This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
107. Sewage is composed of water mixed with kitchen slops, grease, soap, urine, washings from stables and slaughterhouses, rags, leaves, paper, human excreta, etc. The animal and vegetable matters in it rapidly decompose and generate noxious gases, and the combination of these is called sewer gas. These gases are poisonous, and will inevitably produce sickness if they escape into a dwelling, or if they are breathed in any considerable quantity for even a few minutes.
An exceedingly dangerous feature of the air contained in sewers is that it is liable to be loaded with putrefactive germs, which will develop typhoid fever, scarlet fever, and diphtheria, if they find a lodgment in the human breath or food.
A small leak of sewer gas into a house may cause much sickness that will probably be ascribed to other causes. Many people will endure a small amount of bad odor rather than incur the expense of sending for a plumber, and are unwilling to believe that a small defect in the drainage can do so much mischief. But the architect must protect people against their own ignorance and cupidity in this matter.