A risk factor in medicine is something that causes, or helps to cause, a medical condition. For example, smoking or breathing in somebody else's tobacco smoke (called second-hand smoke) is a risk factor for emphysema and lung cancer.
A trigger factor causes the symptoms of a medical condition to begin in a person who already has that medical condition. Example: Smoking or breathing in someone else's smoke can be a trigger factor for asthma. The person already had asthma but the smoke caused an asthma attack.