Well, CO levels definitely get less the farther back you go. I had RJ surf all the way up to the deck, so the front of the board was over the tip of the deck. The only time I got a few readings over 30ppm, which is not dangerous unless you wakesurf at that level for like several days straight, was during a tight right turn before we put on the FAE. During the turn the exhaust seem to escape different, and this was a turn we could have only done with the rope.
A great source of information on distance and CO levels is a study done by the USCG. They measure at 2ft above the ground and 5ft above the ground. The only time they had higher levels was at 2ft from the water level. Here is a figure from the report.
You can see they show 45ppm at 2ft above water at 7.5 and 10mph. This is the worst, but even at 45ppm you would need constant exposure at 45 ppm for about 8 hours, and you would get a headache, thats about it.
From all of my reading, CO levels are considered dangerous at above 35ppm, but that is regarding industrial or occupational danger. In other words, 35-70ppm can cause exposure symptoms if sustained over an 8 hour period. Here is a link to that safe distance report. http://www.uscgboating.org/command/co/files/CO_SafeDistanceReport.pdf
Here is another figure from a report found on the USCG site on CO and boating. It shows the effects of CO at different levels over time. This says at 50ppm, you would have to be exposed for about 4 hours to get to 15% COHb, which will cause a slight headache.
It is between 150-200ppm where the danger starts. Consider that 40% COHb could be fatal. Technically, you need 50%, but if you faint, which you can at 40% COHb, you can die if you are not wearing a life jacket. So 4 hours at 200ppm could get you to 40% COHb.
Now that is VERY possible on the back of your boat, or riding for long periods on the swim deck or sun deck, or even back seat, especially if you have a DD boat. But not wakesurfing. Levels can be sustained on your sundeck at 300-400ppm. You can passout in an hour at that level. Think of some of the long 5mph zones you may have been in. Think if someone is a child, or someone more sensitive.
So, IMHO, and the data supports it, wakesurfing is SAFE, but riding on the back of your boat at slow speeds is NOT. You need an FAE for riding in your boat, not wakesurfing. HTH.
you can find more info here. http://wake9.com/cokills