Standard scuba knowledge tells us not to dive and fly within 24 hours, and in fact the advice could be expanded to include avoiding diving and going into the mountains within 24 hours as well. And diving in the mountains, perhaps in a mountain lake, requires a separate certification course entirely. But why? The answer has to do with relative compression, which occurs when the base pressure to which we compare our depth while diving is something other than 1 bar or atmosphere.
At sea level, the ambient pressure — the pressure that we’re exposed to by the Earth’s atmosphere — is 1 bar. But go up into the mountains, and the pressure begins slowly dropping. Airplane and hot-air balloon pilots use complex equations to calculate the drop in atmospheric pressure as they ascend to various heights, but for divers, a simple table with typical ambient pressure at various elevations will do.
When the ambient pressure is lower than at sea level, the increase in pressure as we descend into water is relatively higher, because water maintains the same density whether at 0 feet above sea level or 2,000 feet above sea level. Descending to 10 meters, or 33 feet, in water still yields an increase of 1 bar of pressure. At sea level you begin with 1 bar at the surface, so at 33 feet, you’re at 2 bars, meaning you’ve doubled the pressure your body is subjected to. At 1 mile above sea level, roughly the elevation of Denver, Colorado, you’re at 5,280 feet, and you’re only exposed to 0.8 bars of pressure. But if you go diving in a nearby mountain lake, the total pressure at 33 feet is 1.8 bars — an increase of 125 percent.
As you ascend, the water pressure lessens as normal, but you experience a relatively sharp drop in total pressure when you surface, increasing the risk of decompression illness. Because of this, divers at altitude must use specific tables adjusted for the change in ambient pressure. Most dive computers also have an altitude setting.
Even going for a drive or a hike in the mountains after a dive at sea level can be risky. By climbing to a greater altitude —anything higher than 300 meters or 1,000 feet is considered altitude — you create an increased pressure difference between the nitrogen in your tissues and your surroundings, forcing your body dispel the nitrogen faster, maybe too fast to do so safely, triggering decompression illness. It is possible to get decompression illness hours after your last dive, and many miles from the nearest dive site.
The same can be said for flying. Airplane cabins are pressurized to the equivalent of roughly 2,400 meters, or 7,800 feet of altitude, which can speed nitrogen off gassing to a point where the body can’t keep up. Bubbles form, and decompression illness occurs.
So the standard advice rings true: don’t fly or go beyond 300 meters or 1,000 feet for at least 24 hours after your last dive. And if you’re planning to dive in mountain lakes above 1,000 feet enroll in an altitude-diving course.