In a healthy individual, the strongest respiratory stimulus for breathing is

During normal breathing, the brain is stimulated to breath with increasing acidity as a result of CO2 concentration from basic metabolic processes. The brain is quite selfish and only really wants to maintain it's pH which should be at a range of 7.3-7.45, and will not tolerate any decrease. 

In patients who have a chronic respiratory disorder with things like COPD. The brain has become accustomed to excessive acidic content, and is now stimulated by the Hypoxic drive or by low oxygen content.