What is the cause of wheezing?

Wheezing is a whistling sound created when airflow is partially blocked in one or more of the lung's airways. Usually people who wheeze also experience some sense of heavy or labored breathing. Wheezing can be quite noisy, but the noise level from wheezing does not correlate with the severity of airflow obstruction. When airflow is very limited, wheezing may not be heard at all. The most common causes of wheezing are asthma and bronchitis.