n fresh air with low pollution levels, our bodies perform the subconscious act of breathing. You inhale and exhale air continuously to stay alive, absorbing oxygen in the lungs for use in biochemical reactions around the body. The atmosphere contains around 21 per cent oxygen, but this level only has to drop below 19.5 per cent to become unhealthy. Oxygen levels between 16 and 19.5 per cent cause tissues to stop functioning properly under exertion, and between 12 and 16 per cent your ability to think clearly stops, even when resting. When the air becomes unbreathable and contaminated with
