its quantity is so small that it has so far defied analysis. In a room contaminated by respiration alone, the odor of this substance begins to be perceived when the carbonic acid has increased to about 7 parts in 10,000, and 10 parts in 10,000 may be considered the maximum amount of carbonic acid allowable in dwellings.

The following table shows how much carbonic acid artificial lights produce per hour:

Petroleum, slit-burner,10candle-light,1.98cubic feet,
Petroleum, round-burner,7.6”2.15”
Oil-lamp,4”1.09”
Candle,1”.39”
Coal-gas, slit-burner,7.8”3.25”
Coal-gas, flat-burner,10”3”

A five-foot gas-burner produces as much carbonic acid per hour as five men.

As the most poisonous element of the breath can not readily be detected by analysis, the amount of carbonic acid is taken as a measure of the impurity of air contaminated by respiration.

Test for carbonic acid in air (Pettenkofer’s method):

Shake up a definite volume of the air in a closed vessel with a definite amount of lime-water. The carbonic acid unites with the lime, forming carbonate of lime. This compound, being insoluble in water, renders it turbid. The degree of turbidity may be judged of by looking through the water at a cross marked in lead-pencil on the inside of a piece of paper pasted on the opposite side of the bottle, and a standard may be fixed by shaking up ordinary external air in a sixteen-ounce bottle, as described below, which will show the degree of turbidity produced by 4 parts of carbonic acid in 10,000. Lime-water can be bought of a druggist, or made by shaking distilled water with slaked lime, allowing it to settle, and pouring off the clear liquid. With a common hand-ball syringe, the end of the rubber tube resting on the bottom of the bottle, pump in air, until the bottle is filled with the air to be tested. Put in half an ounce of lime-water, cork the bottle, and shake it up well. Let it stand for five minutes, and if the water becomes turbid, as if a little milk had been dropped into it, the presence of carbonic acid in the air will be indicated in the following proportions.

Size of bottle.
ounces		Amount of
lime-water.		Parts in
10,000.
161-2 ounceA little less than 4
12”A little more than 5
10”A little more than 6
8”8
6”A little more than 10
4”A little more than 15