A mild atrial septal defect, such as one smaller than 3mm, allows for a normal life without any impact. In fact, some people only discover this condition during medical exams in their seventies or eighties. Therefore, it is safe for these individuals to travel to high altitudes. However, a larger atrial septal defect often causes symptoms like tightness in the chest, palpitations, and difficulty breathing after hypoxia or intense exercise. These patients have poorer compensatory abilities and are advised against going to high altitudes to avoid high altitude sickness. If such patients need to go to high altitude, it is recommended that they consider surgery before doing so.