In order to understand the hazards of confined spaces, its important to have a basic understanding of human toxicology. Toxicity is the relative ability of a substance to produce a harmful effect in an organism. Toxicology is the study of these substances and thier effects.
Many if not all chemicals will produce a harmful effect if administered via the right route of exposure and in the proper dose or concentration. There are three principal routes of exposure- Ingestion, Skin absorption, and inhalation. Exposure by inhalation is the most severe as far as confined space safety is concerned.
If oxygen concentrations fall below 19.5% there is an increased risk of Anoxia. The interaction between oxygen and the human body occurs in the lungs. Look at the diagram of the lungs to the right. The human lungs have 50-100 square meters of surface area for contact to facilitate exchange of gases with the blood stream. The exchange of these gases is accomplished via tiny sacs called "alvioli". Click on the right lung to see an exploded view.