[Ostrogoth]

To further expound on why snorkeling and freediving after scuba can be dangerous, it is helpful to understand that the greatest gas compression occurs in the first 10 meters underwater (Boyle’s law). Scuba divers are trained to ascend slowly (~10m/minute) in order to off-gas expanding nitrogen (and other gasses) and prevent bubbles from forming in tissue (which causes decompression illness). After scuba diving the human body still has a much higher residual nitrogen saturation, so even fairly shallow (less than 10m) rapid descents/ascents in the water column can cause bubbles to form.