11.   Outbreaks of botulism and very low oxygen levels have killed thousands of birds and fish.

12.   The bacteria to blame was Clostridium botulinum, which naturally occurs in the soil and thrives in conditions of low oxygen.

13.   The frigidity and low oxygen levels of the deep sea are known to keep many old items remarkably well preserved.

14.   When these same receptors detect low oxygen levels during sleep, they delay sending a message to the brain stem and the heart patient stops breathing.

15.   When this happens, arteries in the lung detect low oxygen and constrict.

16.   Others suggested that the high mortality was caused by hypoxia -- low oxygen content in the water -- which has been a long-standing problem in the sound.

17.   Competitors face difficult terrain, sudden climactic changes and low oxygen.

18.   In the vacuum pack, the fruit quickly adapted to survive in a new environment of very low oxygen.

19.   Some scientists also wonder about rising water temperatures and low oxygen levels in the water.

20.   The climbers adapted to the low oxygen flow in a variety of ways.