see plants are also living beings, they also need oxygen for respiration. There would be less carbon dioxide available for plants for photosynthesis. Atmosphere will be clean, less polluted, clean water. No fighting, peaceful, cool environment
It allows more oxygenated blood to be delivered to the muscles.
Allan already had a heart attack and can try to prevent another one by changing the diet, exercising, or even quit smoking, but there are non-modifiable risks such as age, sex and generic heritage, so If Allan is over 40 years old, male or comes from a family with heart diseases, Allan needs to be extra careful because those factors cannot be changed no matter how the way of life is changed.