More nitrogen is more nitrogen. More nitrogen is more risk.
While you may not actually reach the NDL on a long 20ft dive you are adding nitrogen to those tissues. If you happen to be at the more sensitive end for DCS (eg an undiagnosed PFO) then your risk of injury is higher on air. If your profile is aggressive, for example multiple ascents while training, or your post dive activities are strenuous, for example moving kit a long way, up ladders or up a hill, then your risk increases.
Nitrogen saturation is a depth and time thing. Long exposures carry more risk, deeper exposures more risk.
DCS is a risk game, nitrox is a mitigation of some risk.
Focus of NDL as a measure of risk is rather indirect.
Why would a shallow site do nitrox fills? Maybe you do training the, then fill cylinders and go off to do proper diving somewhere deeper.
While you may not actually reach the NDL on a long 20ft dive you are adding nitrogen to those tissues. If you happen to be at the more sensitive end for DCS (eg an undiagnosed PFO) then your risk of injury is higher on air. If your profile is aggressive, for example multiple ascents while training, or your post dive activities are strenuous, for example moving kit a long way, up ladders or up a hill, then your risk increases.
Nitrogen saturation is a depth and time thing. Long exposures carry more risk, deeper exposures more risk.
DCS is a risk game, nitrox is a mitigation of some risk.
Focus of NDL as a measure of risk is rather indirect.
Why would a shallow site do nitrox fills? Maybe you do training the, then fill cylinders and go off to do proper diving somewhere deeper.