An explosive depressurisation is one which occurs in less than half a second. This type of decompression usually only occurs in small aircraft flying at very high altitudes. Decompression which occurs this rapidly, at a rate which is greater than the rate by which the lungs can decompress, is likely to cause lung damage. For a specific size of pressure hull breach, the likelihood of the rate of decompression reaching a level where lung damage is possible decreases with an increase in the overall size of the pressure hull.
A decompression of an aircraft which takes less than 0.5 seconds is considered by most authorities to be “explosive”. The cabin air may fill with dust and debris, and fog caused by an associated drop in temperature and change in relative humidity. Crew may be momentarily dazed or shocked, especially if the event was unexpected, and may therefore be slow to fit oxygen masks.
The great danger of depressurisation is crew incapacitation due to Hypoxia. The Time of Useful Consciousness is reduced by the explosive nature of the decompression. Decompression Sickness is another hazard associated with high altitude decompression.
If the cause of the decompression is structural failure, failure of a window for example, there may be a risk of some crew or passengers being buffeted by strong winds, hit by debris, and extreme cold temperatures, or even being sucked out of the aircraft - another reason for wearing a seat belt or harness whenever seated.