To decide as to whether a room is well ventilated or not, some standard of permissible impurity must be fixed, and this standard is now usually taken to be, in a room occupied by human beings, that condition of air which produces in a person having a normal sense of smell, and who enters from the fresh air, a faint sensation of an odor very slightly musty and unpleasant. Upon testing the air of such a room, it will be found that the amount of carbonic acid impurity present--that is, the excess of this acid over the amount in the external air--will be between 2 and 3 parts in 10,000. As the amount of carbonic acid in normal air varies from 2 to 5 parts in 10,000 in different places, and in the same place at different times, it is better to look to the carbonic acid impurity as above defined rather than to the total amount of the acid found present, if strict accuracy is desired; but usually the statement of Dr. Parkes is correct, that the organic impurity of the air is not perceptible to the senses until the total carbonic acid rises to the proportion of 6 parts in 10,000 volumes. When the carbonic acid reaches 9 parts in 10,000 the air is close, and when it exceeds 1 part in 1000 the air is usually decidedly unpleasant. If we take 2 parts in 10,000 as the permissible maximum of carbonic acid impurity, it follows that the amount of fresh air which must be supplied and thoroughly distributed for each person per hour is 3000 cubic feet. If 3 parts per 10,000 be taken as the permissible maximum (which is the standard of Pettenkofer), the amount of air per head per hour must be 2000 cubic feet. While it is impossible, as Dr. Parkes remarks, to show by direct evidence that the impurity indicated by 7, 8, or even 10, parts of carbonic acid per 10,000 is injurious to health, it is advisable to accept his standard, because it is a simple one, and can be practically applied without special apparatus or technical skill, and because there is evidence of the injury to health which continued exposure to air impure, by this standard, ultimately produces.