Oxygen is a very toxic gas and an extreme fire hazard. It is fatal in concentrations of as little as 0.000001 p.p.m. Humans exposed to these oxygen concentrations die within a few minutes. Symptoms resemble very much those of cyanide poisoning (blue face, etc.) In higher concentration e.g. about 20%, the toxic effect is somewhat delayed and it takes about 2.5 billion inhalations before death takes place. The reason for the delay is the difference in mechanism of the toxic effect of of oxygen in 20% concentration. It apparently contributes to a complex process called aging, of which very little is known, except that it is always fatal.
However, the main disadvantage of the 20% oxygen concentration is the fact that it is habit forming. The first inhalation (occurring at birth) is sufficient to make oxygen addiction permanent. After that, any considerable decrease in the daily oxygen doses results in death with symptoms resembling those of cyanide poisoning.
Concentrations higher than 20% decrease the above mentioned delay. High oxygen concentration provokes in prematurely born babies placed in incubators a condition known as retrolental fibroplasia resulting in blindness. Lung irritation has been reported on experimental animals exposed for several days to high oxygen concentrations.
Oxygen is an extreme fire hazard. All the fires that were reported in the continental U.S. for the period of the past 25 years were found to be due to the presence of this gas in the atmosphere surrounding the buildings in question.
Oxygen is especially dangerous because it is odorless, colorless and tasteless, so that its presence cannot be readily detected until it is too late.