The subject of the thread was not downcurrents; what I was referring to was the fact that oxygen toxicity does not happen immediately after a MOD violation. It takes time.
As long as we are talking about it, though, many times a downcurrent is the equivalent of a waterfall. It can be impacted by the terrain. Just as a waterfall is created by water moving through a channel and over an edge, current moving through a channel on the reef and over the edge can cause a downcurrent, and in such a case,moving laterally along the wall can get you out of the channel. That channel can create a funnel effect.
As I said earlier, I think the risk of down currents in places like Cozumel is exaggerated, in part because of false reports. Here are two examples.
- A dive shop owner, one of her DMs, and her boyfriend had a serious incident including significant DCS (she eventually died, and the DM was crippled), and they reported that they had been caught in a down current and taken to 300 feet and beyond. People were panicking--I'm never going to Cozumel if that kind of thing can happen! Well, it turned out to be a total lie to hide the fact that they had intentionally dived to 300 feet, at which point the dive shop owner either passed out or was so narced that she went to 400 feet before being turned around by the DM. Since they were only diving single tanks, they had to surface without doing any decompression stops.
- In a very widely viewed video, a young and inexperienced boy was wearing a GoPro camera on a helmet, and he went out of control on his descent and only stopped descending when the father got to him and put air in his BCD. It was blamed on a downcurrent, but that was BS. The camera shows the boy was plummeting for sure, but it shows him plummeting past divers who were somehow not being affected at all, and his father had to swim hard to get to him and turn him around. Turning him around was accomplished by adding air to the BCD. It sure looked to me like the overweighted youth had simply forgotten to add air to the BCD and panicked. Once again, everyone commented on how dangerous it is to dive in Cozumel because of those nasty downcurrents.