Deep breathing can lead to low CO2 levels and this causes reduced body and brain oxygenation. Even though most people believe in goodness of deep breathing: http://www.normalbreathing.com/CO2-deep-breathing-myth.php
, it can cause health problems.
Deep breathing can be practiced safely if you maintain the same or get increased CO2 levels in the lungs. CO2 is crucial for health since it is a powerful dilator of blood vessels, while overbreathing and deep breathing exercises can cause low CO2 in body cells.
In this video, Dr. Artour Rakhimov analyzes main breathing patterns and explains how to notice deep breathing in other people.