Dyspnoea refers to undue awareness of one's own breathing and increased drive to breathe. It can be normal with strenuous exercise but is pathological if it occurs at lower thresholds. Dyspnoea has many potential causes including cardiac, respiratory, neuromuscular, metabolic issues or toxins. It is classified based on the cardiovascular and respiratory systems with grades ranging from no limitations to total confinement based on the level of physical activity one can perform without symptoms.