Normally, you would only have to add around 100-300PSI to the system. Off the top of my head, 1850PSI is the aprox. system pressure. 300PSI represents 16% of the total pressure.
The air we breathe is aprox. 78% nitrogen and 21% oxygen. In order for a potentially fatal level of nitrogen to be added, the aircrafts system pressure would have to have been considerably less than 380PSI!!!!!!!!!!
This would be correct at sea level, but, at high altitude a much higher percentage of Oxy is required. If an explosive decompression happened at 40,000 feet, the regulator on each flight crews Oxy mask would give 100% Oxy in order the get sufficient partial pressure of Oxy into each pilot's lungs. If some of the gas being inhaled is not Oxy, then it's possible that the partial pressure of Oxy would be insufficient to remain concious. This error could have had very serious consequences if one of the affected aircraft had a decompression at very high altitude.